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Glass Struct. Eng. https://doi.org/10.1007/s40940-022-00206-9 RESEARCH PAPER Glass up-casting: a review on the current challenges in glass recycling and a novel approach for recycling “as-is” glass waste into volumetric glass components Telesilla Bristogianni · Faidra Oikonomopoulou Received: 1 June 2022 / Accepted: 29 September 2022 © The Author(s) 2022 Abstract This paper presents the casting of volumet- this new recycling approach. In continuation, address- ric glass components from glass waste as an alternative ing the technical challenges that are mainly linked to glass-recycling approach. The approach is character- contamination, an overview is provided of the main ized by its ﬂexibility to accommodate a variety of com- experimental ﬁndings on the inﬂuence of cullet con- positions and ability to yield volumetric (solid or thick- taminants and casting parameters on the generation of walled) glass products that can tolerate higher contam- defects, and how these affect the mechanical properties. ination rates without a signiﬁcant compromise to their The experiments study a broad variety of glass compo- properties. The novelty of the proposed glass-to-glass sitions, including soda-lime, borosilicate, aluminosil- recycling method lies in the “as-received” recycling icate and lead/barium glasses, and different levels of of glass waste, using relatively low forming tempera- cullet contamination, of embedded (e.g. frit, wire) or tures (750–1200 °C). This reduces both the need for external (e.g. stones, glass ceramics) character. Based expensive, labour-intensive and logistically complex on the cullet characteristics and imposed ﬁring sched- purifying, segregation and treatment (e.g. removal of ules, different glass quality grades arise and critical coatings) techniques, and the required energy and CO defects are highlighted. Thereafter, the most promising emissions for product forming. Aim of this paper is glass waste sources that can be recycled via this novel to provide an overview of the potential but also of recycling approach are distinguished and directions for the technical and supply-chain challenges and limita- future research are highlighted. tions that still need to be tackled, in order to intro- duce this recycling approach to the market. Addressing Keywords Glass recycling · Glass waste · Cast glass · the supply-chain barriers of glass recycling, the prin- Kiln-casting · Glass defects · Glass strength cipal challenges linked to the collection and separa- tion of glass waste and the established quality stan- dards for the prevailing glass production technologies 1 Introduction are identiﬁed, in order to argue upon the potential of Glass waste treatment and recycling is a currently unre- solved, pressing problem. Apart from container glass T. Bristogianni ( ) · F. Oikonomopoulou being successfully recycled in a closed-loop in Europe, Department of Architectural Engineering and Technology, the rest of the commercial glass waste is, in its major- Delft University of Technology, Delft, The Netherlands e-mail: email@example.com ity, downcycled or landﬁlled. This comes down to the lack of infrastructure for the collection, separation and F. Oikonomopoulou e-mail: firstname.lastname@example.org puriﬁcation of glasses different in composition and 123 T. Bristogianni, F. Oikonomopoulou surface treatment than container glass, the distrust of ﬁeld visits to glass production and glass recycling glass producers to pollute their ovens with recycled cul- plants (e.g. Maltha Recycling Nederland, AGC Bel- let of various compositions, and the quality reduction gium, Magna Glaskeramiek), and interviews with recy- that typical thin-walled glass products experience when cling organizations and companies (e.g. Vlakglasrecy- contaminated by recycled cullet. In pursuit of feasible cling Nedeland, Coolrec, LightRec). solutions to this problem, this review paper follows a The experimental procedures conducted at the TU two-fold research approach: Delft Glass Lab involve the following consecutive steps: (i) First, an overview of the current glass recy- cling situation in Europe is provided, pinpoint- ing the challenges and impediments that prevent i. Cullet evaluation Various types of commercial the closed-loop recycling of -other than container- glass waste cullet are collected by the authors or glass products. Based on the ﬁndings, an immedi- provided by glass producers and glass recycling ate glass recycling solution is suggested that can companies. These streams typically involve the circumvent the identiﬁed impediments: the “recy- main glass compositional families of soda lime sil- cling as-received” casting method, for the produc- ica, borosilicate, aluminosilicate and lead/barium tion of voluminous glass products for architectural silicate, while specialty glasses are excluded from and interior design applications. this study. The glass composition and type of con- (ii) In continuation, an overview of the conducted tamination of the samples are examined by visual melting experiments using the aforementioned inspection and determined, where applicable, by method is provided. This section investigates the X-ray ﬂuorescent (XRF) analyses conducted with impact that different contaminants within the as- a Panalytical Axios Max WDXRF spectrometer. received glass sources may have on the integrity, ii. Cullet preparation The glass cullet is cleaned homogeneity, colour, and strength of the recy- with isopropanol. Easily detectable contaminants cled cast glass components. Focus is given on the of equal or considerably larger size than the glass most challenging contaminants that are typically shards (e.g. metal or plastic pieces) are manually rejected by the glass industry. This overview is removed, while traces smaller than 3 mm are kept conducted by combining a signiﬁcant amount of within the cullet samples. Coatings, fritting and unpublished experimental work (mainly concern- other surface treatments are left as-is. If required, ing melting experiments, chemical composition the glass cullet sample is crushed into ﬁne particles analyses and defect characterization) with prior (e.g. 1–2 mm) or powdered using a Retsch vibra- published data by the authors (focusing on ﬂexu- tory disc mill with hardened steel grinding discs, ral strength). and metal sieves of various sizes. Based on the presented state-of-the-art and the iii. Casting experiments The kiln-casting technique is used to produce recycled specimens in various dif- extended experimental overview, a discussion is ini- tiated on the most promising glass waste streams for ferent sizes, namely 50 × 50 × 50 mm, 100 × achieving top-quality cast components, on methods of 100 × 10 mm, 350 × 350 × 10 mm, 30 × 30 × improving the quality of contaminated glass sources, 240 mm and 30 × 20 × 350 mm. More speciﬁcally and on the future steps to be taken for introducing the about the casting process, each cullet type is posi- “recycling as-received” casting method to the market. tioned in separate disposable investment moulds The discussion results in cullet and recycled cast glass made out of silica plaster (Crystalcast M248) in classiﬁcation schemes, based on their kiln-casting ease, a random or structured manner. The moulds are criticality of resulting defects, strength and colour. then introduced in an ROHDE ELS 200S or ELS 1000S electric kiln, ﬁred at relatively low form- ing temperatures (750–1200 °C) and then control- 2 Methodology lably cooled and annealed in the same kiln accord- ing to their chemical composition and size. The Regarding the mapping methods of current glass recy- employed lower forming temperatures than those cling status, data are collected from literature research, typically followed by the glass industry for glass 123 Glass up-casting: a review on the current challenges in glass recycling forming aim to reduce the energy and CO emis- resource-efﬁcient future: glass, crushed into cullet (bro- sions linked to glass making. ken glass), can be returned to the melting furnace and iv. Specimen preparation and evaluation Integral be indeﬁnitely recycled in a closed-loop without loss of specimens are further cut in size, if required, using properties. Here, a distinction should be made between a water-cooled rotary diamond wheel cutter, and internal and external cullet, and pre-consumer and post- then ground and polished using a Provetro ﬂat consumer cullet. According to (Vieitez Rodriguez et al. grinder and diamond abrasive discs in multiple 2011): steps from 60 to 600 grit. A Keyence VHX-7000 • Internal cullet originates from offcuts and rejects dur- digital microscope (20–200 × zoom) lens is used ing the production and cold end of the manufacturing to evaluate the different types of defects present process (e.g. cutting or pre-stressing), or the transi- in the glass, as a result of the present contami- tion phases of product changes (e.g. thickness and nants and the followed casting parameters. Cross colour changes) and can be immediately absorbed in polarized light techniques are used to qualitatively the melting process as raw material. As this cullet evaluate the presence of residual stresses within never leaves the manufacturing plant, it is not con- the glass. A selection of specimens are further sidered waste. evaluated in terms of ﬂexural strength by con- • Pre-consumer cullet concerns glass waste originat- ducting 4-point bending experiments. The exper- ing from further processing of products containing imental data on ﬂexural strength exclusively con- glass, which have left the speciﬁc facility in which cern prior published work (mainly in Bristogianni the glass has been generated, but have not yet reached et al. 2020; Bristogianni et al. 2021a, b), therefore the consumer market; e.g. from further processing the reader is referred to the relevant publications (e.g. transformation sites and coating lines, losses for detailed information on the experimental con- during laminating, bending) or during transportation ditions applied in each case (see Table 5). It is and placement. important to mention that these tests were con- • Post-consumer cullet is waste glass originating after ducted using a limited number of specimens per the use of glass products at the customer market. The case (e.g. 3–5), and different experimental ﬁxtures composition of post-consumer cullet can be highly and specimen sizes were employed in each study. variable and cannot be well deﬁned, which generally Therefore, the results are only indicative of the limits its recyclability. strength differences between glass types and do • External cullet, which is waste glass that is collected not allow for statistical conclusions or the extrac- and/or reprocessed with the purpose of recycling can tion of design values. include both pre- and post-consumer waste. The use of cullet can generally lead to signiﬁcant cost savings as a result of the reduction in both energy 3 Review of current glass recycling status and raw material requirements: 3.1 Recycling status of glass products in the EU (i) It reduces the amount of accumulated waste and the extraction of raw materials. Speciﬁcally, each There is currently a continuous urge to develop and tonne of cullet saves 1.2 tonnes of raw materi- utilize recyclable and reusable materials in the build- als, including 850 kg of sand (Hestin et al. 2016; ing sector in order to reduce the substantial CO emis- Surgenor et al. 2018). sions and end-of-life waste generated by it—the lat- (ii) It reduces the energy consumption by 2.5–3% ter accounting for approximately 25–30% of all waste for every 10% of cullet added to the melting generated in the EU (Surgenor et al. 2018). This shift batch (Nilsson et al. 2007) and the CO emis- in materialization is further supported by international sions required to melt the glass by 300 kg per guidelines, such as the European Green Deal and EU’s tonne of cullet used (Hestin et al. 2016). 2050 climate neutrality target (European Commission). (iii) It increases the service life of a glass melting fur- Glass, with an annual production of 35.4–36.8 Mt in EU nace by up to 30% due to decreased melting tem- in 2018–2020 according to Glass Alliance Europe, is peratures and a less corrosive batch (Worrell et al. an excellent candidate in the transition to a low-carbon, 2008). 