Molds for slip casting are made of plaster. This is because the mold must absorb water in order to form a clay body.
Slip refers to the liquid clay. Clay comes in many forms; the texture and chemical content of the slip dictate what type of clay body it forms.
Slips with a very fine texture form porcelain, which is white and extremely delicate and must be supported in the kiln during firing.
On the other end of the spectrum is stoneware, which has a very coarse texture and must be fired at a very high temperature.
Ceramic slip, which is also a clay, is what I work in and have the most experience with. It falls in the middle as far as its texture.
Putting your hand in a bucket of slip is like slipping your hand into a muddy ooze, which feels much like the mud pies we all used to make as children, (only with a much finer granular texture, anyway).
HOW DO YOU CAST SLIP?
First, you need a mold made of plaster. The plaster mold absorbs the water from the slip.
Fill the mold with slip. As the mold absorbs the water, the level of the slip in the mold will go down requiring you to top off the mold.
OK. Here's the trick to it all: knowing when to dump the excess slip.
If you leave a mold filled with slip too long, the ceramic piece will be much too thick and heavy.
If you don't leave it in long enough, it will be too delicate and squash and tear as it comes out of the mold.
For my small figures and pieces, I like to leave the slip in the mold for about five minutes before I dump out the excess slip.
On the bigger items, I watch the mold for an eighth of a inch skin to form around the lip of the pour holes before I dump the slip.
Now, a few pieces call for the piece to be poured solid. These include things like plates, bowls, and other utility items. These molds you just keep filling until they stop sucking up slip.
HOW DO YOU OPEN THE MOLD?
After the mold has been drained of all its excess slip, the mold has to sit and dry. The mold pulls the water out of the clay and dries the piece out.
If you leave a piece in the mold too long, it will dry out too much and crack.
If you don't leave it in long enough, it won't let go of the mold and will probably tear or it will collapse under its own weight.
When I pour, I run a fan across the mold to help dry things out. Six to eight hours later, I go back and take the pieces out.
Larger pieces take longer to dry -- as much as a couple of days.
You can tell if a piece is ready to come out of the mold if it has pulled away from the sides of the pour hole.
When it is time to open the mold, remove the bands from around the mold and gently pull the mold apart. If the piece is dry enough, it should release immediately.
Try to pull the mold halves straight away from the piece since the clay is still soft and can be easily scarred or marred. The piece should be handled carefully since it is so soft.
When the piece comes out of the mold it will be a dark gray. Set the piece up and let it finish drying until it turns a light gray.
After the piece has dried to a light gray, it is ready to clean and fire in the kiln.
WHAT IF I MAKE A MISTAKE?
If you make a mistake, don't fret about it. Slip casting takes time, patience, and practice. I've been casting on a regular basis for over three years and I still make mistakes. That's why I have a reclaimer and use it regularly.(Karen) ***********************************************
Doc Holliday Molds "Producing Quality Ceramic Molds for over 32 years" 1518 S. Washington, Wichita, Kansas. U.S.A. 67211 Phone: 316-262-3350 Fax: 316-262-1396