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I want to try an aperture melt into a plaster mold, has anyone here done this? Any issues I should be aware of?

The maximum working temperature of plaster is around 1200F. If you add silica to the mixture you can get to 1500F or so. Firing higher than that can result in having the mold crack and fall apart on you.

Having said that, we have been able to pot melt into a plaster/silica mold by making sure the hole in the bottom of the pot is fairly large and then firing to as low a temperature as feasible.

Okay, that's what I thought. Are there other glass casting mold materials that will take 1,700 F?

That's a question with multiple answers. Are there mold materials that will take 1700-plus degree temperatures? Yup. Metal-casting foundries have several, and if you check with them they can probably point you to them. They won't be plaster-based, they'll be cement- or ceramic-based, and they will handle differently from the plaster-silica you're probably used to.

I personally don't like using a silica additive in a mold surface that's next to glass, especially if I'm heating the glass past the 1500F mark. I tend to have more problems with the glass sticking to the mold that way, and more issues with surface quality that require additional coldworking (grrr).

However...you don't necessarily need to heat the casting mold to 1700F to get a pot melt effect. I pretty routinely reservoir-cast with stacks of scrap glass, a la pot melt, by putting a flowerpot on a rack/furniture above the mold. If I take the top temp to 1525F or 1550F and simply hold it for 4-6 hours, I get much the same effect.

Or, I can do a real pot melt at full temperatures, but instead of doing it into a flat plate, I do it into another pot and make a billet out of it. Then I put that billet into the mold. You do have to be careful about compatibility shifts, since you're doing an additional firing after the high-temp firing.

Cynthia, what do you use for a face material in that case?

I usually substitute a fine grade of alumina for the silica flour. More expensive than silica flour, but gives a much better result. That's only part of the equation, though. You also try to keep the temperatures as low as possible--the higher the temperature (I find), the more trouble I usually have with plaster mold material sticking to the surface of the glass.

Hi, Cynthia,

I have been thinking about dripping glass into both a "plaster" mold and also into a pre-formed depression in sand/bentonite. I have a couple of questions to try and "smarten up" before doing this. I was going to either use one of the R&R mixes or maybe mix #1 pottery plaster, Silica flour, and grog together to make a mold. I think this would survive ok at 1550 F or so? Could I go higher in temp? I don't have alumina but could certainly use that if it were a better choice. How does the alumina affect getting good detail? I also wondered whether it would improve the surface to spray the mold with a mold release like boron nitride? As for the sand/bentonite, I would like to make a mold with simple designs in a sand depression in a steel box and let glass drip into it. I wondered if the sand would dry out and crumble long before the glass dripped into the well? Any info you can share on this would be much appreciated. I have taken several casting classes over the years and different approaches were used but I really don't know why and what the variables are.
Thanks, Michele