123 T. Bristogianni, F. Oikonomopoulou Fig. 1 Illustration of the production and recycling of glass cullet in EU28 in 2017 based on approximate numbers as provided by (Rose and Nothacker 2019; Hestin et al. 2016) However, despite the common notion that glass is appliances (e.g. tableware, ovenware, mirrors), build- 100% recyclable, at present, only the container glass ing waste (eg. glass tiles), electronic waste (e.g. light industry, accounting for ≈ 62%/22 Mt of the EU28 bulbs/tubes, mobile phone screens, Liquid Crystal Dis- glass production (Harder 2018), implements successful plays, solar panels, Cathode Ray Tubes) and indus- closed-loop glass recycling with an average recycling trial/laboratory waste, to name a few (see Table 1). rate of 76% within the EU28 (FEVE 2016, 2019). For The systematic recovery and recycling of such glass the remaining types of glass waste (≈38%/13.5 Mt of products in a closed-loop manner is seldom observed. the total glass production within EU28 (Harder 2018)), With the exemption of a few successful cases, such as the closed-loop or glass-to-glass recycling rate is, at the energy-saving lamp recycling in NL by Stichting present, remarkably less (see Fig. 1). Speciﬁcally, in LightRec and Wecycle, the recycling percentage of this the ﬂat glass industry (≈ 10.2–10.8 Mt annual pro- vast variety of glass products back to glass is close to duction), which includes the Construction & Demoli- zero. In essence, glass waste, particularly originating tion (C&D) and automotive sector, end-of-life, post- from the post-consumer phase, remains a signiﬁcant consumer, glass is seldom recycled back into ﬂat glass and unresolved problem. products; instead it is moderately down-cycled to glass bottles, processed into low-value products or land- 3.2 Principal challenges in recycling glass (other ﬁlled (Hestin et al. 2016). Even in the Netherlands, than containers) where, through the establishment of the nationwide Foundation “Vlakglas Recycling Nederland” (VRN), The limited closed-loop recycling of non-packaging the highest percentage of recycling C&D glass (80%) glass can be attributed to several factors that can be is achieved, the majority of the collected glass is either grouped under (i) the supply-chain barrier and (ii) the down-cycled into bottles (42.8%), or processed into technical barrier. aggregates and insulation products such as glass wool (41.2%). Only 7.5% of the collected ﬂoat glass is recy- 3.2.1 Supply-chain barrier cled back into the same product in the Netherlands (Vlakglas Recycling Nederland 2020). The supply-chain barrier concerns the logistics and leg- Other glass products result mostly in glass waste, islation involved in collecting, treating and recycling either because they follow different compositions than glass products; accordingly, it mainly refers to external soda-lime or because they present a high degree of glass cullet. To this end, the main challenges revolve contamination. These include household utensils and around the widely variable legislations, recycling man- dates and waste management plans per country and the lack of a recycling-back-to-glass provision and prop- 1 erly organized collection, sorting, treatment & recy- According to (FEVE 2020) glass is the most recycled closed- loop packaging material in Europe. cling schemes for glass other than (soda-lime glass) 123 Glass up-casting: a review on the current challenges in glass recycling Table 1 Characteristic chemical compositions, temperatures and applications of the most common glass types derived by (Oikonomopoulou 2019) based on (Shand and Armistead 1958); mean melting points as stated by (Martlew 2005) Glass type Typical Approximate Mean melting Point at Soft. Anneal CTE applications composition 10 Pa.s Point Point 0–300 °C −6 (°C) (°C) (°C) 10 /°C Soda-lime Window panes 73% SiO 1350–1400 730 548 8.5 Container glass 17% Na O Building sector 5% CaO Automotive industry 4% MgO Mirrors 1% Al O 2 3 Borosilicate Laboratory glassware 80% SiO 1450–1550 780 525 3.4 Household ovenware 13% B O 2 3 Lightbulbs 4% Na O Optical glass 2.3% Al O 2 3 Pharmaceutical 0.1% K O packaging Lead silicate Artistic ware 63% SiO 1200–1300 626 435 9.1 Neon-sign tubes 21% PbO TV screens (CRT) 7.6% Na O Absorption of X-rays 6% K O 0.6% Al O 2 3 0.3% CaO 0.2% MgO 0.2% B O 2 3 Alumino-silicate Mobile phone screens 57% SiO 1500–1600 915 715 4.2 Fiber glass 20.5% Al O 2 3 High temperature 12% MgO thermometers 5.5% CaO Combustion tubes 1% Na O The provided compositions and temperatures are given as a reference to indicate the differences between the various glass types. In practice, for each glass type there are numerous recipes resulting into different properties packaging. Challenging logistics can further diminish cullet (Surgenor et al. 2018). To this end, the set-up of the environmental & economic beneﬁts of glass recy- recycling infrastructures in local/regional level can be cling. Speciﬁcally, labor-intensive, non-standardized vital in enhancing the recyclability of glass and ren- manual dismantling processes (e.g. when glass is part of dering it more sustainable. A good example towards a framed window and not a stand-alone product), asso- this direction is Saint-Gobain’s Glass Forever Cullet ciated also with the vast variety and types of end glass Return initiative for post-consumer ﬂat glass recovery products further contribute into making glass recycling and recycling (Saint Gobain), providing its own trans- less economically attractive. Moreover, due to the high portation scheme and trailer collectors to interested par- weight to volume ratio of glass, transportation costs ties, as well as offering a cullet crushing machine that of external cullet to the treatment and recycling plants can be used to separate glass from glazed units. can amount to as much as one third of the total costs of recycling (Rose and Nothacker 2019) and can be responsible for the highest amount of CO emissions produced in eventually substituting raw materials with 3.2.2 Technical barrier For example according to (Hestin et al. 2016), in UK, treatment The technical barrier is linked to the composition of facilities do not set up/collect skips more than 50 km away from the glass cullet and speciﬁcally to the inability of their treatment centre or at least from their storage facilities, for the transport cost outweighs the beneﬁts of using cullet. the current glass production, predominantly of ﬂat 123 T. Bristogianni, F. Oikonomopoulou Table 2 Allowable % of coloured cullet for producing green, Contamination, even from small quantities of impu- brown and clear glass bottles according to (Harder 2018; Vieitez rities, remains the main technical obstacle in glass recy- Rodriguez et al. 2011) cling, particularly concerning the production of ﬂat glass products, such as architectural and automotive Glass colour glass Green Brown Clear (ﬂint) glass. Contamination can be infeasible (e.g. adhesives, cullet (%) (%) (%) lamination, fritting) or technically strenuous (e.g. coat- (horizontal)/end ings, metal frames) to remove; which in turn can dimin- product (vertical) ish the environmental and ﬁnancial beneﬁts of recy- cling cullet. This is subject to the amount of treat- Green >70 <10 < 30 ment and sorting needed. Typically the following treat- Brown < 20 >80 <20 ments occur: initial manual visual inspection and sort- Clear (ﬂint) < 0.2 < 0.3 > 94–98 ing for elimination of foreign matter, crushing via a roller mill, removal of organics by washing or dry- ing and sieving, magnetic separation of ferrous met- glass products, and recycling technologies to accom- als, separation of non-ferrous metals by eddy current, modate variations in chemical compositions and vir- size classiﬁcation with a vibrating screen, cyclonic/air tually any contamination. In particular, cullet of dis- separation of lighter materials, incl. light metals, plas- similar glass compositions cannot be easily mixed, due tics, paper, and automated optical sorting into differ- to variations in thermal expansion coefﬁcient (CTE), ent colours (this step also may remove other opaque melting temperature and annealing temperature and non-glass materials) (Vieitez Rodriguez et al. 2011). rate, as shown in Table 1. This, in combination with Even small amounts of impurities, e.g. ceramics and the discussed supply-chain barrier, renders particu- stones (CSP), or metals such as nickel and iron, can larly challenging the recycling of glass compositions lead to undesirable inclusions in the glass, discoloration and respective products that are produced in relatively or even damage of the ﬂoat tank. In high percentages smaller batches and which lack an established recy- of cullet, the control of composition and hence of the cling collecting scheme. This refers to, for example, physical characteristics of the glass melt can be sig- high-quality/high-value aluminosilicate and borosili- niﬁcantly reduced, which in turn can compromise the cate glasses that require higher working temperatures quality and properties of the ﬁnal product (Scalet et al. than common soda-lime glass and present a consider- 2013). In thin-walled and ﬂat glass products, the accept- ably reduced thermal expansion coefﬁcient. able level of contamination is linked to a great extent Recipe compatibility does not necessarily permit a to their limited thickness, where it results in defects closed-loop glass recycling either, as colour contami- at, or close to, the surface. These defects are known to nation is also considered critical in current glass recy- act as stress concentrators and form a prime cause of cling schemes. This is also reﬂected in the closed-loop failure (Aldinger and Collins 2016; Vieitez Rodriguez glass recycling rates of different coloured containers, et al. 2011;Bartuška 2008), or they can simply result in as shown in Table 2. Green glass, where a mixture of blemishes that compromise the optical quality of trans- different colours can be used in higher amounts, can parent glass. be produced of up to 90–95% recycled glass, whereas Due to all the above, strict quality prerequisites are for brown glass the recycling rate drops to 70% and imposed, particularly for ﬂat glass products. As an for clear glass this is reduced to no more than 60%. example, the allowed contamination rate for external In ﬂoat lines, stringent optical-quality standards linked as well to colour contamination, render compatible-in- According to Vlakglas, the current recycling infrastructure composition external (pre- or post-consumer) cullet, allows for the crushing of glass up to 30 mm in thickness without yet with small quantities of colour additives, to be com- damaging the crushing blade. monly rejected in order to prevent a slightly altered tint Non-glass material components such as ceramics and porce- lain have higher melting points than glass and might not melt in in the end product. According to (GTS 2007), cullet the furnace (depending on their size); metal contaminants may for use in ﬂat glass manufacture should be of known cause damage to the furnace (Vieitez Rodriguez et al. 2011); for origin, consist of clear soda lime silica window glass example old wine bottle caps that had lead, caused liquid metallic and contain less than 5% lightly tinted ﬂat glass. lead downward drilling to the furnace. 123 Glass up-casting: a review on the current challenges in glass recycling Table 3 Typical sources and allowable contamination of cullet used for ﬂat, container and mineral wool products based on (GTS 2007) and (Vieitez Rodriguez et al. 2011) Contaminant Typical source Allowable contamination (ppm) of cullet used for Flat Container Mineral wool Ferrous Cutting blades, wired glass 2 (if particle size < 0.5 g) 50 10 Non-ferrous Spacer bars, drink cans, leaded glass 0.5 (if particle size < 0.1 g) 20 20 Inorganic Porcelain, aggregate, silicon carbide,020 25 cutting wheels Organic Plastics (PVB lamination foil, PVC), 45 (if particle size < 2 g) 3000 3000 wood, cardboard, paper, rubber gasket, foam spacers cullet in ﬂoat glass production is merely 2 ppm for fer- The lack of properly informing and raising aware- rous impurities and 0.5 ppm for non-ferrous metals, ness to the public on the glass types that can be recycled, whereas for container glass it is 50 ppm and 20 ppm further contributes to the contamination problem, as respectively (Vieitez Rodriguez et al. 2011), as shown due to “wish-cycling”, considerable amounts of glass in Table 3. According to (GTS 2007) cullet used for ﬂat have to be eventually down-cycled or landﬁlled due glass manufacture, should not contain any wired glass, to recipe mismatch or external contamination. To this laminated glass, container glass, glass of other com- end, the establishment of national and international pro- positions (lead, borosilicate), dark tinted glass, wind- tocols on the quality of the collected glass, such as the screens and glass ceramics (see Table 4). one published by (Wrap 2008) would be very helpful. These, stringent quality standards, in turn, force large-scale ﬂat glass producers to, in principle, reject the use of external cullet, restricting the industrial abil- 3.3 The potential of recycling glass waste ity to close the recycling loop. With the exception of into volumetric glass products via casting unmodiﬁed and uncontaminated ﬂoat glass, glass can- not ﬂow back into the original ﬂat glass production In effect, closed-loop recycling of post-consumer glass system. As a consequence, ﬂoat lines rely mainly on is only achieved in the packaging industry, as glass bot- internal (e.g. ﬂat glass edge trims) and pre-consumer tles are virtually mono-material and bear little risk of cullet from processors of ﬂoat glass which can guaran- external contamination compared to other glass prod- tee speciﬁc cullet properties, in order to maintain the ucts; at the same time they can also afford higher con- precise chemical composition and ﬁnal glass tint per tamination rates than e.g. ﬂat glass products (see Table production plant (see Table 4). This fact is strongly 3). Other glass products, once entering the consumer illustrated by the percentages of cullet material used market and considered post-consumer glass waste, are in Saint Gobain’s ﬂat glass furnaces in 2018: from the either down-cycled or landﬁlled. This is particularly the 31% total cullet used in their melting process, 19% case for ﬂat glass products, where standards on allow- was internal cullet (not considered waste), 11% was able contamination are the strictest, and special glass pre-consumer cullet and only 1% was post-consumer products, where the vast variety of end products leads cullet (Saint Gobain). to difﬁcult-to-predict levels of contamination and to a wide range of chemical compositions. Several ﬂoat glass manufacturers have revealed that they are conﬁdent in recycling e.g. coated glass, which is discarded dur- ing internal processing back to ﬂoat (pre-consumer glass waste); A common “wish-cycling” approach that hinders recycling due however, they will not accept for recycling the same coated glass to recipe incompatibility is the deposition of domestic lead crystal if returned by the ﬁnal user (post-consumer glass waste), due to tableware, borosilicate light bulbs and TV screens at the collec- fears of additional, external contamination. tion banks of container glass. 