Preparing the master mold

I will need to do a few things with my master mold before it’s ready for the plaster. The biggest of these is going to be separating the gusset into two pieces. Normally this is done after the rubber copy has been made in order to keep the mold lines on the original to a minimum.

I need to do this because the gusset piece will not release the casting without risking a break to one of the legs. The problem with Imp, however, is that those legs (and the necessary pour hole) occupy a really small space. It’s going to be tight working no matter where I cut.

I decide to run my split down the forward tripoded leg. The gusset usually splits to either side of the pour hole. I’ve already had to make a pour hole that does not bisect the gusset because there wasn’t enough room between the two legs on that side. It was tempting to split the gusset along the pour hole and spare that front leg the mold line, but I suspected that design wouldn’t really give me much more wiggle to get the piece out with all its legs intact.

The downside was that it would be a complicated cut. I would need to carefully cut the line down that inner front leg, but it would need to fall quite further back if I was going to miss truncating the raised foreleg on the other side. (This will become more clear in the next post with pictures of the resulting piece.)

I should add here that I hate cutting rubber. The rubber I use is really tough. It doesn’t cut easily and it will quickly dull an Xacto blade. The important initial slice down the leg, using the point of the blade, isn’t hard. After that I spent most of the time thinking, “This is a recipe for lopping off a finger” and “gee, maybe I should have placed the phone next to the workbench in case I have to call for medical help”. The split was rough, but I cut it without incident (other than a few new gray hairs).

With the gusset split, all that was left was to fill the sprues. I’m not really picky about getting these just right, especially since the plasteline doesn’t really stick well to the rubber. Mostly I just want to cut down on the clean-up I will have to do with each plaster piece.

Now I am ready to start. The basic idea behind a rubber master mold is that, one by one, the pieces will be replaced with a plaster version, until all that is left is a plaster mold encasing the rubber original. The rubber original is then removed and the plaster mold is ready for ceramic slipcasting, and the rubber pieces can be reassembled to make the next plaster mold.

When I began the rubber mold, I started with the big side pieces first, and worked my way inside. The plaster molds go the other way; I start on the inside and work my way out. For this mold, my first piece is going to be the smaller, forward gusset section. That piece gets removed and the rest of the master mold is reassembled.

In the next step, I’m going to be pouring my plaster into the cavity left by the missing piece. The rest of my master mold is going to act as a framework with the rubber original creates the image in the plaster.

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