Using glass waste as a ceramic ingredient
Why Glass?
Glass waste has always been an issue in the modern era. Most commonly seen as broken glass on the sides of roads, broken windows, cracked or damaged drinking glasses, and much more. Glass makes up 33% (6) of waste produced today, which only accounts for the glass that makes into the recycling. Shards of glass can be harmful to the environment by harming workers who sort recycling, harming animals foraging for food, and filling landfills (6,7). For a single glass bottle it could take 4 thousand to 1 million years for it to degrade naturally (8,7). High chemical stability of glass makes it highly recyclable since heat does not alter its chemistry (1). Recycling glass can be achieved in 3 main ways. The first is the most logical, using old glass to make new glass. Reworking glass is already done today in most glass manufacturers by the use of cullet, or defective glass from the production line (6). Another more novel way is being done by the company Glass Half Full, grinding glass down to grains and using it for beach restoration projects (13). Due to recycled glass’s chemical stability and availability, it is perfect for these types of projects (1, 6). Glass produced today has a similar composition to feldspars and feldspathic minerals commonly used in ceramics. It contains 71.42% of silica, 26.50% of fluxing oxides and 1.84% of alumina (4). This makes it possible to rework powdered glass into new ceramic products. The two common types of glasses produced today are soda lime glass and borosilicate glass. Soda lime glass uses the fluxing oxides sodium oxide and calcium oxide to dramatically reduce the meeting point of silica so that the glass can be worked into a variety of shapes (12). This glass most closely mimics natural feldspathic minerals and will be the main focus of testing in this review. Soda lime glass compositionally speaking could be a glaze and some hobbyist potters use it to create a crackle effect (figure 1). Borosilicate glass is known for its high thermal shock resistance, which is due to the addition of boron trioxide into the composition (1). This type of glass is produced much less than the soda lime glass and rarely ends up in the recycling process (9).
Figure 1: A small bowl with Red Chunky Terrazzo size 2 from Lakeside Pottery fired to cone 6 (10). Terrazzo are bits of glass often used in decorative concrete.
What is Glaze?
Whether it be in public schools or a weekend trip at a paint-your-own-pottery studio, most people have applied glaze to pots and experienced the glassy layer it creates. What many do not realize is that the materials that make up those glazes are all mined directly from the earth and processed into commercial glazes. According to Professor yoppolo at Kutztown University, glazes are made of 4 main components. Silica, also known as silicon dioxide, is the main component in glaze and is responsible for creating the glassy layer. Silica cannot melt on its own and needs a material known as a flux. Fluxes, generally speaking, are anything that reduces the melting point of another material. Ceramic fluxes are naturally found in feldspars which are minerals that contain silica, fluxing oxides, and alumina. Alumina, also known as aluminum trioxide, is the refractory component in ceramics and balances out the effects of fluxes by increasing the melting point of silica. Clays and kaolins are the main source of alumina in ceramics. Separate from these categories are colorants and modifiers, which make up the 4th component. These are most commonly metal oxides and carbonates from transition metals like iron, cobalt, copper and chromium. These are what impart colors and visual effects to glazes and come from a variety of sources. All of these components come together to create ceramic glazes (12). The ratio of the main 3 components is what determines the cone the glaze fires to. Cones are the metric that ceramicists use to gauge the permeation of heat into the ceramic. These cones, as pictured in figure 2, are specifically formulated to bend when a certain amount of heat has permeated them (11). Each cone formulation has a number assigned, 019 through 01 and 1 through 42 (11). Low fire ceramics, also known as earthenware, are fired between cone 04 and cone 06. Mid range ceramics, also known as stoneware, are fired between cones 5 and 8 (12) .
Figure 2: Left Cone pack containing cones 3 (orange), 5 (white), 6 (red) and 7 (teal). Right cone pack represents the left cone pack post-firing (12).