123 T. Bristogianni, F. Oikonomopoulou Table 4 Principal recyclability streams of ﬂat glass according to processing steps based on (Surgenor et al. 2018) and (Kasper 2006) Glass process Stage Recyclability to Remarks Float Container Mineral wool Annealing Internal × – – Internal cullet—readily recyclable Cutting and edge processing Internal × – – Internal cullet is recycled almost at 100% internally. Cullet from cutting lines of building and car glazing production are generally not contaminated. Glasses of different colors have to be separated to be recycled to ﬂoat glass manufacture Pre-consumer ×× Tempering Internal/ pre-consumer × – – No effect on recyclability if internal process Laminating pre-consumer – ×× Delamination (separating glass and foil) is technically feasible, but related to high expense. The resulting glass cullet usually contains less than 0.1% by weight of PVB Coating (hard/soft), Pre-consumer × – – Coatings can be burnt off in the Mirroring remelting process, so they can be mostly recycled. Metal contamination (e.g. silver from mirrors) can be absorbed in an internal recycling process when it is known and calculable. Once considered post-consumer the same material is down-cycled to other products Post-consumer – ×× Ceramic printing and pre-consumer – – – Recycling of such glass is fritting, enamel currently not possible (e.g. ovendoors, enamelled windscreens, architectural glass) Wired-glass pre-consumer – – – Recycling of such glass is currently not possible Insulating glass units post-consumer – ×× Requires removal of the spacer (assembled, bars and edge seals. Danger of multi-material) contamination due to traces of adhesive or metals and due to differences in chemical composition (colour contamination) renders its recycling back to ﬂoat glass particularly challenging 123 Glass up-casting: a review on the current challenges in glass recycling Table 4 (continued) Glass process Stage Recyclability to Remarks Float Container Mineral wool Automotive glass post-consumer – ×× Requires challenging and (assembled, expensive logistics linked to multi-material) the separation and treatment processes due to variety of colours, contamination by foreign matters, lamination, black enamel, use of different types of glass, which renders such cullet more expensive than new raw materials The above observations stress the importance of to incorporate different glass compositions and accom- improving the recycling rate of particularly post- modate a higher contamination rate at the batch of the consumer glass (excl. container glass) back to glass ﬁnal recycled glass product. Moreover, recycling by products. However, this is a multi-factor challenge that casting can be achieved on a local/regional level, even cannot be tackled in a sole solution. Nonetheless, the by small-scale glass casting studios, without major development of an alternative glass-recycling process alternations to their existing infrastructure; this min- which can be achieved on a local level and which has imizes relevant investments and logistic challenges the ﬂexibility to accommodate a variety of composi- related to waste collection, transportation, and asso- tions and tolerate higher contamination rates in the end ciated CO emissions and costs. glass products, can be a signiﬁcant contribution in tack- Addressing aspects of the technical barrier, com- ling the glass waste problem. Equally importantly, key pared to thin-walled glass elements, volumetric (solid for the expansion of glass recycling is the design of end or thick-walled) cast glass components present a higher products with the provision of using glass waste as the tolerance in size and number of defects from contami- initial material source. nation (Bristogianni et al. 2021a, b; Bristogianni et al. This research introduces the casting of volumetric 2020). This is attributed to the increased volume and glass components from glass waste as an alternative proportionally conﬁned surface area of such compo- glass-recycling approach, characterized by its ﬂexibil- nents that results in the majority of defects occurring ity to accommodate a variety of compositions and abil- in the bulk; such defects are rarely subjected to criti- ity to yield volumetric (solid or thick-walled) glass cal stresses and are thus, considered less critical than products that can tolerate higher contamination rates surface ﬂaws (Bristogianni et al. 2020). Due to the without a signiﬁcant compromise to their properties. kiln-casting method and the use of disposable plaster- The novelty of the proposed glass-to-glass recycling silica moulds it is also anticipated by the authors that method lies in the “recycling as-received” approach higher amounts of corrosive glass recipes (e.g. alumi- at relatively low forming temperatures (750–1200 °C) nosilicate) and metals can be tolerated in the kiln with- that reduces the required operational energy and CO out causing its damaging, as these substances largely emissions. Using impure, external glass cullet as a remain within the moulds. source for new glass components is a notable departure Moreover, glass casting is a versatile process that from the traditional approach of strict glass homogene- allows to switch between, or even mix, different glass ity. recipes in the same furnace and by the same moulds Addressing aspects of the value-chain barrier, to after every annealing cycle, without incurring produc- reduce the expensive, labour-intensive and logistically tion loss. In comparison, a change in the raw material complex purifying and separating techniques for the discarded glass, through the casting method it is intend 123 T. Bristogianni, F. Oikonomopoulou batch at the continuous ﬂoat lines can result in the dis- Figure 3 provides an overview of the different types card and loss of up to 7 days of production for success- of cullet included in this study, and how these are fully altering the material composition in the continu- structured in pre-consumer and post-consumer cate- ous ribbon, according to AGC Belgium. gories, and sub-organized into ﬂoat glass and other A broad variety of glass compositions (Scholtens glass types. The gathered experimental data concern 2019; Bristogianni et al. 2018; Yu et al. 2020; Bris- to a great extent unpublished experimental work by the togianni et al. 2020) and mixtures of different glass authors and gathered information from previously pub- compositions (Anagni 2019) have already been suc- lished work (Bristogianni et al. 2018, 2020, 2021a, b; cessfully recycled by kiln-casting at TU Delft (Fig. 2). Scholtens 2019;Yuetal. 2020) enriched with addi- Recycling via kiln-casting of different glass compo- tional experiments and images. Aim is to provide an sitions back to glass products, has been successfully integral picture on the recycling potential and chal- done by (Magna Glaskeramik), which makes cast glass lenges of each glass category. panes out of 100% recycled glass from industrial pro- For each melting experiment, the glass composi- duction plants and bottle production plants, and Spring tion (XRF analysis), type of contamination, cullet size, Pool Glass which produces high-ﬁre resistant, porous as well as the forming temperature followed, and the building blocks out of ﬂat-panel display (e-waste) type of expected defects are reported (Table 5). Further glass (glassOnline 2018); whereas (Snøhetta 2020) has on, the glass specimens are evaluated based on criteria recently developed glass tiles from microwave oven found relevant during the review analysis: glass. Numerous artists have also experimented with (i) Recycling ease This is linked with the required recycled glass and kiln-casting, such as Hanna Gib- temperature to form each glass. High tempera- son, who has made glass ﬁgures from a variety of glass tures, above 1200 °C, result in mould corrosion waste, incl. container glass, windscreens, mobile phone and often fracture of the glass due to its bond- screens and deposited artistic glass (20–21 Visual Arts ing to the mould. The recycling ease is ranked as Centre) and Tyra Oseng-Rees, who makes fused glass facile, moderate, difﬁcult and challenging, judg- architectural panels from locally sourced waste glass ing on the easiness of achieving an integral, glass bottles (Oseng-Rees and Donne 2015). component at the examined processing tempera- ture. (ii) Crystallization tendency Presence of certain 4 Review of experimental work based compounds may trigger the crystallization of the on the “recycling as-received” casting method melt. (iii) Colouration Colour shifts due to the presence of 4.1 Specimen selection, categorization contaminants are reported. and assessment criteria (iv) Stress induction The stress caused by the pres- ence of defects is characterized in 4 levels, based Based on the current glass recycling state analysis on their impact on the integrity of the compo- (Sect. 3), it is concluded that the experimental efforts nent. This stress can be catastrophic, weakening, on recycling by casting should be focused on pre- tolerated or not-detected. consumer and post-consumer types of glass waste, (v) Flexural strength (if available) The ﬂexural excluding the study of internal cullet, which currently strength values mentioned concern only a lim- is internally recycled in a closed-loop by each man- ited number of conducted tests, and therefore only ufacturer. Container glass is also excluded from this serve as an indication and not as a design value. study, due to its accomplished high closed-loop recy- cling rate in the EU. Extra attention is, however, to It should be noted that -given the aim to keep be given to pre- and post-consumer glass cullet types the forming temperature low for environmental ben- containing contaminants that currently classify them eﬁts, the viscosity of the melts during forming ranges 6 4 2 as non-recyclable, leading to the rejection of the par- between ≈10 –10 dPa·s, which is higher than the 10 ticular cullet streams by the industry. Purer glass cullet dPa·s forming viscosity typically used by the industry. samples will be mentioned in the study, as a point of As a result, the glass specimens do not fully homog- reference and comparison to the contaminated cases. enize, while particles of higher melting point (e.g. 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 2 Glass kiln-cast panels (350*350*10 mm) made at TU Delft from glass waste cullet, namely Cathode-Ray Tube (CRT) front screen (a), transition ﬂoat glass from clear to blue (b), CRT back screen and crystal coloured glass (c), enamel ﬂoat glass (d), automotive glass (e) and oven doors (f) Fig. 3 Overview of glass types included in this study 123 T. Bristogianni, F. Oikonomopoulou Table 5 Overview of experimental results Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) Pre-consumer Float glass/=soda Pure 75.4% SiO , 12.4% – Shards 1120, 10 Miniscule lime silica Na O, 7.6% CaO, bubbles, surface 4% MgO, 0.4% stones in Al O 0.09% reaction to the 2 3, Fe O mould 2 3 Pure, tinted 73.1% SiO , 12.8% – Shards 1120, 10 Miniscule bubbles Na O, 8.1% CaO, 4% MgO, 0.9% Al O , 0.76% 2 3 Fe O 2 3 Soft + hard coating 74.4% SiO , 12.5% Soft: ZnO based Shards 1120, 2 Bubble veils and Na O, 8.2% CaO, Hard: SnO subtle colour 2 2 3.9% MgO, based streaks (defects 0.55% Al O , decrease with 2 3 0.06% Fe O increased dwell 2 3 timeattop temperature) Mirror coating 73.5% SiO , 12.8%, Mirror coat: ZnO, Shards 1120, 10 Bubble veils, cord Na O, 8% CaO, BaO, TiO , and colour 2 2 4.3%, MgO, 0.9% Fe O streaks 2 3 Al O 0.08% 2 3, Fe O 2 3 Dichroic Standard ﬂoat Dichroic coat: Shards 1120, 6 Subtle colour ZnO, SnO , streaks, bubbles NiO Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) Fritted (black) 73.8% SiO , 12.6% Black frit: Bi O , Shards 1120, 10 Green streaks, 2 2 3 Na O, 7.7% CaO, Cr O ,CuO, dark green 2 2 3 4.5% MgO, 1.3% TiO crystalline ﬂakes Al O (Low-iron 2 3 ﬂoat) Fritted (white) White frit: TiO , Medium 1120, 10 Heavy cord, ZnO cullet bubbles Fritted (orange) 75.6% SiO2, 11.2% Orange frit: PbO, Shards 1120, 10 Minor orange Na2O, 8.3% CaO, TiO ,CdO,S streaks, bubbles 3.9% MgO, 0.8% Al O , 0.03% 2 3 Fe O (Low-iron 2 3 ﬂoat) Fritted (blue) Standard ﬂoat Blue frit: PbO, Shards 1120, 6 Blue streaks, dark Al O ,TiO , green crystalline 2 3 2 Co O ,Cr O ﬂakes 3 4 2 3 Wired 72.7% SiO , 12.4% Steel wires Shards 1120, 10 Total colouration Na O, 8.9% CaO, of the glass, 4.7% MgO, 0.7% metal wires Al O ,0.3%S, embedded (rust 2 3 0.1% Fe O develops over 2 3 time) T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) Boro-silicate Soda-borosilicate 80% SiO , 13% – Rods, 1120, 10 Bubble veils (DURAN), pure B O ,3.5% shards 2 3 Na O, 2.7% Rods Al O ,0.5%K O 2 3 