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Structural behaviour and micro-structural characteristics of coloured kilned glass panels

Structural behaviour and micro-structural characteristics of coloured kilned glass panels Glass Struct. Eng. https://doi.org/10.1007/s40940-022-00208-7 RESEARCH PAPER Structural behaviour and micro-structural characteristics of coloured kilned glass panels Vlad-Alexandru Silvestru · Rena Giesecke · Benjamin Dillenburger Received: 6 July 2022 / Accepted: 4 October 2022 © The Author(s) 2022 Abstract In recent years, the architecture, engineer- observed. Furthermore, the reasons for this reduction in ing and construction industry is increasingly embracing structural performance were analysed based on micro- digital design approaches and robotic production meth- scopic investigations. It was observed that both the ods. This is as well noticeable in the façade design sec- kilning process and the density of deposited granular tor, especially when it comes to architectural design powder had an influence on the surface microstructure. tasks dealing with complex geometries or adaptive The presented results are essential for the production of components. Although glass is mainly used in facades polychromatic kilned glass as well as for future appli- due to its transparency, it often determines significantly cations of this type of glass panel in façades. the aesthetic appearance of a building and has to control the transmitted light and the visibility between exterior Keywords Coloured kilned glass panel · Four-point and interior. Recently, a novel method for producing bending test · Tensile bending strength · Micro- polychromatic glass panels by fusing granular glass structural analysis · Glass surface texture · Surface powder of different colours onto the surface of annealed roughness float glass in a kilning process was developed at ETH Zurich. By using a multi-channel tool attached to a robotic arm for placing the granular powder onto the 1 Introduction glass surface, digitally designed patterns can be realised precisely and repetitively. This paper focuses on the Digital design approaches and automatized robotic structural behaviour and the micro-structural charac- production methods enjoyed increasing interest from teristics of this novel type of coloured glass panels for the architecture, engineering and construction (AEC) façade applications. The bending strength of such glass industry in recent years. However, while for materi- panels was determined based on four-point-bending als like concrete and steel, various research projects tests and was compared to that of annealed float glass. were conducted and some representative construction A non-negligible decrease in the bending strength was projects were completed, in the case of glass, such approaches and production methods are rather limited. V.-A. Silvestru ( ) First attempts of using additive manufacturing (AM) Institute of Structural Engineering, ETH Zurich, for glass at an architectural scale were done by Inamura Stefano-Franscini-Platz 5, 8093 Zurich, Switzerland et al. (2018) in the form of glass columns or by Seel e-mail: silvestru@ibk.baug.ethz.ch et al. (2018) in the form of prototypical connections. R. Giesecke · B. Dillenburger Whereas these projects focused on realizing structural Institute of Technology in Architecture, ETH Zurich, Stefano-Franscini-Platz 1, 8093 Zurich, Switzerland components made of glass, digital design approaches 123 V.-A. Silvestru et al. Fig. 1 Views of the pavilion with polychromatic kilned glass panels showing (a) the relationship to the surroundings through the decorative pattern applied and (b) the graded transparency obtained through the colour grading process. Images credit: Digital Building Technologies, ETH Zurich; Photographer: Evangelos Roditis and robotic production methods were applied recently glass either to slump in a mould due to gravity or to be also for achieving novel architectural glass products. formed by movable moulds. Giesecke and Dillenburger (2022) and Giesecke et al. Several decorative surface treatment processes are (2022) developed a method for large-scale robotic fab- available nowadays, ranging from (i) patterned glass rication of polychromatic glass by fusing granular glass (see EN 572-5 2012) done by casting through pat- powder on annealed float glass panels. An application terned rollers, over (ii) enamelled glass done by burn- of this type of glass for a mock-up pavilion built in ing a ceramic frit colour into the annealed glass dur- 2021 is shown in Fig. 1. As it is the case of other pro- ing the tempering process, to (iii) digital printing cessing technologies of glass, understanding the mate- methods done by applying ceramic or organic colours rial behaviour at different temperatures in the transition on the glass surfaces. All these processes show cer- range from melt to solid is of ultimate importance for tain disadvantages. For instance, patterned glass and applying additive manufacturing principles to glass. enamelled glass lead to reduced bending strengths The glass mostly used in the AEC industry, soda- and are either translucent or opaque. Enamelled glass lime silicate glass, has a glass transition temperature of shows limitations in the achievable colours compared around 570 °C, a softening point of around 725 °C and to digital prints on glass surfaces, which allow for a working point of around 1030 °C (Schneider et al. photo-realistic images. Digital prints on glass sur- 2016). In addition, the strain point is situated at around faces exhibit a lower durability compared to enamelled 505 °C, while the annealing point is at approximately glass. From the available decorative surface treatment 550 °C. These temperatures and the related changing processes, only the enamelling requires the glass to viscosities of the glass are used in various processing be reheated. Maniatis and Elstner (2016) conducted steps, including tempering, shaping or surface treat- investigations on the structural behaviour of enamelled ments. For thermal tempering, the annealed float glass glass. is heated to approximately 100 °C above the glass tran- A decorative surface treatment by fusing coloured sition temperature, where it is in a viscoelastic state, powder on flat glass was used by Saint Gobain Glass and subsequently it is quenched from both sides. This Solutions in collaboration with TNO for the facades of allows to obtain a structurally favourable residual stress the Dutch Institute of Sound and Vision in Hilversum state. In case of hot-bending, the flat glass is heated as (see TNO 2007). Granules in three basic colours were well to approximately 100 °C above the glass transi- deposited in a patterned mould and fused with the glass tion temperature to reach a viscosity, which allows the sheets at temperatures of around 800 °C. 123 Structural behaviour and micro-structural characteristics While for fusing the ceramic frit into the glass, tem- any additional coatings). This basic product made from peratures slightly below the softening point are suffi- soda-lime silicate glass is the one mostly used in cient, for a proper mixing of glass coming from differ- facades, often further processed to thermally tempered ent recycled cullet, higher temperatures are necessary. glass, laminated glass or insulating glass units. From Bristogianni et al. (2020) and Bristogianni et al. (2021) a mechanical point of view, EN 572-1 (2016) specifies used temperatures between 820 and 1120 °C to kiln- for annealed soda-lime silicate glass a characteristic cast small-scale beams (30 × 30 × 240 mm) from cullet bending strength of 45 MPa and a Young’s modulus of coming from different previous applications. Related 70 000 MPa. The thermal expansion coefficient of such –6 −1 to cast glass, Oikonomopoulou et al. (2018)showed glass is 9 × 10 K . that the material used for the moulds influenced the For colouring the annealed float glass panels by obtained finishing surface. While permanent stainless fusing, granulate powder from OPTUL Spezialglas steel or graphite moulds allowed for glossy finishes, GmbH was used. Powder granules of the transparent silica plaster and alumina-silica plaster lead to translu- colour FF-BF 0076 Chrome Green with grain sizes cent and rough finishes. To restore transparency and to between 0.36 and 1.00 mm were used in the exper- increase accuracy of cast glass, generally, grinding and iments discussed in this paper. For the pavilion in polishing were required when using the latter mould Fig. 1, other colours, like FF-BF 0078 Dark Green, types in past applications. FF-BF 2135 Yellow, and FF-BF 3100 White were used In contrast to the above methods, polychromatic as well (Giesecke et al. 2022; Giesecke and Dillen- kilned glass allows for an enhanced diversity enabled burger 2022). These granular glass powders were devel- by digitally designed multi-colour patterns that can be oped to match the thermal expansion coefficient of realized in a precisely controlled and repeatable man- annealed float glass to enable fusing without breakage, ner thanks to the robotic dispensing of the colour. How- according to OPTUL Spezialglas GmbH. The speci- ever, the structural performance of this glass type has fied thermal expansion coefficient of 8.2 (± 0.3) × –6 −1 not been addressed yet, limiting its application. There- 10 K , however, is slightly lower than that of soda- fore, this research investigates the structural behaviour lime silicate glass and could eventually lead to residual and the micro-structural characteristics of coloured stresses after the fusing and kilning process. A colour kilned glass panels. The panels were produced in a palette of 40 compatible granular glass colours, both similar manner to the polychromatic glass presented transparent and opaque, are available. The granules by Giesecke and Dillenburger (2022) and by Giesecke come in six different sizes ranging from powder of et al. (2022). 0.00–0.36 mm up to chips of 5.00–8.50 mm (OPTUL Float 2022). This wide range of available colours and grain sizes opens up a vast diversity of potentially achievable design patterns and resolutions. Due to the 2 Materials and methods packing of the granules, it is not only possible to add two-dimensional properties to the glass, but also three- This section briefly describes the materials involved in the presented investigations and provides informa- dimensional relief features (Giesecke and Dillenburger 2022). tion on the applied methodology for producing the coloured kilned glass panels, for determining their structural behaviour and for evaluating their micro- structural characteristics. 2.2 Manufacturing of polychromatic kilned glass panels 2.1 Material properties of the glass panels The polychromatic patterns of the glass panels for the and the granulate pavilion shown in Fig. 1 were digitally designed to visually interact with the surroundings. For the robotic Annealed float glass as regulated in EN 572-1 (2016) dispensing process, granular glass of four colours was and EN 572-2 (2012), with a nominal thickness of placed in a multi-channel tool attached to a robotic arm 8 mm was used for the panels investigated in this or motion system, as shown in Fig. 2a (Giesecke et al. paper (PLANICLEAR® from Saint-Gobain without 2022; Giesecke and Dillenburger 2022). The robotic 123 V.-A. Silvestru et al. Fig. 2 Multi-channel tool setup for robotic application of the coloured granules (a), and different features of the achievable colour patterns: (b) repeatability due to robotic precision, (c) granular distribution, (d) dense colour fusion and (e) variable transition between colour densities. Images credit: Digital Building Technologies, ETH Zurich arm moved along a specific toolpath based on the pat- of 800 °C was chosen based on the fact that at this tem- tern designed and dispensed a controlled volume of perature the fused coloured granulate powder develops material on the air side of the annealed float glass pan- the desired nuances. els. The granules could also be distributed manually, as it was done for the panels investigated in this paper from a structural and micro-structural point of view. How- 2.3 Four-point bending tests ever, the robotic process enabled the precise, repetitive inscription of decorative and functional (e.g. light and To assess the structural performance of the novel poly- view control) material patterns, as well as continuity chromatic/coloured kilned glass panels in comparison of patterns between elements without the use of a tem- to standard annealed float glass panels, a series of four- plate through a digital design-to-production workflow. point-bending tests were conducted. The test speci- For the investigations within this paper, a manual dis- mens and the test setup were designed according to pensing procedure was chosen for obtaining random EN 1288-3 (2000). The four-point-bending test setup, patterns. The granulate powder was distributed directly which includes the influence of the free glass edges on on the glass surface, without using a fusing gel. Both the the bending strength, was preferred to a coaxial double robotic dispensing process and the manual dispensing ring test setup, since the strength values determined in process lead to similar varied colour distributions, rang- these investigations should characterize the glass pan- ing from isolated granular patterns to densely coloured els with free edges used for the pavilion shown in Fig. 1. areas (see Fig. 2c–e). The main dimensions of the test specimens and the test For the kilning process, the annealed float glass pan- setup are provided in Fig. 4a, while Fig. 4bshows the els with the granular glass deposited on top (on the air assembled test setup. A total of twelve specimens were side) were placed in a Nabertherm GF600 kiln (see tested, as listed in Table 1. This number of specimens Fig. 3a), which could host glass panels with dimen- is not sufficient for statistically significant results, but sions of up to 1 m × 2 m. A textile layer made of a allows to provide first information on the mechanical 2 mm thick ceramic fibre paper was used as a substra- properties of the coloured kilned glass panels compared tum for the glass panels. For the kilning process, the to the annealed float glass panels. According to exist- glass panels were heated to 800 °C and cooled in a ing standards, no differentiation in strength is made for controlled annealing process according to the temper- annealed float glass depending on whether the tin side ature vs. time curve shown in Fig. 3b. The temperature or the air side is subjected to tension. For the coloured 123 Structural behaviour and micro-structural characteristics Fig. 3 Nabertherm GF600 glass kiln for fusing the granules into the float glass surface (a) and applied temperature curve for producing the investigated coloured kilned glass panels (b) Fig. 4 Schematic illustration of the geometry of the test setup with built-in specimen (a) and test setup with its components during testing of a coloured kilned glass panel (b) kilned glass, it was expected that the air side, on which The glass panels had nominal dimensions of the green granular powder was dispensed and fused, 1100 mm in length, 360 mm in width and 8 mm in thick- and the tin side, which was in contact with a textile ness. The length and the width were chosen accord- during the kilning process, might have suffered differ- ing to EN 1288-3 (2000), while the thickness was the ent structural changes. Thus, specimens of the kilned same as for the panels in the pavilion mentioned in glass panels were tested both with the coloured air side the introduction. Especially for the kilned glass pan- and with the tin side, respectively, subjected to tension. els, deviations from these nominal dimensions were expected. Therefore, before testing, the actual sizes of each specimen were measured at four positions for the 123 V.