How glass can be a glaze
For glass to be used as a proper glaze without any defects, the composition needs to be adjusted. Ceramic researchers have already begun testing and using soda lime glass in both clay bodies and glazes as a feldspar replacement. Yeşilay uses powdered glass as a feldspar replacement in clay bodies in a mid range firing. Using various percentages Yeşilay found that adding glass to clay bodies functioned as a feldspar by supplying both silica and flux. Yeşilay also found the glass lowered the temperature needed to fully vitrify the clay (4). Mirdali found that using glass in low-fire ceramic glaze creates stable and visually interesting surfaces. It functions as a feldspathic material and can be used as a source for colorant in glazes (3). Colorant oxides are often mined in unethical or environmentally harmful ways; By using colored glass the oxides are effectively reused which leads to a reduction of mined materials in each recipe (5). Gualtieri et al., who appeared in the Journal of Asian Ceramic Societies, used glass waste in a glaze for tile glazing. His glaze used 41 wt.% of glass waste which reduced the melting temperature of the glaze by about 400°F. Also in this review Revelo et al. used glass from cathode ray tubes in ceramic glaze. His glaze contained 20 wt.% of waste glass and conventional frits to create a transparent glaze (2).
In experimentation with glass as a glaze component, a 30:70 ratio of high purity clay and pulverized glass yielded a functional glaze that only crazed slightly on the Kutztown University's shop c;ay. However, When used on a clay body using glass as a flux, Yeşilay’s B2 recipe was used, the glaze did not craze and only had minor surface defects. Using Scanning electron microscopy, it was revealed that the glaze made with glass formed microscopic plagioclase crystals, which seemed to lend a slight matte-ness to the surface finish. This glaze seems to take colorants well, but more testing is required to see the efficacy of different metal oxides in this glaze.
Why glass waste should be in ceramics
Glass’s feldspathic qualities are what make it ideal for use in ceramic materials. Coupled with its wide availability and accessibility, glass can alleviate the strain material mining has put on the earth. According to the research done over the last few years, pulverized glass is able to be used in both clay and glaze as a feldspar substitute. Evidence for the use of glass in glaze is present only for lowfire. In theory, with the addition of alumina via ball clay or kaolins the glass could be turned into a stable mid- to high firing glaze. This addition could be done with a myriad of naturally occurring clays. This could also be achieved with mine tailings and aluminum refinery wastes, as stated in Journal of Asian Ceramic Societies (2). Specifically colored glasses can be used as a coloring agent (3). Recycling colored glass this way can reduce the necessity of mined metallic oxides (5). Recycling in this way can also improve the industrial production of ceramics. Glass waste can be incorporated into ceramic products up to 40% by weight. This reduces the cost of raw material and can reduce the peak temperature needed to fully vitrified products (2). Large scale companies could use these findings to both reduce cost and sequester current waste. Aside from large scale production of ceramics, this technology could make ceramics and glazing more affordable to the hobbyist and to schools. This would allow for ceramic art to be more accessible, especially in public schools.
The abundance of glass waste makes it ideal for repurposing. Glass’s chemical resistance makes it ideal for various recycling projects, but its specific chemical composition is what makes it ideal for recycling into ceramic products. Ceramic items are incredibly common, especially that of dinnerware and sanitaryware. If glass waste can be incorporated into these mainstream products, then we could see a dramatic decrease of glass in landfills and as litter. As this technology improves stronger, better, and more cost efficient wares could soon be used alongside the wares we have today. As a material glass waste can provide cheaper and more accessible options to school and hobbyists. Overall, the use of glass waste in ceramic could vastly improve multiple fields and aspects of life.
In experimentation with glass as a glaze component, a 30:70 ratio of high purity clay and pulverized glass yielded a functional glaze that only crazed slightly on the Kutztown University's shop c;ay. However, When used on a clay body using glass as a flux, Yeşilay’s B2 recipe was used, the glaze did not craze and only had minor surface defects. Using Scanning electron microscopy, it was revealed that the glaze made with glass formed microscopic plagioclase crystals, which seemed to lend a slight matte-ness to the surface finish. This glaze seems to take colorants well, but more testing is required to see the efficacy of different metal oxides in this glaze.
