Monday, March 8, 2010
Preparing a new plaster mold (Part 3)
The only remaining pieces (aside from the large side) are the two back gusset pieces, which are tinted pink in this picture. These two pieces are designed to lift up with the casting, with the horse balanced on top like a pedestal. This is the way most horses are removed.
The two front gusset pieces came away really clean, so they didn't need an adjusting.
But while the first side came off easily and the two front pieces came away clean, the casting was still a little too soft to be considered leather-hard. Normally I would set everything aside at this point to let the casting firm up a little. That's because to break the seal on the other large side, I'm going to need to rock the casting a bit, and I don't want to risk collapsing the barrel, shoulders or chest. But since this casting is already lost, I went ahead and tried to remove it to show what happens when a casting is removed too soon.
Not only was my guy too soft to retain his shape as I jiggled him, the clay on the other side was still wet enough that it hadn't fully released from the mold. That pinned him in at the top if his shoulder and the point of his hindquarter. (You can also see the cloudy film the release residue has created on the surface of the clay.)
Here he is on the gusset pedestal, showing how pulling the casting while it was still pinned to the side pieces caused the piece to rip apart.
The two bits of stuck clay show where the casting had not yet dried enough to release from the mold. Fortunately it was the clay that was too damp, which is relatively easy to clean up with a fine (dry!) sponge. If the mold itself had still been damp, it might be impossible to completely clean the clay from the mold. Patience really is one of the most important qualities in slipcasting!
Even though my casting was badly damaged, I was still able to wiggle the two back gusset pieces free so that I could check for flashing. Sure enough, both hind legs had scraped clay showing where I needed to correct the molds.
It's hard to believe that a casting like this is a success, but it was because I was not after a usable bisque. What I wanted was information, and this first casting provided it. By looking at how the casting was going wrong, and adjusting the mold pieces accordingly, I was able to fix the mold so that future castings were easy to get.
Here is the second casting from that same mold, poured to double-check that I caught any potential problems. As you can see, the mold is clean and he came out whole, so I can declare this a working production mold.
at 8:48 AM