-A. Silvestru et al. Table 1 Overview of the performed four-point-bending test series Specimen type Number of specimens Specimen nomenclature Reference annealed float glass panels with tin side under tension 4 specimens TS-01-FG-ref | TS-02-FG-ref TS-03-FG-ref | TS-04-FG-ref Coloured kilned glass panels with coloured side under compression 4 specimens TS-05-KG-tin | TS-06-KG-tin TS-07-KG-tin | TS-08-KG-tin Coloured kilned glass panels with coloured side under tension 4 specimens TS-09-KG-air | TS-10-KG-air TS-11-KG-air | TS-12-KG-air Fig. 5 Representative edge shapes of the reference annealed float glass panels (a) and the coloured kilned glass panels (b)—pictures of resulting shards after completing the four-point-bending tests width and at eight positions for the thickness. The ref- at a distance of L  200 mm to each other. Soft lay- erence annealed float glass panels had polished edges ers of 3 mm thin ethylene-propylene-diene-monomer (see Fig. 5a). The coloured kilned glass panels, for (EPDM) were applied between all the steel rollers and which float glass with cut edges was used, had after the the glass panels to avoid stress concentrations. kilning process rounded nearly quadrant-shaped edges In addition to the acting force and the machine dis- (see Fig. 5b). For the kilned glass panels, the thick- placement, the vertical deflection of the panels at mid- ness measurements were conducted as far away from span was measured near each of the two longitudinal the edges as the calliper allowed, to exclude the thinner bended edges with linear variable differential trans- areas at the edges; these measurements included both formers (LVDTs), as shown in Fig. 4b. Beside evaluat- zones without and zones with granules, since it was not ing load vs. displacement curves and fracture patterns, possible to completely avoid measurement points with the equivalent bending strength σ was determined b,eq,B granules. Before testing, a 0.03 mm thin polypropylene for the different specimens with Eq. (1) according to (PP) self-adhesive film was applied on the top side of EN 1288-3 (2000). each panel (side under compression during testing) to allow an easier removal of the broken specimens and a 3 · (L − L ) s b σ  k · F · + σ (1) b,eq,B max b,eq,G better evaluation of the failure patterns after testing. 2 · B · h The tests were performed under load control on a Zwick universal testing machine. The load was applied Here, the factor k  1.0 was used, since both specimens with a load rate of 38.4 N/s, which corresponds for for which the fracture initiated on the surface and such the given cross-section of the panels to a stress rate of for which the fracture initiated at the edge were con- 2 MPa/s, as specified in EN 1288-3 (2000). The glass sidered. F was the maximum force measured with max panels were simply supported on rollers at both ends. the load cell, B was the measured average width of the The distance between the supports was L  1000 mm. specimens and h the measured average thickness of the The load was applied as well with rollers positioned specimens. The equivalent bending strength resulting 123 Structural behaviour and micro-structural characteristics from the self-weight of the glass panels σ was In a second step, a confocal laser microscope Zeiss b,eq,G determined with Eq. (2). LSM 780 upright was used to characterise the sur- face condition in terms of texture and roughness. This method allowed to capture sharp images of a defined 3 · ρ · g · L σ  (2) b,eq,G plane of focus, without disturbing light from the back- 4 · h ground or other regions of the specimen. By stack- ing several such images, the three-dimensional tex- Here, the density of glass ρ  2 500 kg/m and the ture of surfaces could be analysed in detail. For the gravitational acceleration g  9.81 m/s were used. investigated glass surfaces a Fluar 5X/0.25 objective The bending strengths were determined as equivalent and a laser with a wavelength of 405 nm were used. values, since for the kilned glass the thickness was less uniform due to the production process, similar to pat- The obtained images had dimensions of 2.83 mm × 2.83 mm with a resolution of 4096 pixel × 4096 pixel. terned glass. Furthermore, the Young’s modulus of the For the results presented in Sect. 3.4, a representa- glass E was estimated from the obtained measurements tive area of 2.0 mm × 2.0 mm was evaluated for (i) the for the different glass panels with Eq. (3), where y air side and (ii) the tin side of the reference annealed 4 mm was chosen as the vertical deflection in the mid- float glass, for (iii) the uncoloured side of the kilned dle of the panels and F was the force recorded (y = 4 mm) glass and for the coloured side of the kilned glass, with at this displacement. (iv) low, (v) medium and (vi) high density of the dis- 3 2 3 pensed and fused granulate. Based on these areas, the 3 · F L L · L (y4mm) s s b b E  · + − (3) local waviness and roughness of the surfaces were eval- 4 · y · B · h 3 6 2 uated along a chosen representative profile in order to allow a quantitative comparison of the surfaces. Fur- thermore, 3D diagrams of a magnified area of 0.5 mm 2.4 Microscopic investigations of glass surfaces × 0.5 mm were plotted to illustrate the different tex- tures of the reference annealed float glass surfaces and The production process for the coloured kilned glass the kilned glass surfaces. For evaluating the roughness, included the fusion of the dispensed granules into the first, the low frequencies (waviness) and the high fre- air surface of the annealed float glass panels. Fur- quencies (roughness) were separated from each other thermore, the glass panels were positioned with the by applying a Gaussian filter with a cut-off of 0.25 mm. tin surface on a textile during the kilning process. Three different characteristic roughness values were Modifications, which might have influenced the bend- calculated from the profiles according to EN ISO 4287 ing strength, were expected for both of the surfaces (2010): on micro-structural level. For evaluating these modi- fications and eventual damages of the glass surfaces • The arithmetic mean deviation of the roughness pro- through the production process, microscopic investi- file R , calculated as the average vertical distance gations were carried out on shards resulting after per- of each measured point from the mean profile line forming the four-point-bending tests. The shards were along the evaluation length of 2.0 mm; cleaned with pressurized air and wiped over with a • The maximum height of the roughness profile R , cloth with acetone. In a first step, a stereo light micro- calculated as the average of maximum peak-to-valley scope Leica M60 able to zoom between 6.3X and 40X distances from a defined number of sampling lengths was used. Two levels of magnitude (10X and 40X) (five such lengths of 0.4 mm each were used); were used to observe micro-structural differences on • The maximum valley depth of the roughness profile the two surfaces of the coloured kilned glass panels R , calculated as the vertical distance between the compared to the reference annealed float glass panels. lowest measured point along the evaluation length The characteristics and the size of observed features and the mean profile line. were analysed. For the coloured side of the kilned glass panels, an additional differentiation was made depend- ing on the density of the dispensed and fused granu- While the first two values are often provided when late. characterizing surfaces, the latter one was considered as 123 V.-A. Silvestru et al. a rough indicator for the depth of strength determining the positioning of the glass panels on the textile during surface features, e.g. cracks. the kilning process resulted as well in non-negligible surface modifications. For a better comparison of the four-point-bending 3 Results and discussion test results and their scatter, numerical values for each test as well as average values and coefficients of varia- The results obtained from the different four-point- tion are provided in Table 2 for the reference annealed bending test series were analysed and compared to each float glass panels, in Table 3 for the kilned glass pan- other from a structural point of view in a first step. els with the coloured side under compression and in Afterwards, findings from micro-structural analyses of Table 4 for the kilned glass panels with the coloured the surfaces were evaluated and related to the structural side under tension. Beside the maximum force and the performance of the different glass panels. maximum deflection, the tables include the measured glass panel widths and thicknesses as well as the cal- culated maximum bending stresses and the Young’s 3.1 Four-point-bending test results moduli. Despite the small number of test specimens per test series, especially for the material glass which The glass panels tested under four-point-bending in the shows a large scatter of mechanical properties, the pro- different series exhibited significant differences regard- vided values allowed a good assessment of the range ing their structural performance. Based on the force vs. of values for the different parameters. displacement curves plotted in Fig. 6, it can be observed In the case of the dimensions, it was observed that that the kilning process applied to obtain the coloured the width of the panels increased after the kilning pro- glass panels had a degrading effect on the mechanical cess. This was in agreement with the reshaped edge performance. A linear development of the curves with geometry and confirmed the softening of the material similar gradients was observed for all specimens, as at the targeted temperature of 800 °C. Based on the it is usual for the linear-elastic material glass. How- measured thickness values, it seemed that the thick- ever, in terms of failure loads and maximum mid-span ness was increased after the kilning process. In reality, deflections, the kilned glass panels with the coloured the thickness of the kilned glass depended on the local side under compression (Fig. 6b) and those with the distribution and amount of the dispensed granular pow- coloured side under tension (Fig. 6c) reached on aver- der and the degree to which the powder fused with the age only around two thirds and less than half, respec- glass (see also microscopic images in Sects. 3.3 and tively, of the values achieved for the reference annealed 3.4). This explained also the higher scatter of the thick- float glass panels (Fig. 6a). This suggested that the fus- ness in case of the coloured kilned glass compared to ing of the granular powder on the air side lead to signifi- the annealed float glass. cant micro-structural deterioration of the surface, while Fig. 6 Force vs. displacement curves for the reference annealed float glass panels (a), for the coloured kilned glass panels with the coloured side under compression (b) and with the coloured side under tension (c) 123 Structural behaviour and micro-structural characteristics Table 2 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the reference annealed float glass panels a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-01-FG-ref 359.7 7.84 1 002.6 17.81 56.8 69 197 TS-02-FG-ref 359.4 7.83 847.7 15.23 48.5 68 847 TS-03-FG-ref 359.5 7.88 882.6 15.38 49.8 71 392 TS-04-FG-ref 359.5 7.85 939.8 16.11 53.3 74 059 Average 359.5 7.85 918.2 16.13 52.1 70 874 Coefficient of 0.1% 0.30% 7.4% 7.32% 7.1% 3.4% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used Table 3 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the coloured kilned glass panels with the coloured side under compression a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-05-KG-tin 362.6 8.19 647.9 10.90 35.9 71 667 TS-06-KG-tin 362.4 8.29 694.2 11.77 37.6 69 281 TS-07-KG-tin 362.1 8.44 630.8 11.04 33.9 64 898 TS-08-KG-tin 362.4 8.01 586.1 10.01 33.1 71 483 Average 362.4 8.23 639.7 10.93 35.1 69 332 Coefficient of 0.2% 3.23% 7.0% 6.61% 5.7% 4.5% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used Table 4 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the coloured kilned glass panels with the coloured side under tension a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-09-KG-air 361.9 8.16 473.2 8.18 26.8 71 919 TS-10-KG-air 362.5 8.10 373.4 6.42 21.7 72 058 TS-11-KG-air 362.2 8.00 432.8 7.42 24.9 73 697 TS-12-KG-air 362.3 7.99 445.6 7.69 25.8 73 920 Average 362.2 8.06 431.3 7.43 24.8 72 899 Coefficient of 0.2% 2.41% 9.8% 10.03% 9.0% 1.4% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used 123 V.-A. Silvestru et al. As already observed from the force vs. displacement at the edge allows to estimate whether the edge curves, the maximum force, maximum deflection and bending strength is lower than the overall bending maximum stress values in Tables 2, 3 and 4 show that strength. In the case of the tested reference annealed the kilned glass panels behave worse than the reference float glass panels, which had polished edges, for annealed glass panels from a structural point-of-view. three specimens the fracture initiated on the glass The lower reached values could be explained based surface (see Fig. 7a–c), while for one specimen it on modifications suffered by the surfaces as a conse- initiated at the glass edge (see Fig. 7d). This indi- quence of the kilning process, which were observed in cated that, as expected, the polishing reduced pro- the microscopic analyses presented in Sects. 