2 Rods, shards C-ﬁber, pure 63.8% SiO ,5.5% – Pellets 1020, 10 Miniscule B O , 11.8% bubbles, cord at 2 3 Na O, 6.4% CaO, the top part of 5.2% Al O , the sample 2 3 3.7% MgO, 3.2% K O Alumino-silicate Alkali-aluminosilicate, 63.1% SiO , 13% – Shards 970, 10 Multiple bubbles pure Al O , 11.1% 2 3 K O, 6.2% MgO, 5.6% Na O, 0.8% ZrO Lithium 61.1-x% SiO , Coating: TiO Shards 1200, 2 Opaque 2 2, boroaluminosilicate, 28.7-y% Al O , Fe O crystallized 2 3 2 3 coated, chemically 4.1% Na O, 2.8% sample, coating strengthened MgO, 1.6% K O, appears in x+y %Li O+ patches B O not traced 2 3 by XRF Lead/Barium Coloured lead crystal 57.7% SiO , 28.7% Various colours Shards 820, 10 Colour streaks, silicate PbO, 9% K O, bubbles 3% Na O, 0.8% Sb O ,0.6%ZnO 2 3 Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) CRT screen 61.5% SiO ,8.1% – Shards 870, 10 Miniscule bubbles SrO, 8% BaO, 7.2% Na O, 6.8% K O, 3.6% ZrO , 2 2 2.3% Al O , 2 3 1.1% CaO Post-consumer Float glass/soda Float Combo Float variations: Float variations, Shards 1120, 6 Glass ceramic lime silica 71.7–72.8% SiO , LAS glass inclusions, 12.2–12.5% ceramics heavy cord, Na O, 8.7–9.3% colour streaks, CaO, 3.5–3.8% bubbles MgO, 0.6–2% Powder 1120, 6 High bubble Fe O3, 0.6–0.9% content, Al O 2 3 increased content of glass ceramic stones Float Metal Float variations Float variations, Shards 1120, 6 Metal inclusions, mirrors, wired colour streaks, glass, local inclusions aluminum of incompatible frames glued on glass ﬂoat, other compositions, glass bubbles compositions Powder 1120, 6 Small inclusions of metal and incompatible glass compositions, high bubble content T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) Oven Doors Low-iron tempered Black frit: Medium 1120, 10 Colour streaks, ﬂoat variations Cr O ,CuO, cullet ﬂat crystalline 2 3 PbO, White frit: inclusions TiO (surface stones when reacting with mould), cord, bubbles Refrigerator glass Float variations. Float variations, Fine cullet 1120, 6 Increased bubble Powdered mix: white frit, sand, content, small 66.5% SiO , 18% dust, stones stones, colour Na O, 9% CaO, streaks 4.6% MgO, 1% Al O , 0.27% 2 3 Fe O ,0.2%K O 2 3 2 Automotive glass Float variations Float variations, Fine cullet 1120, 10 Colour streaks, fritting, stones, ﬂat coatings, crystalline stones, sand inclusions, cord, and dust, PVB bubbles foil, cardboard Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) Boro-silicate Borosilicate mix Borosilicate Borosilicate Shards 1120, 10 Intense (Automated) variations variations, crystallization traces of plastic, formed at the cork, metal top surface, bubbles, heavy reaction with mould Borosilicate mix Borosilicate Borosilicate Flakes 1120, 10 High bubble (Manual) variations variations, glass content, small of other stones compositions, traces of plastic, cork, metal, sand, dust Microwave Variation of soda Glass from Shards 1120, 6 Opalescence, lime silica and variable crystallization, borosilicate. producers of reaction with Tested sample: borosilicate and mould, 81% SiO , 13% soda lime silica incompatible B O ,3.1% composition glass 2 3 Na O, 2.5% compositions Al O ,0.3%K O 2 3 2 Powder 1120, 6 High bubble (Borosilicate) content, inclusions of incompatible glass and opalescence, grinding contamination T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Glass chemical Contaminants Cullet size Forming Specimen image Occuring defects composition (%wt) and temperature processing ( C), dwell time (hr) E-ﬁber 55.1% SiO , 23% Not traceable by Powder 1120, 3 Very high bubble CaO, 13.5% optical content of Al O ,5%B O , inspection variable size 2 3 2 3 1.7% MgO, 0.5% TiO ,0.5%K O 2 2 Lead/Barium CRT screen & funnel Mixture of lead Glass from Coarse 870, 10 Heavily corded silicate silicate and different cullet glass, colour barium strontium producers streaks, bubbles, crystalline inclusions Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) Pre-consumer Float glass/=soda Pure Moderate Only minor Colour remains Not detected 43.7 4 PB,  lime silica crystallization as is 30*30*240 at the surface 47.7 4 PB,  20*30*350 Pure, tinted Moderate to Only minor Colour remains Not detected 62.9 4 PB,  facile crystallization as is 30*30*240 at the surface 63.5 4 PB,  20*30*350 Soft + hard coating Moderate Only minor Local colour Tolerated [3, 4] crystallization shifts of yellow at the surface hue where the coating dissolved Mirror coating Moderate Only minor Local colour Tolerated  crystallization shifts of blue at the surface hue where the coating dissolved Dichroic Moderate Only minor Local shifts in Not detected crystallization the colour at the surface intensity due to presence of different coatings T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) Fritted (black) Moderate Minor Black frit turns to Not detected 41.7 4 PB,  crystallization dark green 30*30*240 at the surface ﬂakes. Colour and in sacks of light particular at green surround the location of the crystalline the crystalline interfaces ﬂakes Fritted (white) Moderate to Minor White frit Tolerated facile crystallization becomes clear at the surface and in particular where the frit interacted with the mould Fritted (orange) Moderate Only minor Orange and Not detected crystallization brown frit at the surface considerably fades out Fritted (blue) Moderate Only minor Blue frit results Not detected crystallization into blue at the surface colour streaks and dark green crystalline ﬂakes Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) Wired Moderate Only minor The glass is Not possible to crystallization entirely observe due to at the surface coloured in the opacity of black from the the sample interaction with the metal wires Boro-silicate Soda-borosilicate Moderate to Only minor Clear, as Not detected 42.5–46.8 4 PB,  (DURAN), pure difﬁcult crystallization expected 30*30*240 at the surface 46.1 4 PB,  20*30*350 54 3 PB,  40*40*150 C-ﬁber, pure Facile Only minor Colour remains Tolerated 73.4 4 PB,  crystallization as is 20*30*350 at the surface Alumino-silicate Alkali-aluminosilicate, Moderate to Only minor Colour shifts to Not detected  pure difﬁcult crystallization light yellow at the surface Lithium Difﬁcult High level of Colouration Not detected boroaluminosilicate, crystallization (yellow, blue, coated, chemically both at surface black) due to strengthened and bulk unmolten coating Lead/Barium Coloured lead crystal Facile Almost none Colouration Not detected 35.3 4 PB,  silicate based on initial 30*30*240 cullet. Yellow, orange and red colours may shift to black T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) CRT screen Facile Almost none Colour remains Not detected 51.2 4 PB,  as is 30*30*240 Post-consumer Float glass/ soda Float Combo Challenging Crystallization Based on initial Catastrophic 46.5 (no 4PB,  lime silica of glass cullet (tinted, glass 30*30*240 ceramics, coated). Glass ceramics) minor surface ceramics turn crystallization to opaque white, beige, grey or light purple Challenging Crystallization Based on initial Weakening  of glass cullet (tinted, ceramics, coated). Glass minor surface ceramics turn crystallization to opaque white, beige, grey or light purple Glass up-casting: a review on the current challenges in glass recycling Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) Float Metal Challenging Only minor Based on initial Catastrophic  crystallization cullet (tinted, at the surface coated). Dark brown/green colour sacks around metal wires Challenging Only minor Based on initial Weakening  crystallization cullet (tinted, at the surface coated). Local dark brown/green colour sacks around metal traces Oven Doors Moderate Minor Black frit turns to Tolerated 37.5 4 PB,  crystallization dark green 30*30*240 at the surface ﬂakes. Colour and in sacks of light particular at green surround the location of the crystalline the crystalline interfaces ﬂakes Refrigerator glass Moderate to Only minor Local colour Tolerated facile crystallization streaks based at the surface on cullet contamination T. Bristogianni, F. Oikonomopoulou Table 5 (continued) Stage Category Cullet type Recycling Crystallization Colouration Stress Induction Flexural strength Source ease tendency at Value Test set up, tested (MPa) beam size temperature (mm) Automotive glass Moderate Minor Deep green Tolerated 41.1 4 PB,  crystallization colouration. 30*30*240 at the surface Black frit turns 30.1 4 PB,  and in to dark green 20*30*350 particular at ﬂakes the location of the crystalline ﬂakes Boro-silicate Borosilicate mix Challenging Intense White opaque Catastrophic  (Automated) crystallization sections due to crystallization Borosilicate mix Moderate Only minor Light aquamarine Tolerated 66.9 4 PB,  (Manual) crystallization 30*30*240 at the surface Microwave Challenging Moderate White Catastrophic crystallization opalescence at the surface Challenging Only minor Amber Weakening crystallization colouration due at the surface to grinding contamination. Local white opalescence E-ﬁber Difﬁcult Only minor Light olive green Not detected crystallization at the surface Lead/Barium CRT screen & funnel Facile Almost none Colour remains Tolerated 33.3 4 PB,  silicate as is 30*30*240 Sources:  Bristogianni et al. 2020,  Bristogianni et al. 2021,  Yu et al. 2020,  Bristogianni et al. 2021b,  Scholtens 2019,  Bristogianni et al. 2018  Matskidou 2022 Glass up-casting: a review on the current challenges in glass recycling stones, glass ceramics, steel) will remain entirely or develop crystallization at the cullet interfaces (Fig. 4b). partially intact within the glass network. When such crystalline formations are exposed to the A key selection of melting experiments is presented surface of maximum tensile stress (e.g. bottom cen- in Table 5. Analysis of this experimental work, as well tral region of a beam specimen during bending), they as complimentary experiments conducted at lower tem- will signiﬁcantly reduce the ﬂexural strength down to peratures that reveal additional transformation stages 25.6 MPa. However, if these formations are only situ- of the glass cullet and its contaminants, are provided in ated in the bulk, the glass sample will have a compara- Sects. 4.2 and 4.3. ble ﬂexural strength to that of a homogeneous compo- nent (Bristogianni et al. 2020, 2021a, b). 4.2 Experimental work on pre-consumer glass waste Coated ﬂoat Coatings typically consist of metal Studies on pre-consumer glass waste allow the isola- oxides that are either applied during the ﬂoat line pro- tion of each glass composition as well as the studying cess by pyrolysis (e.g. hard coatings) or off-line by of a limited number of associated contaminants (e.g. magnetron sputtering (e.g. soft-coatings). Float glass single ﬂoat glass pane containing one type of coat- samples with soft, hard, dichroic and mirror coatings ing). In this manner, straightforward information can have been successfully kiln cast at 1120 °C. Soft (ZnO be obtained on the reaction of each glass composition based) and hard (SnO based) coatings burn off dur- and each contaminant to different ﬁring schedules, and ing this forming process, and even if their original how the occurring casting defects affect the strength colour is dark, this is not reﬂected in the ﬁnal com- and aesthetics of each recycled component. ponent. Traces of these coatings may appear as sub- tle bubble veils, however a fast forming and cool- 4.2.1 Float Glass (Soda lime silica) ing ﬁring schedule may intensify their appearance (Fig. 5a). Considering the very low thickness ratio Pure Kiln-casting experiments employing pure clear between these coatings (only a few μm) and the coated ﬂoat, annealed or fully tempered glass, conducted at glass (typically 6–15 mm), in combination with their 50°–100 °C above the liquidus point (T corresponds burning/evaporation at higher temperatures, minimum to ≈ 10 dPa·s viscosity) result in relatively homoge- impact is expected at the mechanical and physical prop- neous components, with a minimum amount of gaseous erties of the recycled components. Indeed, Yu et al. inclusions under prolonged dwell times at top temper- (2020) experimentally showed that kiln-cast glass out ature (e.g. 4–10 h based on specimen size) are imposed of soft and hard coated ﬂoat cullet had similar strength, (Fig. 4a). The liquidus point varies according to the stiffness and CTE to standard ﬂoat cullet recycled with exact chemical composition and has been estimated at the same method. Dichroic (ZnO, SnO and NiO based) a temperature range of 1045–1080 °C for a selection of coatings dissolve in the glass melt in a similar fashion tested ﬂoat samples. Tinted ﬂoat glass, due to its darker at 1120 °C, but remain prominent at lower process- colour (higher Fe O content) heats up and cools down ing temperatures, e.g. 800–850 °C, although thinning 2 3 in a faster rate than clear ﬂoat, and even if subjected of the coating layer is observed (Fig. 5d–f). Mirrors to the same ﬁring schedule, it may acquire an altered (ZnO, BaO, TiO and Fe O based) kiln-cast with the 2 2 3 thermal history which will be reﬂected in the structure same ﬁring schedule, may slightly colour the specimen of its glass network. Small alterations in the chem- according to the type of protective paint layer used, ical composition and thermal history between ﬂoat while pronounced bubble veils and cord (glassy inclu- glass samples (pure and tinted) will result in notice- sion of different composition from the glass matrix) able differences in strength. As an example, recycled will appear in the glass, at the prior position of the beams kiln-cast from pure fully-tempered ﬂoat shards coating (Fig. 5b). At lower temperatures, e.g. 970 °C, at 1120 °C presented an average ﬂexural strength of the mirror coating and its protective exterior paint will 45.7 MPa, while beams from dark blue ﬂoat produced promote the crystallization of the glass, most probably using the same ﬁring schedule showed an average ﬂex- due to the TiO content, which is a typical nucleating ural strength of 63.3 MPa. Samples kiln-cast at 970 °C agent (Fig. 5c). 