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About to give making molds for small soap dishes etc a go.
I have some clay left over from some pottery classes so I was going to use that. It is PB103 fine white stoneware clay.
Should this be okay to use?
When I make the molds should the back be hollowed out to reduce mass and thus mass sitting on the kiln shelf?

any clay is ok. you want to make the backs hollow, and if they're deep, they should have holes in the lowest spots to let air escape when the glass is slumping down to touch bottom.

if they are very thick in places, they will shield heat (be a heat sink) from getting to the shelf under them, which can cause the shelf to crack. that will also increase the thermal mass of objects in the kiln, slowing down ramps up and down.

look at some existing molds for how to build them.

Thanks Charlie. Much appreciated.

For small soap dishes it really does not have to be that complicated. Roll your clay out with a rolling pin on newpaper so it is all the same thickness.....form it looking at the inside of the clay making sure you have a flat area so the finished glass will sit nicely without rocking. No holes are needed if it is a shallow soap dish! Let it dry well......when you think it is dry put it on top of a firing glass kiln over night to insure it! Sand it smooth when dry and bisque fire. Easy peasy!

what schedule would you use to bisque fire? I would like to make some textures. can I fire in my glass kiln? that would open up a whole new world!!!

[quote="lorimendenhall"]what schedule would you use to bisque fire? I would like to make some textures. can I fire in my glass kiln? that would open up a whole new world!!![/quote]

the bisque temp is dependent upon the clay you're using. it's using in the 1850-1900F range, probably out of reach of your glass kiln. you can fire it lower, it will just be more fragile. your glass kiln probably tops out around 1700 or so, but it's pretty hard on your kiln to take it there.

you are right, top temp is 1700... that's too bad... thanks for the info.

My Skutt says it will go to 1800, although I've never taken it that high. I've bisqued a number of molds in it though. I used low fire clay (something called Wonder White at the local place I get my clay) and though I don't take it to the temperature it says is it's bisque range, I take it higher than how I will use it. I read doing that on here. The first one I took up to 1700. Most of the time my kiln rarely puts out that much heat when I check it while I'm fusing or walk by it, but at 1700 it put out a lot of heat into my garage. It did the trick though and I've been able to use the things I made (several smaller casting molds and a number of slumping molds) a number of times and they work fine. They probably wouldn't last for dozens of firings but for the 3 or 4 I used them for they are still usable. I agree with the comment about taking your glass kiln near it's max temp is rough on it, at least I think so as well. I've often thought about getting a small pottery kiln since they seem to be pretty cheap on Crag's List, but I don't do enough clay baking to really justify it. At least I've not in awhile. I'd like to, but I just don't have the time much any more. But get low fire clay and take it higher than you plan to use it at and you should be ok. I just fired it to 1700, had the kiln shut off and cool at it's own pace.

Mike

You can definitely bisque in a glass kiln (although it's hard on the kiln so you might not want to). Like glass, you're considering the amount of heatwork you're giving the clay, not the actual top temperature.

Ceramicists measure heatwork in "cones," and consider most clays to bisque fire somewhere between cone 08 and cone 06. That means the clay is taken to a temperature somewhere between 1692F and 1855F, depending on whose info you use. (I know some artists who bisque at cone 05, which is even hotter--cone numbers with a 0 in front get hotter as they get smaller--but the hotter you go with bisqueware, the more trouble you'll have getting the kilnwash to stick)

The more heatwork you give the clay, the sturdier the bisqueware. Depends on the clay but in general, cone 08 bisqueware is a bit fragile and won't last as long as cone 06.

Big things I've found are, first, never make your piece more than about a half-inch thick anywhere, and keep it as even as possible. Second, dry it very slowly--the bigger and flatter the piece, the more it's liable to warp and crack as the clay shrinks with drying. If you go slowly, the clay gradually wicks out the moisture and shrinks evenly. I wrap my sculptures in a drycleaner bag, cover that with damp paper towels, cover THAT with another drycleaner bag, and put the whole thing in a cool place for about a week. When the clay stops feeling so cold, it's dry enough to put on top of the kiln to finish drying. (obviously, if you're doing only a few square inches of clay, it's more likely to dry without all that fuss)

Flat pieces can curl on the edges and crack, so after I've wrapped the clay, I (gently) put a kilnshelf on top, to keep it flat.

Firing the clay is a lot like firing casting molds--take the kiln up to 200-250F, vent it, and let the clay dry until you don't see any more moisture coming out the vents (it will fog a mirror). Then you can start your upramp. The faster you go up, the higher temp you'll need to reach, and the more you'll need to hold at top temp, to "sinter" the clay. That generally produces the strongest, longest-lived bisque.

And, of course, you must kilnwash it before use.