3.3 and nounced flaws at the edges and that the edge bend- 3.4. The scatter of the force, deflection and stress val- ing strength was not lower than the overall bending ues evaluated as coefficients of variation was similar strength. for the reference annealed glass panels and the kilned For the kilned glass panels tested with the coloured glass panels with the coloured side under compression side under compression, the crack always initiated from (around 6–7%) and a little higher for the kilned glass the edge (see Fig. 7e–h), while for those tested with panels with the coloured side under tension (around the coloured side under tension always on the sur- 9–10%). This slightly higher scatter was attributed to face (see Fig. 7i–l). The initiation from the edge could the non-uniformly distributed granulate powder and the be explained by the fact that the kilned glass pan- observed devitrification, which lead to a more rough- els were thinner in the edge areas due to the nearly ened glass surface with defects of more diverse mag- quadrant-shaped geometry. Moreover, the lower sides nitude, as exemplified in Sect. 3.4. In terms of cal- of the cut edges, which were in contact with the textile culated bending stresses, the values for the reference during kilning and subjected to tensile stresses dur- annealed float glass panels were a little higher than the ing the testing, might still have exhibited some resid- characteristic bending strength value of 45 MPa gen- ual damages after kilning. The crack initiation on the erally provided in standards with an average value of surface in the case of the panels with the coloured around 52 MPa. The lower average bending stress val- side under tension indicated that the fused granu- ues calculated for the kilned glass panels of around lar powder had a significant influence on the near- 35 MPa with the coloured side under compression and surface micro-structure and implicitly on the bending of around 25 MPa with the coloured side under ten- strength. The location of failure initiation was with sion were in the range of bending strength values found high probability determined by the unevenness of the in literature for patterned glass (e.g. Haldimann et al. kilned glass panels combined with surface damages 2008; Schneider et al. 2016). The estimated Young’s caused by the non-uniformly distributed fused gran- modulus values were for all specimens in the range ules. of the value of 70 000 MPa found in literature. How- The main fan-shaped cracks are highlighted in the ever, no significant influence of the kilning and fusing images from Fig. 7. It was observed that for almost all processes on the Young’s modulus was observed. The specimens the fracture initiated between the loading slightly different Young’s modulus values determined rollers. Only for two of the kilned glass panels with for the kilned glass panels were with high probability the coloured side under tension, the fracture initiated due to the larger scatter of the average thickness values, slightly outside the loading rollers, indicating damages determined from measurements at points both with and of the surface through the fused granular powder. The without powder granules. cracks in Fig. 7, which are not highlighted, resulted when the already fractured specimens hit the test setup and were therefore not considered for the evaluation in 3.2 Fracture patterns of the different glass panels this section. When comparing the fracture patterns of the differ- The fracture pattern of a glass type is relevant for ent series to each other, it was observed that the ref- assessing the risk of injury in case of breakage as erence annealed glass panels and the kilned glass pan- well as for the residual load-bearing capacity in case els tested with the coloured side under compression of combination to laminated glass. Furthermore, the position of the fracture initiation on the surface or 123 Structural behaviour and micro-structural characteristics Fig. 7 Fracture patterns for the reference annealed float glass panels (a–d) and for the coloured kilned glass panels with the coloured side under compression (e–h) and with the coloured side under tension (i–l) 123 V.-A. Silvestru et al. exhibited patterns of similar complexity, with multi- 3.3 Analysis of surfaces from the different glass panel ple cracks distributed in the shape of a fan. The frac- types based on stereo microscope images ture patterns of the kilned glass panels tested with the coloured side under tension, however, were much sim- For explaining the worse performance of the coloured pler, with only a low number of cracks. Moreover, for kilned glass panels in the four-point-bending tests com- the reference annealed glass panels with crack initi- pared to the reference annealed float glass panels, the ation in the surface, the fan-shaped cracks intersect different surfaces were analysed under a stereo micro- in one point, while for the kilned glass panels tested scope. The lower zoom factor of 10X was used to get with the coloured side under tension, a crack par- an overview of the surface characteristics, while the allel to the shorter glass panel edges was observed, higher zoom factor of 40X allowed the identification from which a few further cracks developed towards the and assessment of specific features. Figure 8 shows longer glass panel edges. The limited fragmentation a representative area of a reference annealed glass observed for the kilned glass panels tested with the panel near a broken edge. Significant visible features coloured side under tension indicated a lower bend- or defects (except those resulted due to the fracture in ing strength of these samples, which was in agree- direct vicinity of the edge) could not be observed, nei- ment with the determined strength values provided in ther with the lower zoom factor, nor with the higher Sect. 3.1. one. When assessing the safety and the risk of injury In contrast with the reference annealed float glass in case of breakage of monolithic glass panels, the panels, both the uncoloured and the coloured sides of size and sharpness of the resulting shards is relevant. the kilned glass panels exhibited features that could be While annealed glass, whose fracture pattern exhibits associated with the earlier failure in the bending tests. radial cracks and large, sharp fragments, is considered In case of the uncoloured side, bubbles of different sizes to present a high risk of injury, fully tempered glass distributed all over the surfaces could be observed, as with net-like cracks and small, dice-shaped, less sharp shown in Fig. 9a. A closer evaluation with the higher fragments presents a lower risk. In case of the tested zoom factor (see Fig. 9b) allowed to measure the size kilned glass panels, the resulted shards exhibited sim- of the bubbles, which ranged from a few micrometres ilar characteristics in terms of size and sharpness to to around half of a millimetre in diameter. Most of the those obtained from the annealed glass panels. Based bubbles had diameters of less than 400 µm and could on the large, sharp fragments, the same qualitative risk be categorized as seeds, while some few were slightly of injury can be assumed for the coloured kilned glass larger and could be classified as blisters. Part of the bub- as for the reference annealed glass. bles were entirely sunken under the glass surface, while others were directly on the surface, as the one shown Fig. 8 Air side of the reference annealed float glass panels—representative stereo microscope images obtained with zoom factors of 10X (a) and 40X (b) 123 Structural behaviour and micro-structural characteristics Fig. 9 Uncoloured side of kilned glass panels—representative stereo microscope images obtained with zoom factors of 10X (a)and 40X (b) in Fig. 15a, b. Furthermore, it could be observed that granules started to merge in these areas, while the tex- the uncoloured surface was not as smooth as the refer- turized surface parts without granules seemed to get ence annealed glass surfaces, exhibiting hardly visible a greenish tone and to go from translucent to opaque. scratches and a light texture, probably as a consequence Areas with high density of the dispensed and fused of the textile on which the glass panels were placed dur- granules were considered such where granules could ing the kilning process. Figure 9b also illustrates that not be observed anymore as partially immersed in the the kilned glass panels were not transparent anymore, glass surface, but where the entire surface was coloured but translucent, even in the areas without dispensed with green tones and single dark-shaded green gran- granules. ules could be identified fully submersed in the glass In the case of the coloured side of the kilned glass (see Fig. 10e, f). With the higher zoom factor of 40X, panels, areas with different density of dispensed and the surface exhibited a mosaic-like texture noticeable fused granules were observed. These areas seemed based on the reflexions resulting through the exposure to exhibit variable characteristics and were therefore of the stereo microscope. evaluated separately, as shown in Fig. 10. Areas with A detailed meaningful analysis of the fracture ori- low density were considered such where only single gins was not possible, since the acute-angled shards isolated granules were observed on the glass surface got chipped in these regions due to the impact of the (see Fig. 10a, b). Here, the different granules, which fractured specimens to the test setup below them. were partially immersed into the glass surface, could In addition to the observed modifications on the out- be clearly identified and their dimensions, which var- side surface, an assessment of the depth of these mod- ied up to around 1 mm in diameter, could be evaluated. ifications was made based on microscopic images of With the higher zoom factor of 40X, it was observed the fractured surface. The pictures in Fig. 11 show the that pronounced damages of the glass surface were fractured surface of a shard from a coloured kilned produced in the direct vicinity of the granules. More- glass panel after completing the four-point-bending over, texturized surfaces could be identified as well in test. From Fig. 11a, illustrating areas with isolated the areas without granules. Both the more pronounced fused granules as well as coalescing fused granules, it damages near granules and the less pronounced tex- was observed that the granules remained near the out- ture further away from the granules could be features side surface in these areas. Figure 11b shows an isolated which lead to a decrease of the bending strength. Sur- fused granule captured with an increased zoom factor face zones with different granules partially immersed (this granule was not situated at the fracture origin). in the glass surface and starting to coalesce, as those The dimensions of such a granule were in agreement shown in Fig. 10c, d, were considered as areas with with the size range of the initially dispensed powder. It medium density. The pronounced damages around the was observed that there is a fusing interface between 123 V.-A. Silvestru et al. Fig. 10 Coloured side of kilned glass panels with low (a, b), medium (c, d) and high (e, f) density of dispensed and fused granules—rep- resentative stereo microscope images obtained with zoom factors of 10X (a, c, e) and 40X (b, d, f) 123 Structural behaviour and micro-structural characteristics Fig. 11 Representative images obtained with the stereo microscope with zoom factors of 6.3X (a) and 40X (b) for fractured surfaces of the coloured kilned glass panels illustrating the depth to which the dispensed granules are fused into the glass surfaces the granules and the annealed float glass surface, but no assessed based on local features identified on 3D sur- pronounce mixing was present. Furthermore, Fig. 11b face diagrams, linear waviness diagrams as well as shows that the glass surface on the coloured side of the linear roughness diagrams and average values. When kilned glass panels was not roughened only near the comparing the results in Figs. 12, 13, 14, 15, 16 and fused granules, but also in areas without granules. This 17 to each other, one should consider the 20 times indicated that the kilning process resulted in a textur- smaller scales used in all diagrams for the reference ized surface on a micro-structural level, which lead to annealed float glass. Using the same scale would have the worse performance in the four-point-bending tests resulted in perceiving these surfaces as completely flat compared to the reference annealed float glass. and smooth. This aspect also emphasized the surface texture modifications suffered by the coloured kilned glass in the production process. The reference annealed glass exhibited similar sur- face characteristics both for the air side and for the tin 3.4 Analysis of surface conditions based on confocal side, as shown by Figs. 12 and 13. Singular local peaks microscope images in the 3D surface diagrams represented isolated dust particles that could not be avoided, despite carefully The surface analysis with the stereo microscope cleaning the surfaces before the microscopic investiga- allowed for a qualitative characterization of the sur- tions. The measured average roughness values, which face modifications generated through the production were slightly higher than values found in literature process of the coloured kilned glass. For a more pre- (Datsiou and Overend 2016) for annealed float glass, cise characterisation and quantitative assessment, addi- could be attributed to the facts that (i) the glass panels tional investigations based on confocal microscopy were stored in exterior environment for approximately were conducted. This allowed to produce height maps 6 months before testing and (ii) the microscopic eval- of selected representative areas of the different glass uation was carried out on shard samples resulted after surfaces and to evaluate the surface roughness along performing the four-point-bending tests. characteristic profiles. Six different glass surfaces were When looking at the uncoloured surface of the kilned analysed, being (i) the air side (Fig. 12) and (ii) the glass (see Fig. 14a, b), which was positioned on a tex- tin side (Fig. 13) of the reference annealed float glass, tile material during the kilning process, different fea- (iii) the uncoloured side of the kilned glass (Fig. 14) tures could be noticed. The more obvious ones were and the coloured side of the kilned glass, with (iv) low the surface-near bubbles of different sizes, which were (Fig. 15), (v) medium (Fig. 16) and (vi) high (Fig. 17) already observed with the stereo microscope and dis- density of the dispensed and fused granulate. For each cussed in Sect. 3.3. Figure 14b highlights the surface of these surface types, the texture was quantitatively 123 V.