123 T. Bristogianni, F. Oikonomopoulou Fig. 4 a 50 mm cubic sample kiln-cast at 1120 °C for 2 h from standard ﬂoat glass panes. The limited dwell time at forming temperature resulted to the insufﬁcient removal of bubbles and yellow colour streaks (caused by volatile components in the composition, see red arrow) at the top section of the sample. b Crystalline interfaces (red arrow) within a glass sample kiln-cast at 970 °C for 10 h from standard ﬂoat glass panes, as seen through crossed polarized light Fritted ﬂoat Fritted or enamelled ﬂoat is produced and the TiO content will contribute to the crystalliza- by the deposition and thereafter fusion of a contin- tion of the sample (Fig. 6a). Orange/brown fritted ﬂoat uous or patterned frit layer on a ﬂoat glass surface. kiln-cast at 850 °C for 8 h shows thinning of the coat- The resulting fritted layer can be 15–350 μm thick ing layer, yet the colours remain evident (Fig. 6c). At according to its application process and is composed 1120 °C the ceramic layer (based on PbO, TiO ,CdO of a mixture of glass and ceramic particles (Wilson and S) is almost entirely vanished and only local yel- and Elstner 2018). In terms of chemical composition, low colour streaks appear in the glass sample (Fig. 6d). thickness and colour intensity, these ceramic layers are Darker-fritted samples on the other hand are persistent much more prominent than the previously described at 1120 °C, and the recycled glass samples will con- metal oxide coatings. Testing of various enamelled ﬂoat tain intense colour streaks and incorporated dark-green samples at a temperature range of 850–1120 °C shows crystalline ﬂakes in their mass (Fig. 6e). The crystalline distinct transformations according to the present chem- ﬂakes are attributed to the presence of Cr O that has an 2 3 ical compounds and ﬁring schedule. More speciﬁcally, exceptionally high melting point (2435 °C). Blue frit- the colouration of a white frit based on TiO and ZnO ted ﬂoat recycled at the same temperature shows mainly seems to vanish in samples formed at 1120 °C, how- blue streaks associated with Co O and given that the 3 4 ever evident cord is observed throughout the entire Cr O content is minimal, only local traces of crys- 2 3 volume, at the prior position of the glass cullet inter- talline ﬂakes are observed (Fig. 6f). On the other hand, faces (Fig. 6b). This cord is related to the composi- the black fritted ﬂoat of higher Cr O content results 2 3 tional variations occurring to the cullet interfaces dur- in less green streaks (less frit gets dissolved), while the ing fusion and melting, due to the volatility of ZnO and Cr O based ceramic layer remains to a great extent 2 3 the incorporation of TiO in the melt. Upon inspec- intact in shape, although thinning is observed together tion of the specimens through crossed-polarized ﬁl- with a colour shift from black to dark green. Overall, all ters, a bottom zone of pronounced isochromatic fringes observed defects are tolerated by the glass network and can be observed, in contrast to the remaining speci- do not induce critical stresses. However, structural test- men volume. The addition of titanium oxide in soda ing showed that extended zones of crystalline interfaces lime glasses increases the refractive index of the glass resulting from the recycling of full coated ﬂoat glass (Karlsson et al. 2016), and therefore can be associated surfaces with black enamel can reduce the strength if with the isochromatic fringes detected during exam- exposed at the glass surface and subjected to a high ination under polarized light. At 970 °C, the white tensile stress. The reported average ﬂexural strength colouration of the frit layer will remain in the glass, (41.7 MPa) is lower than the strength of purer soda 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 5 a 50 mm cubic specimen out of soft-coated ﬂoat panes, kiln-cast at 1120 °C for 2 h; The insufﬁcient dwell time at top temperature resulted in distinct vertical bubble veils of yellow hue (red arrow) that resemble the vertical manner with which the glass panes were positioned inside the mould prior to ﬁring. b Recycled mirrors at 1120 °C for 10 h; the coating and protective paint layer of the mirrors dissolved, resulting in cord (red arrow), bubbles and coloration of the glass. c recycled mirrors at 970 °C; the paint and coating reacted with the investment mould, resulting in a crystallized surface of yellow hue. d Deposition of dichroic ﬂoat shards in a 100*100 mm investment mould. e The fusion of dichroic ﬂoat shards for 8 h at 850 °C caused thinning and colour alterations to the original coating. f The dichroic coating is not detectable when the sample (100*100*10 mm) is ﬁred at 1120 °C for 6 h lime specimens, and fracture initiates at a glass zone soda lime glass with 0.4% Fe O and 0.2% S content. 2 3 adjacent to such a crystalline interface. Considering the initial 0.1% Fe O and 0.3% S con- 2 3 tent in the original ﬂoat glass, it can be seen that the increase in iron oxide content is related to the diffusion Wired ﬂoat Wired glass comprises a steel wire of iron from the steel wires to the glass. Moreover, car- embedded within a soda lime silica glass (ﬂoat glass bon released from the wires acts as a strong reducing composition) during forming. The separation of the agent during melting. Iron interacts with the sulphur two materials is particularly challenging. Kiln-casting found in the ﬂoat glass melt and, due to the reduc- of the composite cullet is however possible, resulting in ing conditions, forms a ferric-sulphide chromophore, integral components. Specimens produced at 1120 °C which produces a deep amber colour even in small turn black (dark amber) and non-light transmitting at amounts (Schreurs and Brill 1984, Hubert et al. 2017, thicknesses above ≈10 mm due to the partial diffusion Paynter and Jackson 2018). Apart from the surface dif- of the outer surface of the steel wires into the glass fusion actions, the majority of the wires’ cross section (Fig. 7a). The XRF analysis on the black glass shows a 123 T. Bristogianni, F. Oikonomopoulou Fig. 6 a sample kiln-cast at 970 °C for 10 h using low-iron ﬂoat shards fully coated with white enamel; crystallization has developed at the coated interfaces (red arrow). b Inspection under polarized light of a low-iron ﬂoat white enamel recycled sample at 1120 °C for 10 h; compositional variations in the glass due to the melting of the enamel manifest as cord (red arrow) and isochromatic fringes (bottom part of the glass). c Kiln-cast sample using ﬂoat shards with orange/brown frit at 850 °C for 8 h. d Upon kiln-casting at 1120 °C for 10 h, only minor colour streaks (red arrow) will remain from the orange/brown frit. e Black enamel will remain prominent at 1120 °C (10 h), in the form of green crystalline ﬂakes (red arrow); only minor green streaks will appear around the ﬂakes. f Blue frit will mainly dissolve into blue colour streaks (red arrow) when ﬁred at 1120 °C for 6 h; small crystalline ﬂakes of green colour (yellow arrow) will appear if Cr O is present in the frit 2 3 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 7 a 50 mm cubic specimen kiln-cast at 1120 °C (10 h) out of wired ﬂoat glass. b Microscope image showing a ground surface from the cubic specimen; cracking is generated during post-processing along the metal wire (red arrow), indicating the susceptibility of the glass at this location is still present within the glass, without causing fracture content of bubbles will appear in the samples. These to the surrounding network. Grinding and polishing of bubbles are often structured in bubble veils (Fig. 8a, the glass surface exposes these steel threads and water b), the amount of which corresponds to the initial num- causes their rusting. The exposed wires further dam- ber of interfaces between the cullet pieces, and the age the polishing disks, and as a consequence, deep arrangement of the cullet inside the mould. As an exam- scratches may appear on the machined glass surface ple, beams produced by vertically placing solid rods (Fig. 7b). Although the wires are tolerated within the (ø24 mm) inside the mould resulted in signiﬁcantly less glass, it is expected that their role is weakening, given bubbles than beams where 6–8 mm shards were hori- the difference in thermal expansion coefﬁcient between zontally stacked in the mould, preventing the escape of the two materials. Studies in literature on wired glass gases during forming. Bubble veils exposed to the sur- report reduced strength over standard ﬂoat, due to the face of a structural component can be strength reducing induced stresses at the interface of the two materials if this surface experiences tensile stress. The reported (Mai et al. 1982; Perova et al. 1983). average ﬂexural strength of soda borosilicate kiln-cast at 1120 °C ranges from 42.5 to 54 MPa, according to the beam size and bubble content exposed at the sur- 4.2.2 Other glass types face. Specimens kiln-cast at 1070 °C presented exten- sive bubble veils at the surface of the specimens, which Soda borosilicate (pure) Soda borosilicate, and reduced their strength down to 30 MPa. Although speciﬁcally the chemical composition of Schott the risk of crystallization is low when kiln-casting at ® ® ® DURAN and Corning Pyrex 7740, is the most temperatures above 1070 °C, crystalline structures are common borosilicate type, widely applicable in every- formed at the cullet interfaces of samples produced at day glass products that require resistance to ther- lower temperatures, like 970 °C (Fig. 8c). These crys- mal shock and chemical corrosion (e.g. oven-trays, tallized interfaces are found to be a major weakening microwave turntables, laboratory tubes, pharmaceuti- point when exposed to the surface, reducing the ﬂexural cal vials). This type of glass can be kiln-cast at 1120 °C strength of the glass to 12.4 MPa. into a fairly homogeneous glass, however, given its 4.5 higher viscosity than soda lime silica glass (10 dPa·s 3.5 vs. 10 dPa·s for the speciﬁc temperature), a higher 123 T. Bristogianni, F. Oikonomopoulou Fig. 8 a 30*30*240 mm borosilicate specimen kiln-cast at 1120 °C (10 h); multiple bubble veils (red arrow) appear in the bulk, as reminiscence of the shape of the borosilicate shards and how they were placed inside the mould. b microscope view of the bubble veils. c 50 mm cubic specimen produced at 970 °C (10 h) using borosilicate rods that had been geometrically stacked inside the mould; white crystalline interfaces develop along the rods as they connect to each other with heat Fig. 9 a Polarized view of a kiln-cast C-ﬁber glass specimen at 1020 °C; cord and isochromatic fringes appear at the top part of the glass specimen, as a result of local compositional variation occurring by volatile components. b Crystals forming along the cullet interfaces, in a kiln-cast C-ﬁber glass component at 820 °C 123 Glass up-casting: a review on the current challenges in glass recycling C-ﬁber glass (pure) C-ﬁber glass is a modiﬁed soda- starts only above 1200 °C. At this range, the specimens lime silicate, containing ≈5% of B O ,5%Al O and fully crystallize due to the presence of ceramic crys- 2 3 2 3 3% K O. These additions to the main composition con- tals. If opaque coatings are present, they do not burn tribute to a stronger yet more ﬂexible glass network, and off entirely at these temperatures (Fig. 10b). In fact, reduce the viscosity of the melt. The glass can be kiln- given the increased coating to thin glass thickness ratio cast into highly homogeneous components at 1020 °C, (compared to that of coated ﬂoat glass), the coating however the top surface may exhibit cord due to the residue upon kiln-casting has a prominent role within volatilization of boron and alkali elements at this zone the recycled component. Higher processing tempera- (Fig. 9a), and should be cut off upon post-processing. tures can lead to glass samples with a lower degree of The tested specimens showed a higher ﬂexural strength coating contamination, however research is required on in comparison to soda-lime silica glass, namely an aver- mould materials that are resistant to corrosion and can age of 73.4 MPa. Samples kiln-cast at 820 °C presented allow for the separation of the glass object from the crystallization along the cullet interfaces (Fig. 9b), and mould upon cooling. had a reduced strength of 52.2 MPa. Lead-barium silicate Lead and/or barium silicate Aluminosilicate The recycling of aluminosilicate glasses facilitate the recycling by casting process, as glass compositions employed in thin cover glass appli- these compositions are characterized by lower viscosi- cations on liquid crystal screens (LCD) has been eval- ties, based on their PbO/Ba content. They can be kiln- uated. Such compositions, as seen in Table 1, typically cast at a lower temperature range (≈750–900 °C) and have a considerably higher