Re: What will happen if I melt sea glass with my clay in a bisque fire?

Hmmm... I can't say exactly what would happen in each of these cases without trying it out and without knowing the composition of your clay but here are more than a few reasons the kilnmaster should (and is) worried.

The first set of problems you face involve the different expansion and shrinkage rates of the different materials. Mixing glass and clay is not advised because the materials are going to expand and shrink at different rates when the piece is being dried, heated and cooled during firing. You run a very high risk of breaking your piece even when you are waiting for it to dry to its green or air-dried state. The clay shrinks as it dries but the glass won't, creating cracks around the glass and possibly breaking your piece completely before it even gets into the kiln. When the clay is being bisqued, even more water is being driven out of the clay, causing it to shrink even further... I think you can see where this explanation is going. Just keep in mind that most clays shrink about 10% during these processes.

Second, sand or silicon melts at 2574F (1412C) and glass melts at approximately 2010F (1100C). Depending on what kind of clay you are using, you might reach these temperatures during a bisque firing but it is hard to say. Then, you run into the problem that if the glass does melt, it is going to run down your piece. Now you have a hole in your piece where the glass used to be and a glass puddle in the kiln, sticking your piece to the kiln shelf and/or to other pieces. The melted glass also runs the risk of burning out the kiln elements... which is a VERY expensive proposition.

The sand is a more difficult prediction. It will probably fall off your piece as it dries but if it doesn't, it could still create problems in the firing process: the particles are very big relative to the powders used in glazes and beach sand is a wild card of unknown composition, so unknown results. I think the best approach to glazing is to use materials that react in a known way. Glazes have been developed and refined over hundreds of years and aren't usually designed to be applied during a bisque firing. Glazing is generally done in a separate firing for best results.

Anyway, here is another MadSci archive with some excellent links to more information about the s cience of kilns. I hope you find them useful and keep up your interest in pottery. Which reminds me I have a kiln to unload... ;)

Kieran