-A. Silvestru et al. Fig. 12 Air side of the reference annealed glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 13 Tin side of the reference annealed glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 Structural behaviour and micro-structural characteristics Fig. 14 Uncoloured side of kilned glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 15 Coloured side of kilned glass panels with low density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 V.-A. Silvestru et al. Fig. 16 Coloured side of kilned glass panels with medium density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 17 Coloured side of kilned glass panels with high density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 Structural behaviour and micro-structural characteristics area where a bubble escaped the glass surface during of the kilned glass situated between those of the ref- the kilning process. In addition, some linear-shaped and erence annealed float glass and those of the coloured some dot-shaped damages were noticed and are shown side of the kilned glass, are in good agreement with the in Fig. 14a. These ones could be attributed with high differences in structural performance observed for the probability to the textile material used below the panel. different panel types in the four-point bending tests. As shownbyFig. 14c, d, the uncoloured surface of the kilned glass exhibited an increased waviness and, espe- 4 Conclusions cially in the areas around escaped bubbles, a higher roughness compared to the surfaces of the reference The results presented and discussed in this paper pro- annealed float glass. vided detailed insights on the structural performance In case of the coloured side of the kilned glass, it and micro-structural characteristics of a novel type of was differentiated between areas with low (Fig. 15), coloured kilned glass panels which allows for decora- medium (Fig. 16) and high (Fig. 17) density of the dis- tive polychromatic elements. On the one hand, these pensed and fused granules to evaluate an eventual influ- findings can be used by designers to evaluate the lim- ence of this aspect on the quantitative surface proper- its of this novel type of glass and to use it in suitable ties. A modified texture of the surface was observed in applications. On the other hand, they can be a basis for a all three cases. However, in areas with low density of subsequent optimization of the production process with granules, the granules could be clearly identified in the the aim of improving the structural and micro-structural confocal microscopic image (see Fig. 15a), in the 3D properties of the coloured kilned glass panels. The fol- surface diagram and in the waviness profile. In areas lowing more specific conclusions can be drawn: with medium density, only the waviness profile indi- cated the presence of granules, while for areas with • The bending strength and the reachable mid-span high density, a pronounced influence of the granules on deflection of the reference annealed float glass were the waviness profile was not visible. In terms of rough- reduced through the kilning and fusing processes. ness, the highest values were measured in areas with The magnitude of these reductions depended on low density, followed closely by areas with medium whether the coloured side was under tension or com- density. Areas with high density of granules exhibited pression. lower roughness values, indicating that such a distribu- • The kilning production process resulted in a more tion could be advantageous for the resulting microstruc- irregular geometry of the glass panels with rounded, ture. When looking at the surface texture in more detail, nearly quadrant-shaped edges and uneven thickness. it was observed that at low granule density the glass sur- Especially the thinner edges influenced the fracture face exhibited an orange-skin-like pattern, while with pattern of the kilned glass panels. The fracture ini- increasing density this pattern changed more and more tiated at the glass edge, when the coloured side was to a mosaic-like pattern. These aspects indicated devit- under compression and on the glass surface when the rification, which can generally occur when the glass is coloured side was under tension. The resulting shards held for too long at temperatures slightly above the soft- had similar sizes to those of the reference annealed ening point and can be as well boosted by dust particles float glass. present on the glass surface or in the kiln. Both of these • The microstructure of both the uncoloured and the conditions were met to a certain extent during the pro- coloured surfaces of the kilned glass were qualita- duction process of the investigated coloured glass. A tively modified by the production process, result- start of devitrification was observed also in areas of the ing in bending-strength-reducing features. For the kilned glass, in which no granules were deposited. The coloured side, micro-structural differences were devitrification became more pronounced with higher identified for areas with different density of the fused density of deposited granules. In future investigations, granules, ranging from surface damages around sin- the temperature curve could be varied and the kiln could gle granules to devitrification in areas with high den- be carefully cleaned for evaluating if and to what extent sity. the devitrification process can be avoided. • The roughness and the waviness of the coloured The roughness values determined for the different kilned glass was increased by the production pro- glass surfaces, with the values for the uncoloured side cess. The quantitatively evaluated roughness values 123 V.-A. Silvestru et al. were in good agreement with the different bend- Funding Open access funding provided by Swiss Federal Insti- tute of Technology Zurich. ing strength values determined from the four-point- bending test results, since the higher the local rough- ness of the glass surfaces subjected to tensile stresses was, the lower bending strength values were deter- Declarations mined. Conflict of interest On behalf of all authors, the corresponding The obtained results indicate that the coloured kilned author states that there is no conflict of interest. glass panels could be used in principle with similar lim- itations as annealed float glass or patterned glass, if the Open Access This article is licensed under a Creative Com- reduced strength values are considered. For a statisti- mons Attribution 4.0 International License, which permits use, cally significant determination of the strength value, a sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original larger number of specimens needs to be tested. Qual- author(s) and the source, provide a link to the Creative Com- itatively, the injury risk level indicated by the failure mons licence, and indicate if changes were made. The images modes and the size and sharpness of the shards is sim- or other third party material in this article are included in the ilar to that of annealed float glass. article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the Future investigations dealing with this novel type article’s Creative Commons licence and your intended use is not of coloured kilned glass should on the one hand focus permitted by statutory regulation or exceeds the permitted use, on studying the effects of different kilning tempera- you will need to obtain permission directly from the copyright ture curves and support plate materials on the structural holder. To view a copy of this licence, visit http://creativecomm ons.org/licenses/by/4.0/. performance and micro-structural characteristics. This would allow to optimize the properties of the poly- chromatic glass. Furthermore, the studies should be References extended to panels with diverse colour patterns to assess eventual additional effects and should include more Bristogianni, T., Oikonomopoulou, F., Yu, R., Veer, F.A., Nijsse, detailed investigations of (i) the glass powder and flat R.: Investigating the flexural strength of recycled cast glass. glass composition (e.g. X-ray fluorescence analysis), Glass Struct. Eng. 5, 445–487 (2020). https://doi.org/10. (ii) eventual residual stresses after the fusing and kil- 1007/s40940-020-00138-2 Bristogianni, T., Oikonomopoulou, F., Veer, F.A.: On the flex- ning process for different temperature curves (e.g. anal- ural strength and stiffness of cast glass. Glass Struct. ysis with polarised light) and (iii) fracture mirrors on Eng. 6, 147–194 (2021). https://doi.org/10.1007/s40940- shards without chipping. On the other hand, additional 021-00151-z processing steps, like thermal tempering and laminat- Datsiou, K., Overend, M.: Evaluation of artificial ageing methods for glass. In: Bos, F., Louter, C., Belis, J. (eds.) Challenging ing, should be assessed for the coloured kilned glass in Glass 5 Conference, Ghent University (2016) order to allow an increased safety and a wider range of EN 1288-3: Glass in building—Determination of the bending applications as façade glazing. strength of glass—Part 3: Test with specimen supported at two points (four point bending) (2000) Acknowledgements The authors would like to gratefully EN 572-1: Glass in building – Basic soda-lime silicate glass prod- acknowledge the support of the laboratory staff of the Institute ucts – Part 1: Definitions and general physical and mechan- of Structural Engineering at ETH Zurich in planning and manu- ical properties (2016) facturing necessary parts of the four-point-bending test setup. EN 572-2: Glass in building—Basic soda-lime silicate glass Furthermore, the authors thank Asel Maria Aguilar Sanchez products—Part 2: Float glass (2012) from the Institute for Building Materials and Tobias Schwarz of EN 572-5: Glass in building—Basic soda-lime silicate glass ScopeM for their support and assistance in obtaining the micro- products—Part 5: Patterned glass (2012) scope images analysed in this work. EN ISO 4287: Geometrical Product Specifications (GPS)—Surface texture: Profile method—Terms, defi- Author contributions Conceptualization: Vlad-Alexandru Sil- nitions and surface texture parameters (2010) vestru; Methodology: Vlad-Alexandru Silvestru, Rena Giesecke; Giesecke, R., Dillenburger, B.: Large-scale robotic fabri- Formal analysis and investigation: Vlad-Alexandru Silvestru, cation pf polychromatic relief glass. Int. J. Archit. Rena Giesecke; Writing – original draft preparation: Vlad- Comput. 20(1), 18–30 (2022). https://doi.org/10.1177/ Alexandru Silvestru, Rena Giesecke; Writing – review and edit- 14780771221082259 ing: Vlad-Alexandru Silvestru, Rena Giesecke, Benjamin Dillen- Giesecke, R., Clemente, R., Mitropoulou, I., Skevaki, E., Peter- burger; Project administration: Vlad-Alexandru Silvestru, Rena hans, C.T., Dillenburger, B.: Beyond transparency, archi- Giesecke; Supervision: Benjamin Dillenburger. tectural application of robotically fabricated polychromatic 123 Structural behaviour and micro-structural characteristics float glass. Constr. Robot. (2022). https://doi.org/10.1007/ OPTUL Float: Range of granulate products. Optul Spezial- s41693-022-00071-6 glas GmbH. https://optul.de/produktpalette/granulate.html Haldimann, M., Luible, A., Overend, M.: Structural Use of (2022). Accessed 13 June 2022 Glass. Structural Engineering document SED10. Interna- Schneider, J., Kuntsche, J., Schula, S., Schneider, F., Wörner, tional Association for Bridge and Structural Engineering J.-D.: Glasbau—Grundlagen, Berechnung, Konstruktion. IABSE, Zurich (2008) Springer Vieweg, Berlin (2016) Inamura, C., Stern, M., Lizardo, D., Houk, P., Oxman, N.: 3D Seel, M., Akerboom, R., Knaack, U., Oechsner, M., Hof, P., Printing and Additive manufacturing of transparent glass Schneider, J.: Additive manufacturing of glass compo- structures. Addit. Manuf. 5(4), 269–283 (2018). https://doi. nents—exploring the potential of glass components by org/10.1089/3dp.2018.0157 fused deposition modelling. In: Louter, C., Bos, F., Belis, J., Maniatis, I., Elstner, M.: Investigations on the mechanical Veer, F., Nijsse, R. (eds.) Challenging Glass 6 Conference, strength of enamelled glass. Glass Struct. Eng. 1, 277–288 Delft University of Technology (2018). https://doi.org/10. (2016). https://doi.org/10.1007/s40940-016-0025-2 7480/cgc.6.2161 Oikonomopoulou, F., Bristogianni, T., Barou, L., Veer, F.A., TNO: Glass powder printer. TNO Science and Industry. https:// Nijsse, R.: The potential of cast glass in structural applica- www.tno.nl/media/2871/hr_tno_leaflet-glasprinter.pdf tions Lessons learned from large-scale castings and state- (2007). Accessed 4 September 2022 of-the art load-bearing cast glass in architecture. J. Build. Eng. 20, 213–234 (2018). https://doi.org/10.1016/j.jobe. Publisher’s Note Springer Nature remains neutral with regard 2018.07.014 to jurisdictional claims in published maps and institutional affil- iations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Glass Structures & Engineering Springer Journals

Structural behaviour and micro-structural characteristics of coloured kilned glass panels

