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So now that its time to put the pedal to the metal, what's the scoop on a plaster-silica mold vs. a clay mold...I plan to sculpt the texture of a tree into the mold, and I have about 15 pieces that are about 24" square to make.
Clay, when bisque fired, will probably be great for a mold material that will not break easily...(they will be fired to bisque-ware first)
I'm worried that the plaster might break, and I'd be stuck having to duplicate the design again.
With clay, the mold would be more permanent, and I'm thinking of using the molds afterward by glazing them, and having a wall mural for myself out of the tiles that would be the result.
Also, it would seem that clay is a good medium for a duplicatable mold, for many firings, as long as it has a kiln wash on the surface.
My basic need is to slump 1/4" glass into the mold, and attain the texture of the pattern in the mold.
Once again, thanks for the retorts.
jerry

Ya, plaster can crack, but make an original negative, and make a thousand plaster molds off of it. and no kiln wash to wory about.
Liam

There are two tricky things about bisqueware molds. The first is that the clay will shrink and may warp as it dries and is fired. You have to compensate for this if you want the tiles to be a precise size.

The second thing is that the bisqueware, as it cools from casting temp to annealing temp, will not contract as much as the glass will. That means you can't have deep negative spaces in the glass that have steap walls. If the negative spaces are shallow enough, and if their walls slope so that the glass can lift out of the mold as it shrinks, then the glass will survive. If you want deep and steep negative spaces, you have to build a mold that will either shrink more than the glass or will be soft enough that the glass will crush it as it shrinks around the mold. That's why most complex shapes have to be cast into or around a one off mold that ends up, after firing, to be softer than the glass.

Liam wrote:Ya, plaster can crack, but make an original negative, and make a thousand plaster molds off of it. and no kiln wash to wory about.
Liam

Do you mean an original positive? And what would you make the original out of?

- Bev

You can make the original in clay if you like, apply a separator, and make casts off of it while its still wet, so that it hasn't shrunk with drying. Keep the original damp so that it doesn't shrink and keep making plaster molds off of it.
Liam

Something in my head is unclear. Are you slumping into the clay, or casting? I've never heard of casting onto a clay mold, but I guess it could be done. And there are many things that I have never heard of.

Greg