Draping over ceramic

[gemhuyck]

I have been told to use ss for draping, and ceramic for slumping, but I have seen ceramic molds that indicate you can slump over them. I thought the reason for not using ceramic for draping had to do with the difference in cooling rates for ceramic and glass. I have a ceramic mold which is ruffled and can never seem to get it to slump eveanly. I am wondering if I should be using it as a drapeing mold.

[Bert Weiss]

It is possible to use each for both. What you need to understand and visualize is the effect of shrinkage while cooling. If the glass is draped over ceramic and the glass shrinks more than the mold, if there is nowhere for the glass to go, it will crack. If the glass can ride up without grabbing the mold, it will be fine. I often slump in to a stainless steel mold that is shaped such that the mold shrinks more than the glass and squeezes it so it can not be removed at room temperature. The mitigation is to take the glass out warm, or invert the mold and reheat until the glass falls out. Glass is strong in compression but weak in tension, so that process works without breakage. I have other stainless steel bowl molds that are shaped such that the mold does not grab on to the glass, and these work easily and perfectly.

Of all the configurations, draping over ceramic is the least forgiving. When the glass shrinks more than the mold, and catches the mold, it will crack. If it doesn't catch the mold, it will not break.

[gemhuyck]

Thanks Bert