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Glass Struct. Eng. https://doi.org/10.1007/s40940-022-00208-7 RESEARCH PAPER Structural behaviour and micro-structural characteristics of coloured kilned glass panels Vlad-Alexandru Silvestru · Rena Giesecke · Benjamin Dillenburger Received: 6 July 2022 / Accepted: 4 October 2022 © The Author(s) 2022 Abstract In recent years, the architecture, engineer- observed. Furthermore, the reasons for this reduction in ing and construction industry is increasingly embracing structural performance were analysed based on micro- digital design approaches and robotic production meth- scopic investigations. It was observed that both the ods. This is as well noticeable in the façade design sec- kilning process and the density of deposited granular tor, especially when it comes to architectural design powder had an influence on the surface microstructure. tasks dealing with complex geometries or adaptive The presented results are essential for the production of components. Although glass is mainly used in facades polychromatic kilned glass as well as for future appli- due to its transparency, it often determines significantly cations of this type of glass panel in façades. the aesthetic appearance of a building and has to control the transmitted light and the visibility between exterior Keywords Coloured kilned glass panel · Four-point and interior. Recently, a novel method for producing bending test · Tensile bending strength · Micro- polychromatic glass panels by fusing granular glass structural analysis · Glass surface texture · Surface powder of different colours onto the surface of annealed roughness float glass in a kilning process was developed at ETH Zurich. By using a multi-channel tool attached to a robotic arm for placing the granular powder onto the 1 Introduction glass surface, digitally designed patterns can be realised precisely and repetitively. This paper focuses on the Digital design approaches and automatized robotic structural behaviour and the micro-structural charac- production methods enjoyed increasing interest from teristics of this novel type of coloured glass panels for the architecture, engineering and construction (AEC) façade applications. The bending strength of such glass industry in recent years. However, while for materi- panels was determined based on four-point-bending als like concrete and steel, various research projects tests and was compared to that of annealed float glass. were conducted and some representative construction A non-negligible decrease in the bending strength was projects were completed, in the case of glass, such approaches and production methods are rather limited. V.-A. Silvestru ( ) First attempts of using additive manufacturing (AM) Institute of Structural Engineering, ETH Zurich, for glass at an architectural scale were done by Inamura Stefano-Franscini-Platz 5, 8093 Zurich, Switzerland et al. (2018) in the form of glass columns or by Seel e-mail: silvestru@ibk.baug.ethz.ch et al. (2018) in the form of prototypical connections. R. Giesecke · B. Dillenburger Whereas these projects focused on realizing structural Institute of Technology in Architecture, ETH Zurich, Stefano-Franscini-Platz 1, 8093 Zurich, Switzerland components made of glass, digital design approaches 123 V.-A. Silvestru et al. Fig. 1 Views of the pavilion with polychromatic kilned glass panels showing (a) the relationship to the surroundings through the decorative pattern applied and (b) the graded transparency obtained through the colour grading process. Images credit: Digital Building Technologies, ETH Zurich; Photographer: Evangelos Roditis and robotic production methods were applied recently glass either to slump in a mould due to gravity or to be also for achieving novel architectural glass products. formed by movable moulds. Giesecke and Dillenburger (2022) and Giesecke et al. Several decorative surface treatment processes are (2022) developed a method for large-scale robotic fab- available nowadays, ranging from (i) patterned glass rication of polychromatic glass by fusing granular glass (see EN 572-5 2012) done by casting through pat- powder on annealed float glass panels. An application terned rollers, over (ii) enamelled glass done by burn- of this type of glass for a mock-up pavilion built in ing a ceramic frit colour into the annealed glass dur- 2021 is shown in Fig. 1. As it is the case of other pro- ing the tempering process, to (iii) digital printing cessing technologies of glass, understanding the mate- methods done by applying ceramic or organic colours rial behaviour at different temperatures in the transition on the glass surfaces. All these processes show cer- range from melt to solid is of ultimate importance for tain disadvantages. For instance, patterned glass and applying additive manufacturing principles to glass. enamelled glass lead to reduced bending strengths The glass mostly used in the AEC industry, soda- and are either translucent or opaque. Enamelled glass lime silicate glass, has a glass transition temperature of shows limitations in the achievable colours compared around 570 °C, a softening point of around 725 °C and to digital prints on glass surfaces, which allow for a working point of around 1030 °C (Schneider et al. photo-realistic images. Digital prints on glass sur- 2016). In addition, the strain point is situated at around faces exhibit a lower durability compared to enamelled 505 °C, while the annealing point is at approximately glass. From the available decorative surface treatment 550 °C. These temperatures and the related changing processes, only the enamelling requires the glass to viscosities of the glass are used in various processing be reheated. Maniatis and Elstner (2016) conducted steps, including tempering, shaping or surface treat- investigations on the structural behaviour of enamelled ments. For thermal tempering, the annealed float glass glass. is heated to approximately 100 °C above the glass tran- A decorative surface treatment by fusing coloured sition temperature, where it is in a viscoelastic state, powder on flat glass was used by Saint Gobain Glass and subsequently it is quenched from both sides. This Solutions in collaboration with TNO for the facades of allows to obtain a structurally favourable residual stress the Dutch Institute of Sound and Vision in Hilversum state. In case of hot-bending, the flat glass is heated as (see TNO 2007). Granules in three basic colours were well to approximately 100 °C above the glass transi- deposited in a patterned mould and fused with the glass tion temperature to reach a viscosity, which allows the sheets at temperatures of around 800 °C. 123 Structural behaviour and micro-structural characteristics While for fusing the ceramic frit into the glass, tem- any additional coatings). This basic product made from peratures slightly below the softening point are suffi- soda-lime silicate glass is the one mostly used in cient, for a proper mixing of glass coming from differ- facades, often further processed to thermally tempered ent recycled cullet, higher temperatures are necessary. glass, laminated glass or insulating glass units. From Bristogianni et al. (2020) and Bristogianni et al. (2021) a mechanical point of view, EN 572-1 (2016) specifies used temperatures between 820 and 1120 °C to kiln- for annealed soda-lime silicate glass a characteristic cast small-scale beams (30 × 30 × 240 mm) from cullet bending strength of 45 MPa and a Young’s modulus of coming from different previous applications. Related 70 000 MPa. The thermal expansion coefficient of such –6 −1 to cast glass, Oikonomopoulou et al. (2018)showed glass is 9 × 10 K . that the material used for the moulds influenced the For colouring the annealed float glass panels by obtained finishing surface. While permanent stainless fusing, granulate powder from OPTUL Spezialglas steel or graphite moulds allowed for glossy finishes, GmbH was used. Powder granules of the transparent silica plaster and alumina-silica plaster lead to translu- colour FF-BF 0076 Chrome Green with grain sizes cent and rough finishes. To restore transparency and to between 0.36 and 1.00 mm were used in the exper- increase accuracy of cast glass, generally, grinding and iments discussed in this paper. For the pavilion in polishing were required when using the latter mould Fig. 1, other colours, like FF-BF 0078 Dark Green, types in past applications. FF-BF 2135 Yellow, and FF-BF 3100 White were used In contrast to the above methods, polychromatic as well (Giesecke et al. 2022; Giesecke and Dillen- kilned glass allows for an enhanced diversity enabled burger 2022). These granular glass powders were devel- by digitally designed multi-colour patterns that can be oped to match the thermal expansion coefficient of realized in a precisely controlled and repeatable man- annealed float glass to enable fusing without breakage, ner thanks to the robotic dispensing of the colour. How- according to OPTUL Spezialglas GmbH. The speci- ever, the structural performance of this glass type has fied thermal expansion coefficient of 8.2 (± 0.3) × –6 −1 not been addressed yet, limiting its application. There- 10 K , however, is slightly lower than that of soda- fore, this research investigates the structural behaviour lime silicate glass and could eventually lead to residual and the micro-structural characteristics of coloured stresses after the fusing and kilning process. A colour kilned glass panels. The panels were produced in a palette of 40 compatible granular glass colours, both similar manner to the polychromatic glass presented transparent and opaque, are available. The granules by Giesecke and Dillenburger (2022) and by Giesecke come in six different sizes ranging from powder of et al. (2022). 0.00–0.36 mm up to chips of 5.00–8.50 mm (OPTUL Float 2022). This wide range of available colours and grain sizes opens up a vast diversity of potentially achievable design patterns and resolutions. Due to the 2 Materials and methods packing of the granules, it is not only possible to add two-dimensional properties to the glass, but also three- This section briefly describes the materials involved in the presented investigations and provides informa- dimensional relief features (Giesecke and Dillenburger 2022). tion on the applied methodology for producing the coloured kilned glass panels, for determining their structural behaviour and for evaluating their micro- structural characteristics. 2.2 Manufacturing of polychromatic kilned glass panels 2.1 Material properties of the glass panels The polychromatic patterns of the glass panels for the and the granulate pavilion shown in Fig. 1 were digitally designed to visually interact with the surroundings. For the robotic Annealed float glass as regulated in EN 572-1 (2016) dispensing process, granular glass of four colours was and EN 572-2 (2012), with a nominal thickness of placed in a multi-channel tool attached to a robotic arm 8 mm was used for the panels investigated in this or motion system, as shown in Fig. 2a (Giesecke et al. paper (PLANICLEAR® from Saint-Gobain without 2022; Giesecke and Dillenburger 2022). The robotic 123 V.-A. Silvestru et al. Fig. 2 Multi-channel tool setup for robotic application of the coloured granules (a), and different features of the achievable colour patterns: (b) repeatability due to robotic precision, (c) granular distribution, (d) dense colour fusion and (e) variable transition between colour densities. Images credit: Digital Building Technologies, ETH Zurich arm moved along a specific toolpath based on the pat- of 800 °C was chosen based on the fact that at this tem- tern designed and dispensed a controlled volume of perature the fused coloured granulate powder develops material on the air side of the annealed float glass pan- the desired nuances. els. The granules could also be distributed manually, as it was done for the panels investigated in this paper from a structural and micro-structural point of view. How- 2.3 Four-point bending tests ever, the robotic process enabled the precise, repetitive inscription of decorative and functional (e.g. light and To assess the structural performance of the novel poly- view control) material patterns, as well as continuity chromatic/coloured kilned glass panels in comparison of patterns between elements without the use of a tem- to standard annealed float glass panels, a series of four- plate through a digital design-to-production workflow. point-bending tests were conducted. The test speci- For the investigations within this paper, a manual dis- mens and the test setup were designed according to pensing procedure was chosen for obtaining random EN 1288-3 (2000). The four-point-bending test setup, patterns. The granulate powder was distributed directly which includes the influence of the free glass edges on on the glass surface, without using a fusing gel. Both the the bending strength, was preferred to a coaxial double robotic dispensing process and the manual dispensing ring test setup, since the strength values determined in process lead to similar varied colour distributions, rang- these investigations should characterize the glass pan- ing from isolated granular patterns to densely coloured els with free edges used for the pavilion shown in Fig. 1. areas (see Fig. 2c–e). The main dimensions of the test specimens and the test For the kilning process, the annealed float glass pan- setup are provided in Fig. 4a, while Fig. 4bshows the els with the granular glass deposited on top (on the air assembled test setup. A total of twelve specimens were side) were placed in a Nabertherm GF600 kiln (see tested, as listed in Table 1. This number of specimens Fig. 3a), which could host glass panels with dimen- is not sufficient for statistically significant results, but sions of up to 1 m × 2 m. A textile layer made of a allows to provide first information on the mechanical 2 mm thick ceramic fibre paper was used as a substra- properties of the coloured kilned glass panels compared tum for the glass panels. For the kilning process, the to the annealed float glass panels. According to exist- glass panels were heated to 800 °C and cooled in a ing standards, no differentiation in strength is made for controlled annealing process according to the temper- annealed float glass depending on whether the tin side ature vs. time curve shown in Fig. 3b. The temperature or the air side is subjected to tension. For the coloured 123 Structural behaviour and micro-structural characteristics Fig. 3 Nabertherm GF600 glass kiln for fusing the granules into the float glass surface (a) and applied temperature curve for producing the investigated coloured kilned glass panels (b) Fig. 4 Schematic illustration of the geometry of the test setup with built-in specimen (a) and test setup with its components during testing of a coloured kilned glass panel (b) kilned glass, it was expected that the air side, on which The glass panels had nominal dimensions of the green granular powder was dispensed and fused, 1100 mm in length, 360 mm in width and 8 mm in thick- and the tin side, which was in contact with a textile ness. The length and the width were chosen accord- during the kilning process, might have suffered differ- ing to EN 1288-3 (2000), while the thickness was the ent structural changes. Thus, specimens of the kilned same as for the panels in the pavilion mentioned in glass panels were tested both with the coloured air side the introduction. Especially for the kilned glass pan- and with the tin side, respectively, subjected to tension. els, deviations from these nominal dimensions were expected. Therefore, before testing, the actual sizes of each specimen were measured at four positions for the 123 V.