How to Use Plaster Molds

Plaster molds are fundamental tools in ceramics and casting, primarily utilized for shaping materials like clay or slip (liquid clay) and for creating reproductions of forms. They are also used in the process of making other molds.

Creating Plaster Slump Molds from Clay

A common initial step in using plaster for shaping is creating a mold from an existing form. Often, this starts with a shape made of clay. To make a plaster slump mold from a clay form, you just center a lump of clay, use a rib or trimming tool if desired to alter and smooth the shape. This prepared clay form acts as the original shape you wish to replicate. Then plaster can be poured around this to create a slump mold. The plaster hardens, capturing the negative impression of the clay shape.

Here's a simplified breakdown of creating a slump mold from a clay form:

• Prepare the form: Start with a centered lump of clay, shaping and smoothing it as needed with tools.
• Contain the form: Place the clay shape within a mold box or containment ring.
• Pour the plaster: Carefully mix and pour plaster around the clay form within the containment, ensuring even coverage.
• Allow to set: Let the plaster harden completely.
• Remove the clay: Once the plaster is hard, remove the original clay shape to reveal the negative slump mold.

Using Plaster Molds for Forming and Replication

Once created, plaster molds can be used in several ways, including making other molds or directly shaping materials.

Using a Slump Mold to Make a Hump Mold

A specific application described involves using a plaster slump mold to create a different type of plaster mold known as a hump mold. Once you have a slump mold, you can then pour plaster into it to make a hump mold. This process essentially creates a positive plaster form (the hump mold) from the negative slump mold.

When using plaster to cast directly against another plaster surface, it is essential to use a release agent. Applying a suitable release agent to the surface of the slump mold prevents the newly poured plaster from bonding permanently to it, allowing the new hump mold to be easily separated once hardened.

Shaping Clay and Other Materials

The most common use of plaster slump and hump molds, particularly those derived from clay forms, is for efficiently shaping plastic materials like clay or for slip casting. Plaster's ability to absorb moisture helps clay release cleanly from the mold as it dries slightly.

Here's how these molds are typically used with clay:

• Slump Molds: These are negative molds (like a bowl shape). To use a slump mold, a slab of clay is placed into the mold and gently pressed to conform to the inner shape. As the clay stiffens, it holds the mold's form upon removal.
• Hump Molds: These are positive molds (like a dome or hill shape). To use a hump mold, a slab of clay is draped over the mold and pressed down to take the mold's outer shape.

Here's a quick comparison:

Plaster molds allow for the consistent reproduction of forms, making them invaluable tools for creating multiple identical pieces.