One of the drawbacks with long-distance learning is that it’s really easy to overlook the obvious. For those of us who work in slipcast ceramics, there are a number of things we just take for granted because we deal with them on a constant basis. An email message from someone struggling with mold-making reminded me of this.
Probably the most common problem I’ve seen among people attempting to make their first molds is something as simple as using the wrong plaster. I know in this blog I use the generic term plaster, as do most ceramists when they are talking about making molds. The problem is that we aren’t actually using generic plaster. Ceramic molds – at least the kind used for detailed eathernware castings – are made from a special kind of plaster called “Pottery Plaster No.1“. Using the ordinary plaster available at the local building supply store leads to a lot of frustration. I’ve learned that when someone calls me to say they are having trouble, I need to ask what kind of plaster was purchased.
Pottery Plaster No. 1 is made for this kind of work, but it’s also really finicky. If it gets damp it tends to clump, which ruins it. Molds made with clumpy plaster rarely turn out well. Living in the southeast, I end up losing a lot of plaster to humidity.
I try to minimize this by dividing the plaster into smaller portions which are kept in plastic bags that are then stored in a sealed plastic container. I’ve found that one 50 lb. bag of plaster fits pretty comfortably in two document-holder plastic containers. This way I can pull out just enough plaster for a job, without exposing the rest to the air.
As you can see, I have this set up on a paved driveway. It is important not to let the plaster dust contaminate the clay, and this kind of work is pretty dusty. I like to keep it as far from the studio as possible. I’ve often wondered what my neighbors think as I make up dozens of bags of white powder, but so far none have asked!