-A. Silvestru et al. Table 1 Overview of the performed four-point-bending test series Specimen type Number of specimens Specimen nomenclature Reference annealed float glass panels with tin side under tension 4 specimens TS-01-FG-ref | TS-02-FG-ref TS-03-FG-ref | TS-04-FG-ref Coloured kilned glass panels with coloured side under compression 4 specimens TS-05-KG-tin | TS-06-KG-tin TS-07-KG-tin | TS-08-KG-tin Coloured kilned glass panels with coloured side under tension 4 specimens TS-09-KG-air | TS-10-KG-air TS-11-KG-air | TS-12-KG-air Fig. 5 Representative edge shapes of the reference annealed float glass panels (a) and the coloured kilned glass panels (b)—pictures of resulting shards after completing the four-point-bending tests width and at eight positions for the thickness. The ref- at a distance of L  200 mm to each other. Soft lay- erence annealed float glass panels had polished edges ers of 3 mm thin ethylene-propylene-diene-monomer (see Fig. 5a). The coloured kilned glass panels, for (EPDM) were applied between all the steel rollers and which float glass with cut edges was used, had after the the glass panels to avoid stress concentrations. kilning process rounded nearly quadrant-shaped edges In addition to the acting force and the machine dis- (see Fig. 5b). For the kilned glass panels, the thick- placement, the vertical deflection of the panels at mid- ness measurements were conducted as far away from span was measured near each of the two longitudinal the edges as the calliper allowed, to exclude the thinner bended edges with linear variable differential trans- areas at the edges; these measurements included both formers (LVDTs), as shown in Fig. 4b. Beside evaluat- zones without and zones with granules, since it was not ing load vs. displacement curves and fracture patterns, possible to completely avoid measurement points with the equivalent bending strength σ was determined b,eq,B granules. Before testing, a 0.03 mm thin polypropylene for the different specimens with Eq. (1) according to (PP) self-adhesive film was applied on the top side of EN 1288-3 (2000). each panel (side under compression during testing) to allow an easier removal of the broken specimens and a 3 · (L − L ) s b σ  k · F · + σ (1) b,eq,B max b,eq,G better evaluation of the failure patterns after testing. 2 · B · h The tests were performed under load control on a Zwick universal testing machine. The load was applied Here, the factor k  1.0 was used, since both specimens with a load rate of 38.4 N/s, which corresponds for for which the fracture initiated on the surface and such the given cross-section of the panels to a stress rate of for which the fracture initiated at the edge were con- 2 MPa/s, as specified in EN 1288-3 (2000). The glass sidered. F was the maximum force measured with max panels were simply supported on rollers at both ends. the load cell, B was the measured average width of the The distance between the supports was L  1000 mm. specimens and h the measured average thickness of the The load was applied as well with rollers positioned specimens. The equivalent bending strength resulting 123 Structural behaviour and micro-structural characteristics from the self-weight of the glass panels σ was In a second step, a confocal laser microscope Zeiss b,eq,G determined with Eq. (2). LSM 780 upright was used to characterise the sur- face condition in terms of texture and roughness. This method allowed to capture sharp images of a defined 3 · ρ · g · L σ  (2) b,eq,G plane of focus, without disturbing light from the back- 4 · h ground or other regions of the specimen. By stack- ing several such images, the three-dimensional tex- Here, the density of glass ρ  2 500 kg/m and the ture of surfaces could be analysed in detail. For the gravitational acceleration g  9.81 m/s were used. investigated glass surfaces a Fluar 5X/0.25 objective The bending strengths were determined as equivalent and a laser with a wavelength of 405 nm were used. values, since for the kilned glass the thickness was less uniform due to the production process, similar to pat- The obtained images had dimensions of 2.83 mm × 2.83 mm with a resolution of 4096 pixel × 4096 pixel. terned glass. Furthermore, the Young’s modulus of the For the results presented in Sect. 3.4, a representa- glass E was estimated from the obtained measurements tive area of 2.0 mm × 2.0 mm was evaluated for (i) the for the different glass panels with Eq. (3), where y air side and (ii) the tin side of the reference annealed 4 mm was chosen as the vertical deflection in the mid- float glass, for (iii) the uncoloured side of the kilned dle of the panels and F was the force recorded (y = 4 mm) glass and for the coloured side of the kilned glass, with at this displacement. (iv) low, (v) medium and (vi) high density of the dis- 3 2 3 pensed and fused granulate. Based on these areas, the 3 · F L L · L (y4mm) s s b b E  · + − (3) local waviness and roughness of the surfaces were eval- 4 · y · B · h 3 6 2 uated along a chosen representative profile in order to allow a quantitative comparison of the surfaces. Fur- thermore, 3D diagrams of a magnified area of 0.5 mm 2.4 Microscopic investigations of glass surfaces × 0.5 mm were plotted to illustrate the different tex- tures of the reference annealed float glass surfaces and The production process for the coloured kilned glass the kilned glass surfaces. For evaluating the roughness, included the fusion of the dispensed granules into the first, the low frequencies (waviness) and the high fre- air surface of the annealed float glass panels. Fur- quencies (roughness) were separated from each other thermore, the glass panels were positioned with the by applying a Gaussian filter with a cut-off of 0.25 mm. tin surface on a textile during the kilning process. Three different characteristic roughness values were Modifications, which might have influenced the bend- calculated from the profiles according to EN ISO 4287 ing strength, were expected for both of the surfaces (2010): on micro-structural level. For evaluating these modi- fications and eventual damages of the glass surfaces • The arithmetic mean deviation of the roughness pro- through the production process, microscopic investi- file R , calculated as the average vertical distance gations were carried out on shards resulting after per- of each measured point from the mean profile line forming the four-point-bending tests. The shards were along the evaluation length of 2.0 mm; cleaned with pressurized air and wiped over with a • The maximum height of the roughness profile R , cloth with acetone. In a first step, a stereo light micro- calculated as the average of maximum peak-to-valley scope Leica M60 able to zoom between 6.3X and 40X distances from a defined number of sampling lengths was used. Two levels of magnitude (10X and 40X) (five such lengths of 0.4 mm each were used); were used to observe micro-structural differences on • The maximum valley depth of the roughness profile the two surfaces of the coloured kilned glass panels R , calculated as the vertical distance between the compared to the reference annealed float glass panels. lowest measured point along the evaluation length The characteristics and the size of observed features and the mean profile line. were analysed. For the coloured side of the kilned glass panels, an additional differentiation was made depend- ing on the density of the dispensed and fused granu- While the first two values are often provided when late. characterizing surfaces, the latter one was considered as 123 V.-A. Silvestru et al. a rough indicator for the depth of strength determining the positioning of the glass panels on the textile during surface features, e.g. cracks. the kilning process resulted as well in non-negligible surface modifications. For a better comparison of the four-point-bending 3 Results and discussion test results and their scatter, numerical values for each test as well as average values and coefficients of varia- The results obtained from the different four-point- tion are provided in Table 2 for the reference annealed bending test series were analysed and compared to each float glass panels, in Table 3 for the kilned glass pan- other from a structural point of view in a first step. els with the coloured side under compression and in Afterwards, findings from micro-structural analyses of Table 4 for the kilned glass panels with the coloured the surfaces were evaluated and related to the structural side under tension. Beside the maximum force and the performance of the different glass panels. maximum deflection, the tables include the measured glass panel widths and thicknesses as well as the cal- culated maximum bending stresses and the Young’s 3.1 Four-point-bending test results moduli. Despite the small number of test specimens per test series, especially for the material glass which The glass panels tested under four-point-bending in the shows a large scatter of mechanical properties, the pro- different series exhibited significant differences regard- vided values allowed a good assessment of the range ing their structural performance. Based on the force vs. of values for the different parameters. displacement curves plotted in Fig. 6, it can be observed In the case of the dimensions, it was observed that that the kilning process applied to obtain the coloured the width of the panels increased after the kilning pro- glass panels had a degrading effect on the mechanical cess. This was in agreement with the reshaped edge performance. A linear development of the curves with geometry and confirmed the softening of the material similar gradients was observed for all specimens, as at the targeted temperature of 800 °C. Based on the it is usual for the linear-elastic material glass. How- measured thickness values, it seemed that the thick- ever, in terms of failure loads and maximum mid-span ness was increased after the kilning process. In reality, deflections, the kilned glass panels with the coloured the thickness of the kilned glass depended on the local side under compression (Fig. 6b) and those with the distribution and amount of the dispensed granular pow- coloured side under tension (Fig. 6c) reached on aver- der and the degree to which the powder fused with the age only around two thirds and less than half, respec- glass (see also microscopic images in Sects. 3.3 and tively, of the values achieved for the reference annealed 3.4). This explained also the higher scatter of the thick- float glass panels (Fig. 6a). This suggested that the fus- ness in case of the coloured kilned glass compared to ing of the granular powder on the air side lead to signifi- the annealed float glass. cant micro-structural deterioration of the surface, while Fig. 6 Force vs. displacement curves for the reference annealed float glass panels (a), for the coloured kilned glass panels with the coloured side under compression (b) and with the coloured side under tension (c) 123 Structural behaviour and micro-structural characteristics Table 2 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the reference annealed float glass panels a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-01-FG-ref 359.7 7.84 1 002.6 17.81 56.8 69 197 TS-02-FG-ref 359.4 7.83 847.7 15.23 48.5 68 847 TS-03-FG-ref 359.5 7.88 882.6 15.38 49.8 71 392 TS-04-FG-ref 359.5 7.85 939.8 16.11 53.3 74 059 Average 359.5 7.85 918.2 16.13 52.1 70 874 Coefficient of 0.1% 0.30% 7.4% 7.32% 7.1% 3.4% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used Table 3 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the coloured kilned glass panels with the coloured side under compression a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-05-KG-tin 362.6 8.19 647.9 10.90 35.9 71 667 TS-06-KG-tin 362.4 8.29 694.2 11.77 37.6 69 281 TS-07-KG-tin 362.1 8.44 630.8 11.04 33.9 64 898 TS-08-KG-tin 362.4 8.01 586.1 10.01 33.1 71 483 Average 362.4 8.23 639.7 10.93 35.1 69 332 Coefficient of 0.2% 3.23% 7.0% 6.61% 5.7% 4.5% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used Table 4 Measured cross-section dimensions and mechanical properties determined from the four-point-bending tests for the coloured kilned glass panels with the coloured side under tension a a Specimen Panel width Panel thickness Maximum Maximum Maximum stress Young’s (mm) (mm) force deflection (MPa) modulus (N) (mm) (MPa) TS-09-KG-air 361.9 8.16 473.2 8.18 26.8 71 919 TS-10-KG-air 362.5 8.10 373.4 6.42 21.7 72 058 TS-11-KG-air 362.2 8.00 432.8 7.42 24.9 73 697 TS-12-KG-air 362.3 7.99 445.6 7.69 25.8 73 920 Average 362.2 8.06 431.3 7.43 24.8 72 899 Coefficient of 0.2% 2.41% 9.8% 10.03% 9.0% 1.4% variation The provided dimensions are averages of four different values for the width and eight different values for the thickness. For the determination of the coefficient of variation all the measured dimension values were used 123 V.-A. Silvestru et al. As already observed from the force vs. displacement at the edge allows to estimate whether the edge curves, the maximum force, maximum deflection and bending strength is lower than the overall bending maximum stress values in Tables 2, 3 and 4 show that strength. In the case of the tested reference annealed the kilned glass panels behave worse than the reference float glass panels, which had polished edges, for annealed glass panels from a structural point-of-view. three specimens the fracture initiated on the glass The lower reached values could be explained based surface (see Fig. 7a–c), while for one specimen it on modifications suffered by the surfaces as a conse- initiated at the glass edge (see Fig. 7d). This indi- quence of the kilning process, which were observed in cated that, as expected, the polishing reduced pro- the microscopic analyses presented in Sects. 