PPP: Using Glass on Ceramic
Inventing a Glass GlazeThe first 100 tests should have been enough of an education on thermal expansion to force me to forget ceramics and take up basket weaving. I read the books and understood that it wouldn’t work, but I couldn’t understand why there couldn’t be a compromise. So instead of attempting to make the glass fit the ceramic, I decided to make the glaze “fit” the glass first. Since glass melts at a very low temperature, it would make more sense to discover the initial melting temperatures of a glaze where there would still be enough tension to specifically contain the shape of the glass before a complete melt.
Five years and hundreds of additional tests later, I determined that a cone 6 high fire glaze was the point at which the borosilicate glass responded wonderfully. When used on a tile with a still matte glaze underfired at 1500?F the glass settles within the 04 tile glaze and suspends itself beautifully. Whether it is the eutectics of the glaze combinations or a borosilicate bonding with the high-fire glaze that compromises the two glazes, I cannot say, but the glass modifies itself nicely within the surface of low-fire glazes.
When the glass is used on the surface of unglazed porcelain or ceramics, the fit is much the same unless the glass is stronger than the ceramic. For this reason, I use only clays that fire at cone 2 or above. The glass must be heated as quickly as the ceramic body allows so that it can be effectively stopped at the exact melting temperature needed. However, a gentle lowering is required once the melting has been established. The end result is a glass that anneals itself like a glaze, soft-looking with a rolling texture on the surface that captures and reflects the light around it. The new look is positively brilliant in the sun. Indoors, the glass absorbs the light in any room, captivating the eye with a gentle glow only colored glasses give.
Next, I used the same idea to lower the melting temperature of the same high-tension glaze, and to my pleasant surprise, sections of glass up to 10 mm consistently held fast with very little popping or fractures. I then used stained glass remnants for jewelry, crushed glass on box lids or the outside of pots melted just enough to relax sharp edges and capture the light. The jewelry I have made has been with stained glass remnants and sold well to those who liked the unique look. Before long, I was digging bottles out of the backs of bars and frequenting the dollar stores for the elusive glitter of cheap glass.
Avoiding CompromiseAs simple as the process is, there is little room for compromise. The glaze needs to be the thickness of cream and should be applied in a quick layer with the glass added immediately. Once it is dry, you can position the piece any way in the kiln—it does not need to be lying flat.
The firing process must be closely monitored, and larger kilns usually aren’t suitable. If the ceramic retains the heat too long, the glass becomes a lifeless blob, or worse, some portions are melted while others are not. Glass kilns work best, but any quick fire kiln works well as long as the heat distribution around the glass is even.
The type of glass used is also important. A borosilicate glass retains much of its body, even at the melting point, and is more forgiving, but the lower-fire glasses need the discipline of exactness. Different colored glasses will melt at slightly different temperatures, so some experimentation may be needed. The thickness and application of the glass also matter.