For my face coat, which is usually about an eighth of an inch thick, I start with a standard 50/50 pottery plaster/silica flour mix. Then I add three things: EPK, which hardens the mix and enables sharper detail, talc, which aids in release and aluminum hydrate, which is probably also acting as a release but seems to brighten the glass considerably.

I vary the proportions depending on the level of detail in the model, but it’s generally 1 (EPK): 1(aluminum hydrate): 1.5(talc). I just finished the BEcon conference on casting, and several large-format casters I spoke with suggested diatomaceous earth instead of talc…so I’ll try that at some point.

Anyway, I sift those things into the water first, let them fall, then start sifting in the plaster/silica mix. I slake for at least five minutes, then mix. If I’m really concerned about detail on a final piece, I’ll use a “squirrel cage” mixer on an electric drill, but most of the time I’m too lazy and just use my hands. If it’s well-slaked, it works pretty well.

The mix is fairly loose at that point, and I use a paper cup to dribble it over the model. Once it’s covered, I’ll take a big, soft haik brush and push the mix into undercuts and detail. I try not to let the brush, fingers or anything else push through the investment to the model surface–too much of that and I get scum/sticking in that area.

Then I just keep pushing the mix up the model until it solidifies.I don’t worry about making it pretty and smooth. I think the next layer sticks better if there are rough areas.

Second layer is almost always fiberglass dipped in straight plaster/silica and laid into areas that need reinforcing. At the same time, if there are what I call core areas (areas where the glass will need to contract sharply around the plaster), I’ll make “spongemix,” which is plaster/silica mixed with either vermiculite or chopped kitchen scraps. (Anything organic–carrot peelings, oatmeal, rancid nuts, etc–as long as it doesn’t expand when wet). That gets packed into core areas inside the face coat. The face coat is too thin to hold those areas, usually, and the organic stuff burns out, leaving a spongy interior that will crush together when pressed.

The third layer is plaster/silica mixed with reinforcement (either fiberglass “chop” or grog).

Then I steam out as little as possible–I learned from the Rowe twins to make hollow wax models to keep the mold in the steamer for very little time.

I also make sure NOT to contact the surface of the mold with my fingers unless I just have to, to correct a bubble. My technique, pate de verre, is very abrading to the surface of the mold because I’m packing grains of glass hard against the plaster, so I need all the help I can get.

It seems to work. When a mold comes out of the kiln I can slip a thin-bladed kitchen knife into one side and literally lift the mold off the glass almost in one piece (even soda-lime frit pieces, which are the worst for scumming/sticking). The investment will break off in the undercuts, but pretty much pops off with a couple of jabs or a hard spray of water.

I’ll get a slightly roughened surface with pate de verre, about like 400-grit sandpaper, but it takes about 15 minutes with 400- and 600-grit wet-dry to make it very smooth and velvety. If I use soda-lime billet, there’s almost no coldworking needed, and the best surface usually comes with Gaffer–literally like a baby’s bottom.

Someone on the warmglass board pointed out that the mix I’m using is very similar to a good kilnwash, so that’s probably why it doesn’t stick. (And now that I’ve said all this the next mold will stick like crazy, you know that, right?)

But try it on some test samples first, and vary the facecoat proportions. If your surface isn’t too finely detailed, you can also apply a very thin coat of kilnwash over the mold–make a solution of about 1 part kilnwash to 6 parts water, quickly fill the mold and just as quickly dump it out. You should barely be able to see the kilnwash at all. That also produces a nice surface, but it’s more trouble and if you’re not fast it can clog up your detail. If my model is very smooth and doesn’t have a lot of fine detail, I can sometimes brush this mix on for the same effect.

Hope all that helps.

I’ve done some documentation on my blog for the step by step, but my projects are so simple in comparison. I’ll send you a link to one of them, just for grins.