Bibliography
Fu J, et al. (2023) An in-depth understanding of the effects of colorants on the structure and properties of borosilicate glasses for pharmaceutical packing. Ceramics International 49(22):35186–35192.
Hossain SS, Roy PK (2020) Sustainable ceramics derived from solid wastes: A Review. Journal of Asian Ceramic Societies 8(4):984–1009.
Kılınç Mirdalı N (2017) Inorganic wastes in glaze recipes and their effects on microstructure. Journal of the Australian Ceramic Society 53(2):713–718.
Yeşilay S (2018) Production of stoneware clay bodies by using industrial soda-lime-silica glass waste. Journal of the Australian Ceramic Society 55(3):747–758.
Yunusov MYu, Babaev ZK, Matchonov ShK, Buranova DB, Matchonov ShSh (2022) On the use of cobalt-containing metalurgical waste to obtain colored glass and glass enamel. Glass and Ceramics 79(7–8):300–305.
Jacoby M (2019) Why glass recycling in the US is broken. Chemical & Engineering News. Available at: https://cen.acs.org/materials/inorganic-chemistry/glass-recycling-US-broken/97/i6#:~:text=Glass%20can%20be%20 recycled%20 endlessly,%2C%20the%20environment%2C%20and%20 consumers. [Accessed March 25, 2024].
Phogat SK (2023) let’s dispose of broken glass responsibly: A call for change. LinkedIn. Available at: https://www.linkedin.com/pulse/lets-dispose-broken-glass-responsibly-call-change-phogat/ [Accessed March 25, 2024].
Csanyi C (2016) How long does it take for a glass bottle to degrade in a landfill? Seattle Pi. Available at: https://education.seattlepi.com/long-glass-bottle-degrade-landfill-5235.html [Accessed March 25, 2024].
Tayao IG Can you recycle pyrex glass? What do you do with them?. Wastercomau. Available at: https://waster.com.au/can-you-recycle-pyrex-glass/ [Accessed March 25, 2024].
Combining and firing glass on pottery Firing Glass on Pottery or Ceramic. Available at: https://lakesidepottery.com/HTML%20Text/Tips/firing-fusing-glass-on-pottery-ceramic.htm [Accessed April 2, 2024].
Pyrometric cones: The Edward Orton Jr.. Ceramic Foundation: Westerville Orton Ceramic. Available at: https://www.ortonceramic.com/pyrometric-cones [Accessed April 1, 2024].
yoppolo gwendolyn (2020) Kutztown University Ceramics Guidebook (gwendolyn yoppolo, Kutztown, Pennsylvania).
Glass Half Full Our story. Available at: https://glasshalffull.co/our-story [Accessed April 1, 2024].
The abundance of glass waste makes it ideal for repurposing. Glass’s chemical resistance makes it ideal for various recycling projects, but its specific chemical composition is what makes it ideal for recycling into ceramic products. Ceramic items are incredibly common, especially that of dinnerware and sanitaryware. If glass waste can be incorporated into these mainstream products, then we could see a dramatic decrease of glass in landfills and as litter. As this technology improves stronger, better, and more cost efficient wares could soon be used alongside the wares we have today. As a material glass waste can provide cheaper and more accessible options to school and hobbyists. Overall, the use of glass waste in ceramic could vastly improve multiple fields and aspects of life.
In experimentation with glass as a glaze component, a 30:70 ratio of high purity clay and pulverized glass yielded a functional glaze that only crazed slightly on the Kutztown University's shop c;ay. However, When used on a clay body using glass as a flux, Yeşilay’s B2 recipe was used, the glaze did not craze and only had minor surface defects. Using Scanning electron microscopy, it was revealed that the glaze made with glass formed microscopic plagioclase crystals, which seemed to lend a slight matte-ness to the surface finish. This glaze seems to take colorants well, but more testing is required to see the efficacy of different metal oxides in this glaze.