This weekend I've been testing making molds with silicons. I want to cast a mirror frame I've carved out of wood. I doubted at the time whether to guild it but decided to leave the wood to be seen. Now I want to cast it and see if I can have a gilded version too.
I'd used plasticine to back up the frame. It sticks it down so it prevents it from floating, and raises it enough to let the silicone runs under just a little. That way the casting needs little cleaning up. I used plaster of Paris to cast. But the combination of an intricate and delicate shape with gypsum as medium proofed to be too much to ask.
It's too weak & came out broke, but I was pleased to see it showed all the detail in the original frame. So the technique works. All i need to do now is find a way to make the mold it in such a way, that the cast will have more strength and lesser change of breaking.
In this 2nd setup I've let the plasticine run out from under the frame. This way the mold will have more contend. When poured with plaster all the plasticine you see will be filled with it. Hopefully it will add strength to the frame, but it will mean more clean-up after it's released from the mold. This mold is drying as we speak, so I can work on it tomorrow.
I choose gypsum to cast, cos one of it's characteristics is it takes any paint, unlike polyurethane (which is stronger). The broken pieces are a great way to test the gilding. Gilding benefits from a base color to give it extra depth of shine. I'm using fake gold but the same principal applies to that material too. Not too happy bout how it looks so far (broken pieces with cracked finish :)), but no doubt trial and error will make for a better result. Till then,
I've added 25% PVA glue and with the added backing it came out in one piece. Yhippy! but I noticed the glue makes the liquid plaster more viscose, and any air bubbles that are in there are less likely to be able to raise to the surface. A 3th cast is drying, this time I made sure to fill all nooks and crannies with a brush before topping the mold up. Fingers crossed...