softening and melting point are resistant to crystallization (Bristogianni et al. 2018; than the other listed glass families, requiring therefore Kosmal et al. 2017). Lead silicate glass originating higher processing temperatures. Nonetheless, the kiln- from the cullet rejection stream of glass art foundries (≈ casting of chemically strengthened thin glass, and in 25–30% PbO content), if contaminated with colorants, particular of an alkali rich composition (16.7% con- cannot be reintroduced in the furnace and is disposed tent) with a moderate alumina content (13%) was found as chemical waste due to its lead content. This stream, possible at a relatively low temperature (970 °C). The however, is a favourable source for producing uniquely sample resulted in a transparent glass of yellow hue and patterned and pristine glass elements for architectural increased amount of bubbles due to the high viscos- glass applications. Attention should be given however ity of the glass at the chosen processing temperature to colour instabilities, especially above 1000 °C, when (Fig. 10a). Given however the corrosive character of melting lead glass under reducing conditions (e.g. from aluminosilicate melts, castings at higher temperatures organic contamination in the cullet or kiln atmosphere) (e.g. 1250 °C) were proven challenging when standard (Inano et al. 2018). Reducing atmosphere results in silica-based investment moulds were employed, as cor- the generation of elemental Pb already below 700 °C, rosion of the later would readily occur. The recycling and segregation of the metal from the glass melt as the of new generation smartphone screens imposes, how- viscosity reduces (Inano et al. 2018;Bartuška 2008). ever, additional challenges, apart from the high melting Consequently, small lead particles are dispersed in the temperature and corrosive character of aluminosilicate glass or even settled at the bottom of the sample, shift- glasses. Chemically strengthened lithium boroalumi- ing its colour into grey. It is also found that yellow or nosilicate glasses with embedded nano-ceramic crys- orange coated lead glass will turn to black upon remelt- tals or glass ceramic reinforcement are developed for ing (Fig. 11a, b). This is associated to the reaction of increased drop and scratch resistance (SCHOTT 2022, lead with the Cadmium Sulﬁde (CdS) and Cadmium Apple Inc. 2021). Similar technology is applied for Selenide (CdSe) colourants. Other colours (e.g. blue, the back-cover of smartphone devices, with the glass green) have been found stable at forming temperatures being additionally treated with an opaque, metal oxide such as 870 °C (Fig. 11c). based coating. Experimental testing with such samples Another common source of lead silicate and barium suggests multiple difﬁculties in achieving a transparent silicate glass waste is the Cathode Ray Tubes (CRT) glass by kiln-casting, at temperatures below ≈1300 °C. that are not produced anymore and thus their closed- Due to the high alumina (> 20%) and low alkali con- loop recycling is not an option. Lead glass (≈25% tent (< 10%) of such recipes, full fusion of the cullet PbO) used for the funnel part of Cathode Ray Tubes 123 T. Bristogianni, F. Oikonomopoulou Fig. 10 a Recycling of an alkali-rich aluminosilicate at 970 °C. b Coating residue (yellow colour) in a crystallized low-alkali alumi- nosilicate sample produced at 1200 °C has an inner coating of iron oxide and an outer coating 4.3 Experimental work on post-consumer glass waste of graphite (Méar et al. 2006). The graphite coating (grey black in colour) is loosely applied at the glass This section concerns the experimental work conducted surface, can be partially removed with isopropanol, on as-received glass waste that was sorted by recycling and is not traceable upon its kiln-casting at 870 °C. companies in The Netherlands and Belgium. The tested At the same temperature, the iron coating (dark red) samples give an overview of common external contam- thins down if situated at the bulk of the glass, but will inants that can be expected in each type of glass, as a retain its shape if in contact with the investment mould result of the method used for their collection and sort- (Fig. 11d). The traced coating at the bottom surface can ing. As an example, a much more variable mixture of be easily removed by post-processing the ﬁnal glass glass products is expected to be found in a construc- component. The front part of the CRT consists of a tion and demolition glass container (e.g. windows with barium-strontium silicate of dark grey colour. coatings and adhered frames, mirrors, ceramic panels) Flexural strength testing on pure lead glass (28.7% than an automotive glass container. Sorting by product, PbO, coloured) and pure Ba/Sr CRT glass showed an as for example encountered in the recycling of elec- average of 35.3 MPa and 51.2 MPa respectively. The tronics (e.g. microwaves, television screens), could in low strength of lead glass can be explained by the low theory result in the sorting of the same type of glass dissociation energy and packing density of its glass net- per product, however, cheaper glass replacement parts work ( et al. 2020). This adds on the susceptibility of of other than the original glass recipe often are used lead glass towards scratching (Ainsworth 1954). Ba/Sr for the repair of electronic devices, and complicate the silicate glass is a more favourable choice in this sense recycling process. Manual versus automated separation for structural glass applications. The strength and E of the glass can have a large impact on the cullet qual- modulus of this glass type is found to be comparable ity. For example, by manual separation of the glass as to this of soda lime silica. Yet, its lower forming tem- followed in the sorting of electronic waste (e.g. at Cool- perature and resistance to devitriﬁcation beneﬁts the rec), stones, clay and organic materials often encoun- production process of kiln-casting. tered in the cullet of automated sorting, can be avoided. However, the presence of different glass compositions (due to cheaper alternatives as described above) cannot be detected. Metal or plastic parts adhered on the glass 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 11 Lead silicate shards with orange and yellow coating prior a and after b ﬁring at 1200 °C for 10 min; the orange/yellow colour turns into black. c Kiln-cast lead silicate specimen at 870 °C; blue, pink and white colours remained stable, while black colour streaks correspond to previously introduced orange cullet. d Undissolved iron oxide coating on a specimen produced at 870 °C using lead silicate cullet from the funnel of a CRT screen are also commonly found in such samples as a result 4.3.1 Float glass (Soda lime silica) of improper separation of the glass from the device. The automated sorting process can avoid metals, CSP Float combo These cullet samples consist of a com- contamination of above 3 mm size, and foreign to soda bination of transparent (clear and tinted) shards of ﬂat lime silica glass compositions, but this results in a con- glass. More speciﬁcally, variable yet compatible soda siderable quantity of shoot-out glass. The utilization lime silica ﬂoat glass compositions are encountered, of such shoot-out qualities for kiln-casting is also dis- including tinted ﬂoat compositions, and ﬂoat with coat- cussed below. ings. These samples are also heavily contaminated by Lithium Aluminosilicate (LAS) glass ceramics of 123 T. Bristogianni, F. Oikonomopoulou Fig. 12 a A distinct volume of white (light transparent yellow prior to heating) glass ceramic (red arrow) within the glass volume of a ﬂoat combo sample recycled at 1120 °C, is the cause of catastrophic fracture of the sample. b Powdered ﬂoat combo powder recycled at 1120 °C; the sample fails to homogenize at this temperature and multiple glass ceramic inclusions (red arrow) spread throughout the sample causing micro-cracks to the glass network. Sample prepared by Matskidou (2022) transparent yellow or brown colour. If the LAS cul- glass) were manually removed. The kiln-casting of the let pieces are manually sorted out of the sample, then sorted glass shard sample at 1120 °C cracked at sev- the kiln-casting of integral glass pieces is possible at eral locations upon cooling (Fig. 13a), yet the fracture 1120 °C. These samples are heavily corded and char- origins were not associated with the presence of metal acterized by random colour streaks, yet their ﬂexural parts from the wired glass, but with the inclusion of strength (46.5 MPa) can be comparable to the lower other than soda lime silica glass types in the sample (e.g. end of strength range found for recycled kiln-cast spec- borosilicate, see Fig. 13b). Grinding down the shards imens using pure ﬂoat glass cullet. The kiln-casting into ﬁne cullet did not prevent the occurrence of cracks. of the original cullet sample at the same temperature Powdering the sample, resulted in an integral kiln-cast is not possible, as the presence of the glass ceramics component, however inspection under the microscope leads to the immediate shuttering of the sample upon and polarized light revealed localized stress and associ- cooling (Fig. 12a). Processing of the sample into ﬁne ated micro-cracks (Fig. 13c). Moreover, the powdered cullet (e.g. 1–3 mm) or powder may lead to seemingly cullet sample also resulted to an excessive amount of intact kiln-cast components, however inspection under bubbles, which are weakening the glass sample. the microscope reveals the presence of multiple glass ceramic inclusions, which cause micro-cracks to the Oven doors This cullet stream consists of manu- surrounding glass network (Fig. 12b). The original cul- ally sorted low-iron tempered ﬂoat glass (from var- let is therefore unsuitable for kiln-casting at the exam- ious producers) containing white/black frit, originat- ined temperatures. ing from the glass doors of domestic ovens. Upon kiln-casting of the cullet at 1120 °C, the white frit Float metal The “Float Metal” cullet quality is the (TiO based) will dissolve in the melt, while the black result of the shoot-out process from the metal separator (Cr O , CuO, PbO based) will turn into dark green 2 3 sorter. The sample may include ﬂoat glass shards with crystalline ﬂakes of eskolaite (Cr O ) that are embed- 2 3 mirror coatings, wired glass, ﬂoat glass with adhesively ded in the glass. The crystalline ﬂakes are often sur- bonded metal components, lose metal parts, and glass rounded by green colour streaks, showing the partial of different compositions. For the recycling by kiln- dissolution of the frit (Fig. 14a, b). Frequent cord is casting experiments, metal parts (lose or attached on encountered as well, as a result of the kiln-casting of 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 13 Kiln-cast specimens at 1120 °C using ﬂoat metal cullet (a, b) and powder (c), prepared by Matskidou (2022). Both specimens cracked due to the inclusion of foreign glass Fig. 14 Oven door glass specimens kiln-cast at 1120 °C. The black frit results in crystalline ﬂakes of eskolaite (a, red arrow), which are often surrounded by green colour streaks. Variations in the black frit used between manufacturers may lead to blue or black colour streaks as well (b) various ﬂoat glass compositions, and the dissolution of Refrigerator glass This sample is in the form of ﬁne the white frit. The crystalline ﬂakes are tolerable when cullet of ﬂoat glass. White fritting is often encoun- situated in the bulk, but may react with the investment tered on the glass surface. Stones, sand and dirt are also mould during kiln-casting resulting to surface stones found in the sample. The resulting kiln-cast specimens that reduce the strength of the component. The aver- at 1120 °C present colour streaks, multiple bubbles and age ﬂexural strength of such samples was found at several stones (< 3 mm) that do not cause damage to 37.5 MPa, approximately 18% lower than the strength the glass, unless situated at the top surface that shrinks of pure ﬂoat kiln-cast samples at the same temperature freely during cooling (Fig. 15 a, b). The glass mixture (45.7 MPa). seems to be more resistant to crystallization than typi- cal ﬂoat glass, as almost no traces of crystalline patterns are found at the top surface of the glass component. 