Simple Investment Mould Materials

There are a lot of differing recipe options for making plaster moulds. A simple general purpose investment mould making material and method follows:

Equal parts of powdered silica (sometimes called silica flour or flint), plaster of Paris and water by weight.  For example:

1 kilo silica
1 kilo Plaster Paris
1 kilo water

(Do not measure by volume)

Mix silica and plaster of Paris dry in separate bucket by hand.  If you can use a closed container that is best.  Otherwise use breathing protection and do the mixing outside.  Silica is very bad for your health.

Measure the water into a separate bucket with enough volume for three times the amount of water. Slowly sprinkle the entire contents of the dry mix into the bucket of water.  Do not dump it in!

Let the mixture sit for 2 minutes (slaking).  Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.

When mixed thoroughly, pour carefully and slowly into a corner of the mould box or container to reduce the occurrence of bubbles within the investment material or against the master.

When the pour is finished, tap the mould container to encourage any bubbles to the surface.

You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.

For pate de verre, you can use the mould almost immediately.  For casting, it is important to have a dry mould.

Let the whole air dry. Depending on the temp, humidity and density this can last from several days to several weeks. A way to tell how dry the investment is, is by weighing the mould when it has just hardened. When it has lost on third of its weight (the water component), it is ready for kiln drying. This removes the chemically bound water from the investment material. 

This is only an outline of what to do.  Investment moulds are extremely complicated in their chemistry, physics, and use.