What are ceramic molds made of? How do they work?

A ceramic mold is made of plaster. It is made to create a shape or object using clay or slip (liquid clay).

A mold can have one piece, referred to as an open pour, open cast or press mold. Most commonly, molds have two or more pieces which fit together like a puzzle creating a casing to hold liquid clay or ‘Slip’. Open pour or press molds are one piece molds that can be used with slip or by pressing clay into them.

The clay takes on the shape of the cavity of the mold and is then removed, processed and fired.

Ceramic molds can be very heavy with some of the larger ones weighing a couple hundred pounds. The average, everyday hobbyist usually confines their selection of molds to under 50 to 75 pounds.

Being plaster, though heavy, they are quite fragile and they will shatter or chip if hit or set roughly on hard surfaces. They can warp when they get over saturated and are not tightly banded during the drying process. If over used, they soften and the detailing in the cavity can wear away.

Life of a mold is considered 40-50 castings. But with proper care, molds can create several hundred castings.

How They Work

Being plaster, they are highly absorbent. The plaster draws the water out of the slip and the clay platelets stick to the wall of the mold cavity. After a specified amount of time, a shell is formed of adequate thickness to be handled. The slip is cast off or drained from the mold and then then the mold is set aside to mature. As the mold continues to absorb the water, the shell hardens. When it is firm, the parts of the mold will easily release and open allowing you to carefully lift the item. It will still be somewhat soft, so care must be exercised. The item is set aside to continue drying and then it is ready for the next step in mold production.

See articles on care of molds, how to cast molds and others discussing the process of making ceramics.