123 T. Bristogianni, F. Oikonomopoulou Fig. 15 Kiln-cast specimen at 1120 °C using refrigerator glass cullet. Colour streaks (read arrow), bubbles, and stones (blue arrow) are present in the glass (a). The stones at the surface may induce localized micro-cracks at the surrounding glass (b, polarized view) Fig. 16 Kiln-cast specimens at 1120 °C using automotive glass. a Micro-cracks in the glass extend from the periphery of the stone (red arrow), upon polishing of the sample. b Stone in the bulk of the glass, surrounded by a green dissolution sack and clustering of bubbles; no cracks are observed in this case Automotive glass Car windshield glass waste com- the frit) ﬂoating within the glass network (Fig. 16a, prises ﬂoat glass from various producers, often tinted or b). Especially stones, often as big as 3–4 mm, are coated, and is ground in ﬁne cullet form. It is consider- responsible for the weakening of this recycled glass. ably contaminated by stones, sand, dust, PVB foil, and When situated close or at the surface of the compo- metal traces, which are difﬁcult to manually remove nent, they can easily lead to cracking during post- given the small size of the cullet. The resulting glass, processing or loading. The ﬂexural strength was found when kiln-cast at 1120 °C has a dark green colour, with to be much lower than other tested soda lime silica several stones and crystalline ﬂakes (resulting from samples, at 41.4 MPa for small beams (30*30*240 mm) 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 17 a, b The recycling at 1120 °C of borosilicate glass cullet that was automatically sorted was not successful due to the contamination of the sample by foreign glass. c Borosilicate cullet manually sorted had a much higher quality and lead to a good quality glass when kiln-cast at 1120 °C (10 h); minor inclusions (red arrows) are tolerable and at 30.1 MPa when the sample’s size would increase with minor dirt, organic residue, and burning stains (20*30*350 mm) and thus more defects were present at on their surface. The breaking of the pieces in small the surface. Cracks originating from stones embedded shards and kiln-casting at 1120 °C results in a sam- in the bulk were not observed. ple with multiple cracks upon cooling, local opales- cence and local surface crystallization from the glass- mould reaction (Fig. 18a). Opalescence, which exhibits 4.3.2 Other glass types a white blue tint in reﬂective light and orange through transitive light (Fig. 18b) suggests the phase separa- Borosilicate mixture Two different samples of tion occurring when two glasses of different refractive borosilicate cullet were evaluated, one originating from index are mixed (Shelby 2015). XRD analysis on the an automated sorting process and one from a man- kiln-cast sample revealed the presence of cristobalite, ual process. Both waste samples were kiln-cast at but no other traces of crystalline material such as glass 1120 °C. The automated-sorting samples presented ceramics. The cracking is therefore entirely attributed catastrophic cracking and intense crystallization upon to the intermixing of soda lime silica and borosilicate cooling (Fig. 17a, b). This is attributed to the presence glass. Grinding the shards into cullet (3–6 mm) resulted of soda lime silicate contamination in the cullet. The in a seemingly integral piece, however, cracks in the manually sorted glass cullet included traces of metal, form of knots and local opalescence were also observed cork and plastic contamination. The resulting sample (Fig. 18c). Powdering the shards and kiln-casting at the was quite homogeneous with small inclusions of metal same temperature showed improved results, however in the bulk (Fig. 17c), and showed a relatively high ﬂex- still small cracks and opalescence occurred, together ural strength (66.9 MPa), however due to the limited with a high bubble content (Fig. 18d), rendering all number of tested samples, this result is not conclusive. attempts of recycling of this glass stream unsuccessful. Microwave turntable glass Microwave turntables are typically made by an alkali-borosilicate composition. E-ﬁber glass E-ﬁbers are low in alkali and have However, in case of failure, consumers often replace the a much higher content of CaO and Al O than C- 2 3 original turntables with cheaper alternatives, which are ﬁbers, which make their recycling by casting more usually made from soda lime silica glass. This results in difﬁcult due to the increase in the viscosity of the the contamination of this particular glass waste stream, melt and higher tendency of crystallization. Post- and the two types of glass cannot be sorted manually. consumer ﬁbers, are expected to be contaminated with The received samples contain big shards of glass turnta- organofunctional silane coatings (Thomason 2019). bles of both compositions (cannot be distinguished), The tested sample was provided in ﬁne-powdered form 123 T. Bristogianni, F. Oikonomopoulou Fig. 18 a 50 mm cubic kiln-cast specimen at 1120 °C (10 h) using microwave glass shards; cracking, white opalescence and surface crystallization are detected. b Inspection of an opalescent zone of the previously described sample under polarized light; excessive stress concentration is indicated by the isochromatic fringes. c Kiln-cast specimen at 1120 °C (10 h) using microwave ﬁne cullet; small semi-circular cracks (red arrow) and localized opalescence occur in the sample. d: Inspection of a knot under polarized light; the observed cracks are linked with the stress manifested by the isochromatic fringes so contamination by metal, stones or other materials in size (17–650 μm, increasing from bottom to top, was not traceable by optical inspection. Kiln-casting at see Fig. 19b). Higher processing temperatures and/or 1120 °C of 50 mm cubic samples resulted in an exces- the addition of ﬂux compounds are required to achieve sive amount of bubbles and corrosion of the invest- more homogeneous components. ment mould that would lead to cracking of the glass. Reinforcement of the mould with kiln-wash (a paste of Lead/Barium silicate This sample originates from the manual sorting of CRT screens from different man- alumina, kaolin and silica, Bullseye Glass Co. 2000) and kiln-casting of thinner elements resulted in inte- ufacturers. Both the funnel (Lead containing) and panel (Barium Strontium containing) glass are included in gral components, of almost opaque green colour due to the increased amount of bubbles that diffused the light the cullet. Samples of this cullet kiln-cast at 870 °C were heavily corded due to the compositional variations by multiple reﬂection (Fig. 19a). The bubbles varied and presented multiple crystalline inclusions of black 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 19 a, b Recycling of E-ﬁber glass powder at 1120 °C resulted in an almost opaque green sample due to the extensive population of bubbles Fig. 20 Kiln-cast beam from manually sorted CRT screens, prepared at 870 °C (10 h) colour, possibly from the reduction of PbO (Fig. 20). size, quantity, location (bulk vs. surface), and combi- One beam was tested in ﬂexure, reporting a low ﬂexural nation of defects further determines the quality grade strength of 33.3 MPa. of the component (Fig. 21). More speciﬁcally, starting from the cullet character- istics, these are determined by the chemical composi- 5 Discussion tion of the glass, its size and its contaminants: 5.1 Quality of recycled components and the effect • Chemical composition: The chemical composition of contamination deﬁnes the recycling ease and glass network strength. Recycling is facilitated when glasses of lower- The melting experiments on various types of glass viscosity are employed, such as lead silicates, Ba/Sr waste reveal a broad pallet of recycled glass qualities. silicates and C-Fiber glass, and lower process- The cullet characteristics and thermal history are the ing temperatures can be used (e.g. 820–1020 °C). main parameters affecting the properties of the result- Glasses containing a high amount of aluminium ing kiln-cast glass and occurring defects. The occur- oxide (e.g. Aluminosilicates, E-ﬁbers) and glasses ring defects can be classiﬁed in stress inducing and with low alkali content (e.g. borosilicates) have a strength reducing, and their impact on the glass com- higher viscosity that makes their recycling more ponent can be tolerable, weakening or catastrophic. The difﬁcult (Fig. 22). Crystallization can be prevented 123 T. Bristogianni, F. Oikonomopoulou Fig. 21 Parameters affecting the quality grade of the recycled kiln-cast glass components when the glasses are processed at a temperature • Contamination contaminants can be categorized into above their liquidus point. For typical ﬂoat glass embedded and external. Embedded contamination compositions this was found to be around 1060 °C, refers to infeasible or strenuous to remove ele- while for C-glass, lead and Ba/Sr silicates was even ments (e.g. coatings, fritting, wired glass, adhe- lower. In high-viscosity melts, however, the corre- sively bonded components) that are linked with the sponding liquidus temperature may not be realistic glass product and, if applicable, with its incorpora- for the kiln-casting method. At the tested tempera- tion within a system (e.g. window frame, electronic ture range (820–1200 °C), the aluminosilicates con- device). External contaminants are relevant to the taining embedded nano-ceramic crystals were the post-consumer waste glass streams and to the quality only samples to entirely crystallize due to their com- of waste collection, sorting and recycling into cullet. position. Regarding the strength of the glass network, A more sophisticated recycling system could reduce this is deﬁned by the bond strength of the composing or avoid the presence of external contaminants, in compounds and molar volume (spacing and organi- antithesis to the embedded contaminants, which are zation of the molecules). Lead silicate samples, due technically and economically challenging to remove. to the weakness of the PbO bond and the large size of the Pb atoms, showed the lowest ﬂexural strength, The thermal history employed for the recycling by when compared to other recycled glass compositions kiln-casting of each glass type can favour the removal of pure grade. C-ﬁber glass presented, on the con- of contaminants (e.g. coatings) or intensify their effect trary, the highest strength, due to the advantageous (e.g. promote crystal formation around a defect). For effect of the 5% Al O content in combination with 2 3 example, a short dwell time at forming temperature (2 h the ﬂexibility of its glass network (Bristogianni et al. at 1120 °C) showed inadequate removal of coating- 2020). associated bubbles in soft coated ﬂoat or mirror spec- • Cullet size The size of the cullet, in combination imens versus a prolonged heating (10 h). Forming with contaminants surrounding each cullet piece, below the liquidus point would lead to the formation will determine the meso-level structure of the recy- of crystalline interfaces between the cullet pieces. The cled glass. Large shards may result in distinct struc- removal of frit, even of light colour, was not successful tures (glassy or crystalline) within the glass, while at such temperatures (e.g. 970 °C) either. smaller cullet will result in random patterns. Pow- The combined effect of the cullet characteristics dering of the cullet will increase the homogeneity of and thermal history will result in stress inducing and the recycled sample but also the bubble content due strength reducing defects. Strength reducing defects to the higher amount of entrapped air between the can be bubbles or undissolved frit ﬂakes that inter- powder particles. rupt the glass network. Such defects, if situated in the 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 22 Recycling by kiln-casting ease based on the chemical composition of the tested glasses Fig. 23 Ranking of the impact each type of ﬂaw may have on the hosting glass network Fig. 24 a Fractured surface of an automotive glass sample, kiln-cast at 1120 °C; a Cr O based crystalline ﬂake (green colour) interacted 2 3 with the mould during forming, resulting into a white 3-dimensional surface stone that became the fracture origin of the sample during loading in ﬂexure. b Fracture origin of a recycled borosilicate beam at 1070 °C; the repetitive interruption of the surface by bubbles lead to the weakening and eventual failure of the sample during bending bulk of the component, are not intrusive. Stress induc- glass during loading due to stress concentration. Dark ing defects have a more severe character, based on the coloured frits containing Cr O will result in crystalline 2 3 level of deviation of the thermal expansion and stiffness ﬂakes suspended in the glass. Such ﬂakes if in contact between the glass matrix and the defect. Based on the with the investment mould during casting may develop kiln-casting results of various ﬂoat glasses, formed just into stones that weaken the component (Fig. 24a), yet above their liquidus point, a ranking of the severity of if situated in the glass bulk, they are tolerated. The different contaminants is made (Fig. 23). Glass ceram- effect of light-coloured frits and coatings is minimal, ics and foreign glass inclusions of lower CTE lead to provided that sufﬁcient forming time is applied to guar- catastrophic results, regardless the size of the cullet. antee the incorporation of the coatings/frits to the glass Stones will be tolerated in small size < 2 mm, but will melt and the removal of excess bubbles. weaken the sample when present at larger diameters, as The classiﬁed as catastrophic or highly weaken- they induce cracks to the surrounding glass (see auto- ing contaminants were all of external character and motive glass example). Metal inclusions in the form of were found in the post-consumer glass waste streams, wire or small traces will not lead to immediate crack- while embedded contamination resulted mainly in tol- ing of the glass matrix, but will locally weaken the erable defects. Post-consumer glass waste also showed 123 T. Bristogianni, F. Oikonomopoulou a higher content of tolerable defects due to the variabil- glass cullet type. In addition, opaque or marble-looking ity of the cullet. In these cases, although each defect, glasses can be achieved by casting below the liquidus for example, a bubble, has a negligible impact on the point, yet considerable strength reductions need to be strength, its frequent repetition (e.g. see bubble content taken into account as