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I'm finally to the point of getting started on a big job that's been brewing for a few monthes. I'm almost finished with building a new, bigger kiln for the project. Several questions come up, for which I'd appreciate input. There will be about 15 molds of silica/p;aster, about 24x26 inches, for the finished glass size. My kiln is 28"x42".

1) Can I use any type of clay ? Should I have the clay fired before making the molds, in case one of the plaster molds (or glass pieces from the mold breaks down the road. Also, it may be fun to paint into the clay, and use that as a wall piece. Like all good sleuths, I'd be more confident with a backup.

2) Any thoughts on shrinkage of the molds, and shrinkage of the glass, to allow for a finished glass size that will fit into an opening that is fixed. I'd rather not grind glass for a week!

3) I've dried the molds before in the bottom of my smaller kiln. Is it feasible to build a post, or shelf, to dry many in the same load? Should there be more drying time for a bigger mold? Any worry about this size mild breaking while placing it into the kiln, or taking it out....Some way to support the mold while loading.

4) Not having done a casting this large, would it be OK to use a 1-2" edge from the clay to the end of the mold, and the same 1-2" depth in the mold material floor?

5) I'll be using starfire for the glass to slump. Any suggestions on a formula to slump/clast the plate into the mold. It will be 1/4" plate.

6) Can I even fire a couple of the molds at the same time, with glass?

I'm sure there will be more questions, but I'd be happy to answers to these to get rolling. Thanks to each of you. Jerry

I'

Any clay is OK. Just remember that you will have to toothbrush the mold to get the last of the clay off. If you want a backup, remember that clay shrinks 10-14% when fired. So the fired backup will be smaller. I would make a plaster backup if possible.

I have not heard of plaster mold shrinkage. Would be interested to find out about it.

Greg

You've got to be more precise about describing your process. I think you are talking about using clay as a model to make plaster/silica molds. If you can make the plaster molds from fired clay, why not just use bisqueware as your mold and forget the plaster? Yes the clay will shrink as it dries. Different clays shrink at different rates. Your ceramic supply should be able to tell you how much a given clay body will shrink.

2. Plaster/silica molds don't shrink. However, the glass that fills them shrinks a certain percentage from the size it is at casting temperature down to room temperature. So a bigger piece will shrink more than a smaller one. If you don't want to cold work then you have to figure it out by trial and error.

3. You can fill your kiln up with molds to dry them if you want. I think most people are finding that air drying them over a week or so is better. Larger molds take longer to dry. A kiln full of molds will take a long time. Most people put plaster molds on a bed of sand to make it easy to level them. Your mold should be strong enough on its own to load without problem.

4. I don't understand this question.

5. If you are slumping 1/4" glass you don't need mold walls at all. Starphire behaves pretty much like regular float. Call the manufacturer if you want a precise annealing point.

6. I don't understand this question either. Are you talking about bisquing some clay molds while you slump glass on other molds? If you're talking about firing plaster molds at the same time you slump glass, there's no point in doing that. You will break down the plaster and weaken the molds when you fire that high. Or maybe you mean can you slump on a mold that hasn't been dried out yet? Yes you can do that. You just have to ramp up slowly and soak for a long time at 350 - 400 F to burn out the water.

My advice on any of this is to run some tests before you start in on the final piece. It sucks to screw up a whole load of molds and glass. Remember too, it's easier to remove glass to fit it into an opening than it is to add glass. Renting a tile saw is pretty easy.