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Firing Glass on Pottery - Lakeside Pottery Ceramic school and Art Studio in Stamford Connecticut
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 on a pot, where to place it, how does glass interact with glazes, and is glass on ceramic is 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.
What kind of glass to use?Glass has wide range of melting temperature and chemical makeup. We have experimented with many glass types such as broken bottles, sea glass, misc. recycled glass, mosaic class, 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. Breaking colored bottles also was a failure for two reasons. The glass shards are sharp and can hurt the user and, as mentioned above, two of the same color bottles can yield different end results. Therefore, it is important to find a source for your glass that can provide consistent results.
Where to get the glass?
We purchase our glass from a company in the Utah called American Specialty Glass where they have wide range of choices of color and glass chip size. All of their product is recycled. You can find them at: www.americanspecialtyglass.com
Firing temperature and other firing issues
How to fire glass with pottery? We fire our pottery in electric kiln to Cone 6 (2223 degree F). We tried firing glass in our kiln at a lower temperature (cone 5) with American Specialty Glass and other types and 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 will 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 on to the kiln shelf and possibly drip 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.
How much glass to use?
The material coefficient between 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 latter, some of the corners of the tray crack and break off.
Interaction of glass with glazes
Think of glass as glaze. Therefore, every glass / glaze combo will effect 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 safety?
The fired glass surface has minute cracks and can hold substances (food, dust, dirt) in it. Therefore it is not food safe!
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