a consequence of this action. in E-ﬁber glass in Fig. 19b, or bubble veil in Fig. 24b) Figures 23, 25, 26 suggest that a wide variety of glass and combination with other defect types results in properties and qualities can occur from the recycling by the eventual weakening of the component. The above casting method. Systematic mechanical testing of the observations intensify the need of meticulous sorting of resulting recycled glass variants is required to provide post-consumer glass waste. A product labeling system reliable design data to engineers and product designers, similar to the one implemented by the plastic recycling to assist them on applying the created materials in new industry with identiﬁcation codes based on composi- products according to their characteristics. tion, the installation of distinct glass recycling bins and an increase of the consumers’ awareness can assist in controlling this problem. The experimental results also 5.2 Improving the quality by engineering composite highlight the plausibility of recycling by kiln-casting glasses into voluminous components glass containing contam- inants of embedded character. Such contaminants prove The recycling by kiln-casting of post-consumer glass to have a milder impact on the strength of the recycled waste resulted in considerably lower glass quality than component. of that achieved by recycling pre-consumer (free of Based on the criticality, repetition and location of the external contaminants) glass waste. However, taking encountered defects in each recycled glass type, and into account that voluminous cast glass components in combination with the bending test results, a grad- mainly fail due to surface ﬂaws (Bristogianni et al. ing on the strength of the studied glasses is presented 2021a, b), a strategy of composite glasses can be formu- in Fig. 25. This ranking takes into account the glass lated to improve the strength. Such composite glasses quality achieved when kiln-casting the cullet above its can consist of a core produced by the lower grade, post- liquidus point (up to 100 °C higher), where a higher consumer glass waste (in cullet or powder form) and degree of homogeneity can be expected. It can be seen a surrounding surface of purer quality glass (Fig. 27). that most of the glasses ranking as “unsuitable” con- The surrounding surface can consist of pre-consumer cern post-consumer glass streams that contain external glass of the same compositional family, provided that contaminants of critical nature. If such contaminants it does not contain embedded contaminants that can were to be removed from the cullet, then consider- be stress inducing (e.g. metal wires). Milder embed- able improvement in the quality would be expected. ded contaminants (e.g. coatings, light-colour frits) in Apart from the type and population of defects present in the pre-consumer glass waste have a calculable amount each glass, the chemical composition plays an impor- that can be taken into account when engineering com- tant role on the strength of the glasses. As an exam- posite glasses. Apart from engineering distinct quality ple, the lead silicate specimens have a relatively low zones, overall reinforcement of a poorer quality glass strength despite the lack of weakening or repetitive with a purer glass can improve as well the performance tolerable defects in their network. Recycling by cast- of the glass component. The manner of structuring the ing at temperatures below the liquidus point (examples two cullet grades inside the mould, the viscosity dif- excluded from Fig. 25), although beneﬁcial in terms ferential between the two glasses, the cullet size differ- of energy savings, leads to signiﬁcant strength reduc- ence and the chosen forming temperature are crucial tion, rendering the resulting recycled glasses suitable parameters in determining the type of composite glass for low-strength applications only. (distinct quality zoning versus overall reinforcement). Light transmittance and colouration of the recycled Prior ﬂexural testing on composite specimens (Bristo- glasses are also important criteria, when investigat- gianni et al. 2021a, b) showed that by adding 30% of a ing possible product applications. In Fig. 26, the pro- pure compatible soda lime silica pre-consumer cullet to duced glasses (forming temperature set above the liq- the post-consumer automotive glass cullet, overall rein- uidus point) are ranked from clear to dark, providing an forcement was achieved leading to strength improve- overview of the transparency level to be expected per ment by 29%. Moreover, a 25% addition of tinted high 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 25 Quality grading of tested glass waste types, based on the strength obtained for castings performed slightly above the glasses’ liquidus point Fig. 26 Light transmittance ranking of the recycled kiln-cast glasses. Non-suitable or challenging to recycle by kiln-casting glasses have been excluded from the ranking performing ﬂoat glass (63.2 MPa) as a distinct surface from opaque to clear, or dark to clear colour can be layer to a lower performing ﬂoat (45.7 MPa), improved achieved, serving complex demands on shading or pri- the ﬂexural strength of the later by 19%. The further vacy (Fig. 28). exploration of this concept can lead to the utilization of post-consumer glass that is currently discarded due to its lower quality, while achieving a reliable structural 6 Conclusions performance due to the purity of the added reinforcing glass cullet. Supply-chain and technical barriers currently prevent Composite cast glasses can be of value not only the closed-loop recycling of -other than container- glass in structural applications, but also in architectural and waste, of both pre- and post-consumer level. Legis- interior design products. Abrupt or gradient transitions lation and logistics on waste management turn the 123 T. Bristogianni, F. Oikonomopoulou Fig. 27 Composite kiln-cast component consisting of a clear soda lime silica zone and a heavily contaminated zone from a poorer quality post-consumer soda lime silica powder (b). The layering of the glass component is achieved by structuring the different glass sources inside the investment mould based on their viscosity and size (a). c Close-up of the pure to contaminated glass transition; high population of crystals is detected in the contaminated zone that are tolerable if situated in the bulk Fig. 28 Composite kiln-cast components showing abrupt and gradient transitions between opaque or dark tinted and clear glass recycling into a challenging and economically non- frames), diminishing the environmental and ﬁnancial attractive process. The refusal of glass producers to beneﬁts of recycling cullet. In addition, further con- accept recycled cullet reinforces the problem. From a tamination occurs (e.g. metal, CSP, glass ceramics) as a technical perspective, contaminants in the cullet can result of the disposal and sorting process of glass waste, cause stress concentrations or optical blemishes in thin- which downgrades the quality of the cullet. Many of the walled, highly homogeneous glass products that render above obstacles can be circumvented if an alternative them unsuitable for use. Removing these contaminants recycling path is adopted, which involves the recycling is often infeasible (e.g. adhesives, lamination, fritting, “as is” of glass waste into voluminous cast glass com- colour) or technically strenuous (e.g. coatings, metal ponents. Such components can be cast from different 123 Glass up-casting: a review on the current challenges in glass recycling Fig. 29 Wide range of kiln-cast glasses evolved from the recycling of various different glass waste streams. Systematic validation is required to identify the mechanical properties and quality grade of each recycled cast glass and provide the industry with design data their liquidus point. A broad range of material qualities resulted from this experimental investigation. Lower viscosity glasses, such as lead silicates, Ba/Sr silicates and C-Fiber glass facilitated the most the recycling process, followed by the ﬂoat soda lime sil- ica glass family. Glasses with low alkali content (e.g. borosilicates) or a high content of aluminium oxide (e.g. Aluminosilicates, E-ﬁbers) required higher tem- peratures for their recycling back to glass, imposing challenges to the kiln-casting process, such as mould corrosion or increased energy demands. The chem- ical composition also played a role on the strength of the recycled components, with lead silicate glass showing the lowest ﬂexural strength (35.5 MPa) - Fig. 30 Structural cast glass components out of different glass among fairly homogeneous glass specimens produced waste streams, to be used in an interlocking wall system from pre-consumer cullet, and C-ﬁber glass the highest (73.4 MPa). Pre-consumer ﬂoat soda lime silica glass ranged from 45.7 MPa to 63.3 MPa according to com- positional variations. The strength of glass was reduced glass compositions and can tolerate a higher rate of in the case of post-consumer glass waste recycling. contamination, especially in their bulk. As an indication, automotive glass presented a ﬂexural Experimental exploration of the feasibility of this strength of 30.1 MPa. In fact, catastrophic or highly alternative recycling route involved the kiln-casting at weakening contaminants such as glass ceramics, for- relatively low forming temperatures (750–1200 °C) of eign glass compositions of lower CTE, and CSP, were a variety of commercial glass compositions, includ- all found to be of external character and encountered ing soda lime silica, borosilicate, aluminosilicate and in the post-consumer glass waste streams. Embedded lead/barium glasses, containing different levels of cul- contamination (e.g. coatings, fritting) resulted mainly let contamination, of embedded (e.g. frit, wire) or exter- in tolerable or negligible defects of gaseous or glassy nal (e.g. stones, glass ceramics) character. The glass character, with the exception of metal wiring (in wired cullet types were assessed based on their recycling ease, glass) and dark-frit. Although the latter two did not and on the strength and defects of the glass that would impact the integrity of the component, degradation result from their kiln-casting at a temperature just above 123 T. Bristogianni, F. Oikonomopoulou of the surrounding glass network is expected with Ruud Hendrikx is acknowledged for the XRF and XRD char- acterizations. The authors would also like to express their grati- time, as well as stress concentrations during loading tude to Cor Wittekoek (Vlakglasrecycling) and Danny Timmers that will reduce the strength. The experimental results (Maltha Glasrecycling Nederland) for their help. The authors are suggest that if certain external contaminants were to thankful to Maltha Recycling Nederland, AGC Belgium, Vlak- be removed from the post-consumer cullet, then this glasrecycling Nedeland, Coolrec, Sibelco, Royal Leerdam Crys- tal and Schott for their contribution in glass cullet. would automatically shift the grading of the recycled products from “unsuitable” to moderate or even strong. Regarding glass types that resulted in weak or moder- Declarations ate strength recycled components, these could either be applied in lower-strength applications (e.g. tiles or Conﬂict of Interest On behalf of all authors, the corresponding interior partitions respectively) or reinforced by com- author states that there is no conﬂict of interest. posite action. Composite glasses, consisting of a strong Open Access This article is licensed under a Creative Com- surface out of pure, pre-consumer glass cullet of high mons Attribution 4.0 International License, which permits use, quality, and a core our of a post-consumer glass of lower sharing, adaptation, distribution and reproduction in any medium quality, were proven to have an increased strength ver- or format, as long as you give appropriate credit to the original sus glasses kiln-casting only by using the lower quality author(s) and the source, provide a link to the Creative Com- mons licence, and indicate if changes were made. The images grade. or other third party material in this article are included in the The broad pallet of strength and optical qualities article’s Creative Commons licence, unless indicated otherwise achieved from the recycling of the various types of in a credit line to the material. If material is not included in the glass waste streams (Fig. 29) suggests the large appli- article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, cation potential of these glasses in various markets, you will need to obtain permission directly from the copyright among which the ﬁeld of structural glass, architec- holder. To view a copy of this licence, visit http://creativecomm ture and interior design is perhaps the most straight- ons.org/licenses/by/4.0/. forward (Fig. 30). Towards this goal, the establishment of recycling infrastructures in local/regional level are suggested, in combination with labelling systems that References promote the easy separation of different glass compo- sitions and the engagement of the public. 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Glass Structures & Engineering – Springer Journals
Published: Nov 24, 2022
Keywords: Glass recycling; Glass waste; Cast glass; Kiln-casting; Glass defects; Glass strength
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