double stacking kiln shelves in glass fusing
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Ceramic artists have a big advantage over glass artists when it comes to using space in our kilns efficiently. It is common practice to double stack, or more, when you fire smaller ceramic items, usually in the bisque stage. It works just fine.
Glass is not so accomodating. Glass requires a much more even distribution of heat above, below, and all around it, in order to behave in a cohesive manner. If that distribution is interfered with, the result will be an uneven fire.
When you double stacked your small glass pieces, you interupted the flow of hot air circulating in your kiln. Heat rises, so, much like an old oven, the top “shelf” got much hotter than the lower one. I am assuming your kiln has lid elements in addition to the ones in the side. That is one big difference between ceramic and glass kilns. Again, this is because of the need to control an even heat when fusing glass. If this is your case, than the problem is made worse because the glass on the top shelf is also now closer to the lid elements than the ones on the lower shelf.
Basically, there really is no good way to do what you are trying to do. I would advise to accept the fact that a glass artist does not have the luxury of double decking, and you will be much happier with your nicely fused single layer of equally heated glass beauties. Good luck!
Cynthia Ann Swan, Terra Pax Studio, Los Angeles, CA