Mixing Plaster

Prior to mixing the plaster, you will need a bucket of the right size. To determine this, a general rule of thumb is to pick a bucket that is slightly larger than the cast to ensure enough room for stirring. The amount of water needed to fill a cast should be around 2/3 of the cast’s volume.

The water temperature will affect the setting time of the plaster, the hotter the water, the faster the plaster will set. In practice, lukewarm water is generally used.

With your bucket of water, you will now add plaster. Plaster should be added slowly and evenly to avoid clumps. Continue to add plaster until a thin layer or island is formed on top. Once the island is formed, let it sit for 2 minutes before stirring with a stir stick.

This plaster was a good consistency and yielded a blemish free cast.

To ensure the plaster is the right thickness, stick a gloved finger or clean stir stick into the mixture. If the plaster that sticks to your finger is opaque, then the mixture is ready.

Here the plaster is far too thin. Finger looks like it was dipped in water.

The finger is covered by an opaque layer of plaster. Perfect.

Pouring Plaster

Once the plaster is ready to be poured, slowly pour into the negative without offsetting the mandrel position. You can use a small container to scoop out small portions of plaster at a time or if you are confident, remove the mandrel and place it back after the cast is filled. Gently tap the side of the cast during filling to remove air bubbles.

If there is any plaster left in the bucket, lay the bucket on an angle to make it easier to remove later.

Here we discovered that a whisk is not an ideal plaster mixing tool. This plaster is also far too thick.

It was very difficult to pour and trapped air pockets that had to be filled in later.