3.3 and nounced flaws at the edges and that the edge bend- 3.4. The scatter of the force, deflection and stress val- ing strength was not lower than the overall bending ues evaluated as coefficients of variation was similar strength. for the reference annealed glass panels and the kilned For the kilned glass panels tested with the coloured glass panels with the coloured side under compression side under compression, the crack always initiated from (around 6–7%) and a little higher for the kilned glass the edge (see Fig. 7e–h), while for those tested with panels with the coloured side under tension (around the coloured side under tension always on the sur- 9–10%). This slightly higher scatter was attributed to face (see Fig. 7i–l). The initiation from the edge could the non-uniformly distributed granulate powder and the be explained by the fact that the kilned glass pan- observed devitrification, which lead to a more rough- els were thinner in the edge areas due to the nearly ened glass surface with defects of more diverse mag- quadrant-shaped geometry. Moreover, the lower sides nitude, as exemplified in Sect. 3.4. In terms of cal- of the cut edges, which were in contact with the textile culated bending stresses, the values for the reference during kilning and subjected to tensile stresses dur- annealed float glass panels were a little higher than the ing the testing, might still have exhibited some resid- characteristic bending strength value of 45 MPa gen- ual damages after kilning. The crack initiation on the erally provided in standards with an average value of surface in the case of the panels with the coloured around 52 MPa. The lower average bending stress val- side under tension indicated that the fused granu- ues calculated for the kilned glass panels of around lar powder had a significant influence on the near- 35 MPa with the coloured side under compression and surface micro-structure and implicitly on the bending of around 25 MPa with the coloured side under ten- strength. The location of failure initiation was with sion were in the range of bending strength values found high probability determined by the unevenness of the in literature for patterned glass (e.g. Haldimann et al. kilned glass panels combined with surface damages 2008; Schneider et al. 2016). The estimated Young’s caused by the non-uniformly distributed fused gran- modulus values were for all specimens in the range ules. of the value of 70 000 MPa found in literature. How- The main fan-shaped cracks are highlighted in the ever, no significant influence of the kilning and fusing images from Fig. 7. It was observed that for almost all processes on the Young’s modulus was observed. The specimens the fracture initiated between the loading slightly different Young’s modulus values determined rollers. Only for two of the kilned glass panels with for the kilned glass panels were with high probability the coloured side under tension, the fracture initiated due to the larger scatter of the average thickness values, slightly outside the loading rollers, indicating damages determined from measurements at points both with and of the surface through the fused granular powder. The without powder granules. cracks in Fig. 7, which are not highlighted, resulted when the already fractured specimens hit the test setup and were therefore not considered for the evaluation in 3.2 Fracture patterns of the different glass panels this section. When comparing the fracture patterns of the differ- The fracture pattern of a glass type is relevant for ent series to each other, it was observed that the ref- assessing the risk of injury in case of breakage as erence annealed glass panels and the kilned glass pan- well as for the residual load-bearing capacity in case els tested with the coloured side under compression of combination to laminated glass. Furthermore, the position of the fracture initiation on the surface or 123 Structural behaviour and micro-structural characteristics Fig. 7 Fracture patterns for the reference annealed float glass panels (a–d) and for the coloured kilned glass panels with the coloured side under compression (e–h) and with the coloured side under tension (i–l) 123 V.-A. Silvestru et al. exhibited patterns of similar complexity, with multi- 3.3 Analysis of surfaces from the different glass panel ple cracks distributed in the shape of a fan. The frac- types based on stereo microscope images ture patterns of the kilned glass panels tested with the coloured side under tension, however, were much sim- For explaining the worse performance of the coloured pler, with only a low number of cracks. Moreover, for kilned glass panels in the four-point-bending tests com- the reference annealed glass panels with crack initi- pared to the reference annealed float glass panels, the ation in the surface, the fan-shaped cracks intersect different surfaces were analysed under a stereo micro- in one point, while for the kilned glass panels tested scope. The lower zoom factor of 10X was used to get with the coloured side under tension, a crack par- an overview of the surface characteristics, while the allel to the shorter glass panel edges was observed, higher zoom factor of 40X allowed the identification from which a few further cracks developed towards the and assessment of specific features. Figure 8 shows longer glass panel edges. The limited fragmentation a representative area of a reference annealed glass observed for the kilned glass panels tested with the panel near a broken edge. Significant visible features coloured side under tension indicated a lower bend- or defects (except those resulted due to the fracture in ing strength of these samples, which was in agree- direct vicinity of the edge) could not be observed, nei- ment with the determined strength values provided in ther with the lower zoom factor, nor with the higher Sect. 3.1. one. When assessing the safety and the risk of injury In contrast with the reference annealed float glass in case of breakage of monolithic glass panels, the panels, both the uncoloured and the coloured sides of size and sharpness of the resulting shards is relevant. the kilned glass panels exhibited features that could be While annealed glass, whose fracture pattern exhibits associated with the earlier failure in the bending tests. radial cracks and large, sharp fragments, is considered In case of the uncoloured side, bubbles of different sizes to present a high risk of injury, fully tempered glass distributed all over the surfaces could be observed, as with net-like cracks and small, dice-shaped, less sharp shown in Fig. 9a. A closer evaluation with the higher fragments presents a lower risk. In case of the tested zoom factor (see Fig. 9b) allowed to measure the size kilned glass panels, the resulted shards exhibited sim- of the bubbles, which ranged from a few micrometres ilar characteristics in terms of size and sharpness to to around half of a millimetre in diameter. Most of the those obtained from the annealed glass panels. Based bubbles had diameters of less than 400 µm and could on the large, sharp fragments, the same qualitative risk be categorized as seeds, while some few were slightly of injury can be assumed for the coloured kilned glass larger and could be classified as blisters. Part of the bub- as for the reference annealed glass. bles were entirely sunken under the glass surface, while others were directly on the surface, as the one shown Fig. 8 Air side of the reference annealed float glass panels—representative stereo microscope images obtained with zoom factors of 10X (a) and 40X (b) 123 Structural behaviour and micro-structural characteristics Fig. 9 Uncoloured side of kilned glass panels—representative stereo microscope images obtained with zoom factors of 10X (a)and 40X (b) in Fig. 15a, b. Furthermore, it could be observed that granules started to merge in these areas, while the tex- the uncoloured surface was not as smooth as the refer- turized surface parts without granules seemed to get ence annealed glass surfaces, exhibiting hardly visible a greenish tone and to go from translucent to opaque. scratches and a light texture, probably as a consequence Areas with high density of the dispensed and fused of the textile on which the glass panels were placed dur- granules were considered such where granules could ing the kilning process. Figure 9b also illustrates that not be observed anymore as partially immersed in the the kilned glass panels were not transparent anymore, glass surface, but where the entire surface was coloured but translucent, even in the areas without dispensed with green tones and single dark-shaded green gran- granules. ules could be identified fully submersed in the glass In the case of the coloured side of the kilned glass (see Fig. 10e, f). With the higher zoom factor of 40X, panels, areas with different density of dispensed and the surface exhibited a mosaic-like texture noticeable fused granules were observed. These areas seemed based on the reflexions resulting through the exposure to exhibit variable characteristics and were therefore of the stereo microscope. evaluated separately, as shown in Fig. 10. Areas with A detailed meaningful analysis of the fracture ori- low density were considered such where only single gins was not possible, since the acute-angled shards isolated granules were observed on the glass surface got chipped in these regions due to the impact of the (see Fig. 10a, b). Here, the different granules, which fractured specimens to the test setup below them. were partially immersed into the glass surface, could In addition to the observed modifications on the out- be clearly identified and their dimensions, which var- side surface, an assessment of the depth of these mod- ied up to around 1 mm in diameter, could be evaluated. ifications was made based on microscopic images of With the higher zoom factor of 40X, it was observed the fractured surface. The pictures in Fig. 11 show the that pronounced damages of the glass surface were fractured surface of a shard from a coloured kilned produced in the direct vicinity of the granules. More- glass panel after completing the four-point-bending over, texturized surfaces could be identified as well in test. From Fig. 11a, illustrating areas with isolated the areas without granules. Both the more pronounced fused granules as well as coalescing fused granules, it damages near granules and the less pronounced tex- was observed that the granules remained near the out- ture further away from the granules could be features side surface in these areas. Figure 11b shows an isolated which lead to a decrease of the bending strength. Sur- fused granule captured with an increased zoom factor face zones with different granules partially immersed (this granule was not situated at the fracture origin). in the glass surface and starting to coalesce, as those The dimensions of such a granule were in agreement shown in Fig. 10c, d, were considered as areas with with the size range of the initially dispensed powder. It medium density. The pronounced damages around the was observed that there is a fusing interface between 123 V.-A. Silvestru et al. Fig. 10 Coloured side of kilned glass panels with low (a, b), medium (c, d) and high (e, f) density of dispensed and fused granules—rep- resentative stereo microscope images obtained with zoom factors of 10X (a, c, e) and 40X (b, d, f) 123 Structural behaviour and micro-structural characteristics Fig. 11 Representative images obtained with the stereo microscope with zoom factors of 6.3X (a) and 40X (b) for fractured surfaces of the coloured kilned glass panels illustrating the depth to which the dispensed granules are fused into the glass surfaces the granules and the annealed float glass surface, but no assessed based on local features identified on 3D sur- pronounce mixing was present. Furthermore, Fig. 11b face diagrams, linear waviness diagrams as well as shows that the glass surface on the coloured side of the linear roughness diagrams and average values. When kilned glass panels was not roughened only near the comparing the results in Figs. 12, 13, 14, 15, 16 and fused granules, but also in areas without granules. This 17 to each other, one should consider the 20 times indicated that the kilning process resulted in a textur- smaller scales used in all diagrams for the reference ized surface on a micro-structural level, which lead to annealed float glass. Using the same scale would have the worse performance in the four-point-bending tests resulted in perceiving these surfaces as completely flat compared to the reference annealed float glass. and smooth. This aspect also emphasized the surface texture modifications suffered by the coloured kilned glass in the production process. The reference annealed glass exhibited similar sur- face characteristics both for the air side and for the tin 3.4 Analysis of surface conditions based on confocal side, as shown by Figs. 12 and 13. Singular local peaks microscope images in the 3D surface diagrams represented isolated dust particles that could not be avoided, despite carefully The surface analysis with the stereo microscope cleaning the surfaces before the microscopic investiga- allowed for a qualitative characterization of the sur- tions. The measured average roughness values, which face modifications generated through the production were slightly higher than values found in literature process of the coloured kilned glass. For a more pre- (Datsiou and Overend 2016) for annealed float glass, cise characterisation and quantitative assessment, addi- could be attributed to the facts that (i) the glass panels tional investigations based on confocal microscopy were stored in exterior environment for approximately were conducted. This allowed to produce height maps 6 months before testing and (ii) the microscopic eval- of selected representative areas of the different glass uation was carried out on shard samples resulted after surfaces and to evaluate the surface roughness along performing the four-point-bending tests. characteristic profiles. Six different glass surfaces were When looking at the uncoloured surface of the kilned analysed, being (i) the air side (Fig. 12) and (ii) the glass (see Fig. 14a, b), which was positioned on a tex- tin side (Fig. 13) of the reference annealed float glass, tile material during the kilning process, different fea- (iii) the uncoloured side of the kilned glass (Fig. 14) tures could be noticed. The more obvious ones were and the coloured side of the kilned glass, with (iv) low the surface-near bubbles of different sizes, which were (Fig. 15), (v) medium (Fig. 16) and (vi) high (Fig. 17) already observed with the stereo microscope and dis- density of the dispensed and fused granulate. For each cussed in Sect. 3.3. Figure 14b highlights the surface of these surface types, the texture was quantitatively 123 V.