When I design a new animal for Porcelain Menagerie, I always start with an original sculpture in oil based clay. Oil based clay, unlike water based clay, does not dry out because it has oils and wax mixed in it. If the clay doesn’t dry out it is easier to make a mold of because it won’t become brittle and the mold process can take a long time. I can also keep these originals in case the mold wears out and I need to make a new one of the same sculpture.
Once the original sculpture is finished, I can start to section it into pieces where the plaster will be able to pull away without tearing the clay. This can show the difference between a simple two piece mold and a more complicated 5 piece mold. As each section of plaster is cast it takes a negative impression of that part of the sculpture. The otter needed two separate molds for the parts of the sculpture that would have made the mold for the body more complicated. I made separate molds for the tail and the hind feet.
As each piece of the mold is cast, they have to be given keys like the buttons here that make the mold lock together. The sides with keys are then soaped with mold release to keep the plaster from sticking together.
I use a surform rasp to shape and smooth the plaster for each piece so they fit together neatly. Plaster pieces that don’t fit together well can have large seams between the pieces of the mold and cause leaks when the liquid clay slip is poured in.
Once the pieces are soaped and keyed I can cast another piece of the mold. I usually section off part of the sculpture by making a clay wall. This will stop the plaster from flowing to the other sections of the otter. Here you can see that this section will pull away neatly from the plaster without any tearing. Tearing the clay can happen when a section of the mold runs over an “undercut” this is a place on the sculpture that curves to far under something making the plaster go under or around it. Plaster is not a flexible mold material and does not pull away as easily as a rubber mold might. However, it is the best material for making molds for things made in clay, because it absorbs moisture from the clay.
The boards around the mold are called coddle boards. These hold the clay wall in place, and create an even square or rectangle shape for the mold. Using c-clamps and cleats on the end of the board they are easy to fit together and adjustable to any size. Coddle boards are usually made of melamine, or laminated wood that will release from the plaster easily.
Plaster is mixed using a 1/3 plaster to water ratio. I usually use motor oil rubbed on the insides of my buckets for plaster. This keeps the plaster from sticking to the sides of the bucket and ruining it over time. I estimate the amount of plaster needed to fill the volume of the space for the piece I want to cast. Once the bucket has the right amount of cold water, I slowly sift in plaster until it mounds up above the water line. This is the easiest way to tell the ratio is right by eyesight, but you can also use 1 pound of plaster for ever pint of water. The plaster sits like this for about 1 min., this is called slaking and allows the plaster to settle with the water. Then I stir the plaster by hand until it starts to set up. At this point the plaster is thicker to the touch but still fluid.
The liquid plaster is poured into the space in the coddle boards. Plaster goes through an exothermic reaction, meaning heat is released to change the plaster from liquid to solid. Once the plaster is cooled, after about an hour, the coddle boards can be removed.
The finished mold then has liquid casting slip poured into it and the pieces are assembled. The river otter proved to have a wonderful linear form that has a very unique elegance.
After all that, the otter is finished with a glaze and ready for my shop! Each piece in the shop is made starting with a mold. Each new addition like this otter is made by hand and by me every step of the way.
Purchase this piece, or contact me about custom orders for the otter at