The result of incorporating glass into your pottery work is beautiful and rewarding. The colors are vivid; the light reflection creates a jewel effect; the crackling in the glass is unique, and the edges of the glass pool interacts interestingly with glazes. There are some important things to know in order to achieve successful results. Issues such as: what kind of glass to use, what temperature to fire it to, how much glass to place in a pot, where to place it, how does glass interact with glazes, and is glass on ceramic food safe?
Glass has been combined with pottery and ceramic work for years at Lakeside Pottery and we love the results. We teach its use to our students, our ceramic instructors / artists incorporate it into their work, and recently we started using it with children's projects. The following will walk you through answering all the above questions with the hope that the mystery of using glass with pottery is removed and more ceramic artists will use it.
IMPORTANT NOTES:
1) All items made with glass are NOT FOOD SAFE and should be used for decorative purpose only
2) To achieve what you see here, you need to fire the glass to Cone 6
3) Test and document any glass source with your different glazes to verify results before placing on an actual pot.
What kind of glass to use?
Glass has a wide range of melting temperatures and chemical makeup - see Appendix A & B below for more details. We have experimented with many glass types such as broken bottles, sea glass, misc. recycled glass, mosaic glass, and have had mixed results. The most important lesson learned was that even though two pieces of glass looked the same, if they came from a different source, once fired, they had different results -- one remained green and the other turned white, for example. We also discovered that sea glass is expensive and had the same results as any glass
It is important to test the glass you intend to use in your kiln atmosphere. Document the results and continue to use the same glass source firing it in the same kiln (cone 5-6) and the same glass/glaze combination previously verified and documented. See Appendix A below covering glass colors.
Note: Red glass will often not turn red after firing - it can become brown or black if mixed with a glaze or other glass colors. Using it alone and not too thin will increase the chances of getting a wonderful red
Where to get the glass?
We purchase our glass from a company in Utah called American Specialty Glass (ASG) where they have a wide range of choices of color and glass chip size. All of their product is recycled. You can find them at: www.americanspecialtyglass.com. You may want to purchase ASG's sample boxes and experiment with their full menu of color options.
See list below which reflects we have been using for several years from ASG:
1) Turquoise Terrazzo size 3 (the most used with the best and consistent results with all of our glazes)
2) Yellow Chunky Terrazzo size 3 (second best with consistent results with all of our glazes)
3) Dark Blue Terrazzo size 3
4) Purple Terrazzo size 3
5) Light Green Terrazzo size 3
6) Red Chunky Terrazzo size 2
Firing temperature and other firing issues
How to fire glass with pottery? We fire our pottery in an electric kiln to Cone 6 (2223 degrees F) after the pots are bisqued first to Cone 06. We tried firing glass in our kiln at a lower temperature (Cone 5) with American Specialty Glass, and other types; in all cases, the glass did not melt sufficiently. See Appendix B below for glass melting temperature details.
When placing the glass, one must think about what will happen to the glass at it's liquid state. It is helpful to think of the glass and where you place it as if it were an ice cube that would melt. Meaning, as it melts the pooling will increase it's width and if the kiln shelf is not level, the liquid glass will move to the lowest point. Also, do not place on vertical or sloping surfaces. If you place it on a pot with a crack (e.g., "Scratch") with the hope that the thickness of the melted glass will hide the crack, be aware that you will be disappointed - the glass will leak and empty itself to the kiln shelf and possibly drip on to other pots on the shelf below.
How to apply glass to pottery?
The glass can be applied on top of a glaze or, for a different effect, glaze can be scraped off where the glass will be placed right on the bisque. See the process below
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Scraping method |
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No Scraping |
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| Glazed bowl |
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Scrape off glaze |
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Place glass where scraped |
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| Glass on top of glaze |
Clean off glaze |
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Click picture for detailed view |
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How much glass to use?
The material coefficient (thermal expansion) rate of clay and glass is different. Glass will craze (crackle) pending the kind of clay, type of glass and the amount used. The amount of glass used depends on the effect you wish to get. Try it on test tiles (see example of a test tile below). Note that in some cases, depending on the clay you use, the clay will shrink more than the glass and therefore can create stress on the glass and the pot with the following two potential problems.
1) If too much glass is used, the glass crackled surface might not settle level and some cracks can be higher than others with sharp edges exposed and leading to potential injury.
2) If using too much glass on handbuilt items such as trays, the tension, can break the project. We see it on trays where it looks perfectly OK when taken out of the kiln and some hours later, some of the corners of the tray crack and break off.
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| On left, test tile. Above - too much glass in corners - tray could break. |
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Interaction of glass with glazes
Think of glass as a glaze. Therefore, every glass / glaze combo will affect the outcome. It is a good idea to run tests with each combination. For example, red glass can become red used with one glaze and brown/black with another glaze. The glass colors we found to be consistent with almost all glazes are the yellow and the light blue purchased from American Specialty Glass.
Is glass in pottery food safe?
NO! The fired glass surface has cracks that can possibly hold substances (food, dust, dirt) in it. Therefore, it is not food safe! See article about food safe and matte glazes
Appendix A - Glass Pigments
| Glass Compounds |
Colors |
| iron oxides greens |
browns |
| manganese oxides |
deep amber, amethyst, decolorizer |
| cobalt oxide |
deep blue |
| gold chloride |
ruby red |
| selenium compounds |
reds |
| carbon oxides |
amber/brown |
| mix of mangnese, cobalt, iron |
black |
| antimony oxides |
white |
| uranium oxides |
yellow green (glows!) |
| Sulfur compounds |
amber/brown |
| copper compounds |
light blue, red |
| tin compounds |
white |
| lead with antimony |
yellow |
Appendix B - Glass Melting Point
Glass has different melting points depending on what the composition of the glass actually is. Standard soda lime Silica (the most common kind of natural glass) melts at something on the order of 1500 °C, or about 2700 °F. Pure silica needs fluxes to melt at lower temperatures. Glass is made from a variety of substances, depending on the intent of use. Mostly sand, lime and soda are what most glasses are made of. There are many types of glasses ex, bulletproof, tempered, tinted, stained, etc.
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| Hot plate with scraping glaze off and clay texture |
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| Kids projects where the melted glass appears like water |
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