-A. Silvestru et al. Fig. 12 Air side of the reference annealed glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 13 Tin side of the reference annealed glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 Structural behaviour and micro-structural characteristics Fig. 14 Uncoloured side of kilned glass panels—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 15 Coloured side of kilned glass panels with low density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 V.-A. Silvestru et al. Fig. 16 Coloured side of kilned glass panels with medium density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) Fig. 17 Coloured side of kilned glass panels with high density of dispensed and fused granules—representative confocal microscope image (a), 3D surface texture (b), waviness profile (c) and roughness profile (d) 123 Structural behaviour and micro-structural characteristics area where a bubble escaped the glass surface during of the kilned glass situated between those of the ref- the kilning process. In addition, some linear-shaped and erence annealed float glass and those of the coloured some dot-shaped damages were noticed and are shown side of the kilned glass, are in good agreement with the in Fig. 14a. These ones could be attributed with high differences in structural performance observed for the probability to the textile material used below the panel. different panel types in the four-point bending tests. As shownbyFig. 14c, d, the uncoloured surface of the kilned glass exhibited an increased waviness and, espe- 4 Conclusions cially in the areas around escaped bubbles, a higher roughness compared to the surfaces of the reference The results presented and discussed in this paper pro- annealed float glass. vided detailed insights on the structural performance In case of the coloured side of the kilned glass, it and micro-structural characteristics of a novel type of was differentiated between areas with low (Fig. 15), coloured kilned glass panels which allows for decora- medium (Fig. 16) and high (Fig. 17) density of the dis- tive polychromatic elements. On the one hand, these pensed and fused granules to evaluate an eventual influ- findings can be used by designers to evaluate the lim- ence of this aspect on the quantitative surface proper- its of this novel type of glass and to use it in suitable ties. A modified texture of the surface was observed in applications. On the other hand, they can be a basis for a all three cases. However, in areas with low density of subsequent optimization of the production process with granules, the granules could be clearly identified in the the aim of improving the structural and micro-structural confocal microscopic image (see Fig. 15a), in the 3D properties of the coloured kilned glass panels. The fol- surface diagram and in the waviness profile. In areas lowing more specific conclusions can be drawn: with medium density, only the waviness profile indi- cated the presence of granules, while for areas with • The bending strength and the reachable mid-span high density, a pronounced influence of the granules on deflection of the reference annealed float glass were the waviness profile was not visible. In terms of rough- reduced through the kilning and fusing processes. ness, the highest values were measured in areas with The magnitude of these reductions depended on low density, followed closely by areas with medium whether the coloured side was under tension or com- density. Areas with high density of granules exhibited pression. lower roughness values, indicating that such a distribu- • The kilning production process resulted in a more tion could be advantageous for the resulting microstruc- irregular geometry of the glass panels with rounded, ture. When looking at the surface texture in more detail, nearly quadrant-shaped edges and uneven thickness. it was observed that at low granule density the glass sur- Especially the thinner edges influenced the fracture face exhibited an orange-skin-like pattern, while with pattern of the kilned glass panels. The fracture ini- increasing density this pattern changed more and more tiated at the glass edge, when the coloured side was to a mosaic-like pattern. These aspects indicated devit- under compression and on the glass surface when the rification, which can generally occur when the glass is coloured side was under tension. The resulting shards held for too long at temperatures slightly above the soft- had similar sizes to those of the reference annealed ening point and can be as well boosted by dust particles float glass. present on the glass surface or in the kiln. Both of these • The microstructure of both the uncoloured and the conditions were met to a certain extent during the pro- coloured surfaces of the kilned glass were qualita- duction process of the investigated coloured glass. A tively modified by the production process, result- start of devitrification was observed also in areas of the ing in bending-strength-reducing features. For the kilned glass, in which no granules were deposited. The coloured side, micro-structural differences were devitrification became more pronounced with higher identified for areas with different density of the fused density of deposited granules. In future investigations, granules, ranging from surface damages around sin- the temperature curve could be varied and the kiln could gle granules to devitrification in areas with high den- be carefully cleaned for evaluating if and to what extent sity. the devitrification process can be avoided. • The roughness and the waviness of the coloured The roughness values determined for the different kilned glass was increased by the production pro- glass surfaces, with the values for the uncoloured side cess. The quantitatively evaluated roughness values 123 V.-A. Silvestru et al. were in good agreement with the different bend- Funding Open access funding provided by Swiss Federal Insti- tute of Technology Zurich. ing strength values determined from the four-point- bending test results, since the higher the local rough- ness of the glass surfaces subjected to tensile stresses was, the lower bending strength values were deter- Declarations mined. Conflict of interest On behalf of all authors, the corresponding The obtained results indicate that the coloured kilned author states that there is no conflict of interest. glass panels could be used in principle with similar lim- itations as annealed float glass or patterned glass, if the Open Access This article is licensed under a Creative Com- reduced strength values are considered. For a statisti- mons Attribution 4.0 International License, which permits use, cally significant determination of the strength value, a sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original larger number of specimens needs to be tested. Qual- author(s) and the source, provide a link to the Creative Com- itatively, the injury risk level indicated by the failure mons licence, and indicate if changes were made. The images modes and the size and sharpness of the shards is sim- or other third party material in this article are included in the ilar to that of annealed float glass. article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the Future investigations dealing with this novel type article’s Creative Commons licence and your intended use is not of coloured kilned glass should on the one hand focus permitted by statutory regulation or exceeds the permitted use, on studying the effects of different kilning tempera- you will need to obtain permission directly from the copyright ture curves and support plate materials on the structural holder. To view a copy of this licence, visit http://creativecomm ons.org/licenses/by/4.0/. performance and micro-structural characteristics. This would allow to optimize the properties of the poly- chromatic glass. Furthermore, the studies should be References extended to panels with diverse colour patterns to assess eventual additional effects and should include more Bristogianni, T., Oikonomopoulou, F., Yu, R., Veer, F.A., Nijsse, detailed investigations of (i) the glass powder and flat R.: Investigating the flexural strength of recycled cast glass. glass composition (e.g. X-ray fluorescence analysis), Glass Struct. Eng. 5, 445–487 (2020). https://doi.org/10. (ii) eventual residual stresses after the fusing and kil- 1007/s40940-020-00138-2 Bristogianni, T., Oikonomopoulou, F., Veer, F.A.: On the flex- ning process for different temperature curves (e.g. anal- ural strength and stiffness of cast glass. Glass Struct. ysis with polarised light) and (iii) fracture mirrors on Eng. 6, 147–194 (2021). https://doi.org/10.1007/s40940- shards without chipping. On the other hand, additional 021-00151-z processing steps, like thermal tempering and laminat- Datsiou, K., Overend, M.: Evaluation of artificial ageing methods for glass. In: Bos, F., Louter, C., Belis, J. (eds.) Challenging ing, should be assessed for the coloured kilned glass in Glass 5 Conference, Ghent University (2016) order to allow an increased safety and a wider range of EN 1288-3: Glass in building—Determination of the bending applications as façade glazing. strength of glass—Part 3: Test with specimen supported at two points (four point bending) (2000) Acknowledgements The authors would like to gratefully EN 572-1: Glass in building – Basic soda-lime silicate glass prod- acknowledge the support of the laboratory staff of the Institute ucts – Part 1: Definitions and general physical and mechan- of Structural Engineering at ETH Zurich in planning and manu- ical properties (2016) facturing necessary parts of the four-point-bending test setup. EN 572-2: Glass in building—Basic soda-lime silicate glass Furthermore, the authors thank Asel Maria Aguilar Sanchez products—Part 2: Float glass (2012) from the Institute for Building Materials and Tobias Schwarz of EN 572-5: Glass in building—Basic soda-lime silicate glass ScopeM for their support and assistance in obtaining the micro- products—Part 5: Patterned glass (2012) scope images analysed in this work. EN ISO 4287: Geometrical Product Specifications (GPS)—Surface texture: Profile method—Terms, defi- Author contributions Conceptualization: Vlad-Alexandru Sil- nitions and surface texture parameters (2010) vestru; Methodology: Vlad-Alexandru Silvestru, Rena Giesecke; Giesecke, R., Dillenburger, B.: Large-scale robotic fabri- Formal analysis and investigation: Vlad-Alexandru Silvestru, cation pf polychromatic relief glass. Int. J. Archit. Rena Giesecke; Writing – original draft preparation: Vlad- Comput. 20(1), 18–30 (2022). https://doi.org/10.1177/ Alexandru Silvestru, Rena Giesecke; Writing – review and edit- 14780771221082259 ing: Vlad-Alexandru Silvestru, Rena Giesecke, Benjamin Dillen- Giesecke, R., Clemente, R., Mitropoulou, I., Skevaki, E., Peter- burger; Project administration: Vlad-Alexandru Silvestru, Rena hans, C.T., Dillenburger, B.: Beyond transparency, archi- Giesecke; Supervision: Benjamin Dillenburger. tectural application of robotically fabricated polychromatic 123 Structural behaviour and micro-structural characteristics float glass. Constr. Robot. (2022). https://doi.org/10.1007/ OPTUL Float: Range of granulate products. Optul Spezial- s41693-022-00071-6 glas GmbH. https://optul.de/produktpalette/granulate.html Haldimann, M., Luible, A., Overend, M.: Structural Use of (2022). Accessed 13 June 2022 Glass. Structural Engineering document SED10. Interna- Schneider, J., Kuntsche, J., Schula, S., Schneider, F., Wörner, tional Association for Bridge and Structural Engineering J.-D.: Glasbau—Grundlagen, Berechnung, Konstruktion. IABSE, Zurich (2008) Springer Vieweg, Berlin (2016) Inamura, C., Stern, M., Lizardo, D., Houk, P., Oxman, N.: 3D Seel, M., Akerboom, R., Knaack, U., Oechsner, M., Hof, P., Printing and Additive manufacturing of transparent glass Schneider, J.: Additive manufacturing of glass compo- structures. Addit. Manuf. 5(4), 269–283 (2018). https://doi. nents—exploring the potential of glass components by org/10.1089/3dp.2018.0157 fused deposition modelling. In: Louter, C., Bos, F., Belis, J., Maniatis, I., Elstner, M.: Investigations on the mechanical Veer, F., Nijsse, R. (eds.) Challenging Glass 6 Conference, strength of enamelled glass. Glass Struct. Eng. 1, 277–288 Delft University of Technology (2018). https://doi.org/10. (2016). https://doi.org/10.1007/s40940-016-0025-2 7480/cgc.6.2161 Oikonomopoulou, F., Bristogianni, T., Barou, L., Veer, F.A., TNO: Glass powder printer. TNO Science and Industry. https:// Nijsse, R.: The potential of cast glass in structural applica- www.tno.nl/media/2871/hr_tno_leaflet-glasprinter.pdf tions Lessons learned from large-scale castings and state- (2007). Accessed 4 September 2022 of-the art load-bearing cast glass in architecture. J. Build. Eng. 20, 213–234 (2018). https://doi.org/10.1016/j.jobe. Publisher’s Note Springer Nature remains neutral with regard 2018.07.014 to jurisdictional claims in published maps and institutional affil- iations.

Journal

Glass Structures & Engineering – Springer Journals

Published: Nov 2, 2022

Keywords: Coloured kilned glass panel; Four-point bending test; Tensile bending strength; Micro-structural analysis; Glass surface texture; Surface roughness

References

  • Investigating the flexural strength of recycled cast glass
    Bristogianni, T; Oikonomopoulou, F; Yu, R; Veer, FA; Nijsse, R
  • On the flexural strength and stiffness of cast glass
    Bristogianni, T; Oikonomopoulou, F; Veer, FA
  • Large-scale robotic fabrication pf polychromatic relief glass
    Giesecke, R; Dillenburger, B
  • Beyond transparency, architectural application of robotically fabricated polychromatic float glass
    Giesecke, R; Clemente, R; Mitropoulou, I; Skevaki, E; Peterhans, CT; Dillenburger, B
  • 3D Printing and Additive manufacturing of transparent glass structures
    Inamura, C; Stern, M; Lizardo, D; Houk, P; Oxman, N
  • Investigations on the mechanical strength of enamelled glass
    Maniatis, I; Elstner, M
  • The potential of cast glass in structural applications Lessons learned from large-scale castings and state-of-the art load-bearing cast glass in architecture
    Oikonomopoulou, F; Bristogianni, T; Barou, L; Veer, FA; Nijsse, R