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My name is Jianu Vladimir and I am a Romanian artist working in pâte de verre. I face some problems that I have been trying to solve on my own for about one year now but with no significant progress and I hope that someone more experienced than me might help me.
My most important problem is that the heat resistant plaster that I use for the molds cracks and thus leads to the appearance of lines on the models. I use a mix of gypsum and very finely grinded heat resistant bricks and I would like to find another solution for the heat resistant plaster (another recipe). In exchange for a useful recipe I can make (and send to you) an artistic model of your choice style “Art Nouveau”.
Thank you very much for your time, I remain at your disposition for any additional information.

The traditional formula is one part silica and one part plaster (gypsom). If you can find it, 200 mesh silica flour works well.

In some areas it can be difficult to obtain silica, but you may be able to find and use flint instead. Flint is a silica based material.

There are different grades of plaster, some stronger than others. The stronger plasters here are called gypsum cements. Perhaps you could find some of those. There are also castable refractories, which are stronger and more heat resistant still, but they may be too hard for easy removal from the finished glass.

If you are buying a premixed heat resistant plaster it may already have silica or flint mixed in. In that case adding more refractory may be weakening the mold. When Brad writes that we usually mix 1 part plaster with 1 part silica, that is ordinary pottery plaster without any heat resistant materials added.

Another idea is to add a bit of shredded newspaper to the mix. This will burn out as the mold heats up and leave it porous so that water can escape easily. Any details you can tell us about your exact process and what you've tried will help us give you more detailed answers.