BronzeSite Plaster Investment, Lost Wax Method

BronzeSite Plaster Investment Procedure

Derek G. Kerslake
©July 1998.

Investment plaster (measured by weight)

Note: This is a very fine, porous investment which requires an exterior metal sleeve for the entire procedure. Sheet metal pop riveted or stove pipe work well once sealed with wax to contain liquid investment as it is poured around the wax model.

4 parts silica flour (flint silica from ceramic supplier)
1 part plaster of paris (hardware store, building supply, hobby shop or ceramic supplier)

Mix the silica and plaster in a clear plastic garbage bag by closing the bag and rolling the mix thoroughly. Wear plastic gloves to protect hands from the caustic plaster of paris and a dust mask to avoid inhaling the silica and plaster dust. Now the investment plaster is stored in a dry location until needed.

To prepare the liquid investment for pouring around your wax model, mix:

1 part water
2 parts investment powder described above

Add investment powder to room temperature water until an island develops in the centre of the mixing container.
In both cases, let the plaster sit for 3 minutes, then mix thoroughly for 3 minutes before pouring investment down the inside edge of the metal flask surrounding the wax model and circulatory attachments and feeders.

Dewaxing investment

It is a good idea to steam the wax out of the investment by inverting investment flask and placing it on a metal rack over boiling water in a covered container for about 30 minutes. When the water cools, remove solidified wax from the waters surface for reuse.

Calcining (baking) the mould

Once again invert the flask, pouring cup up and place the flask in a kiln. It is very important not to heat the investment above 250 degrees F. until all the free water has evaporated. This procedure may require days depending upon the size of the project. Place a cool metal plate over the opening in the investment for 5 seconds and remove. When no moisture appears on the cool plate, slowly raise the temperature to 450 degrees F. and check again. When no mist appears on the cool plate, continue slowly raising the temperature to 1200 degrees F. or until the interior of the investment is a very dull red when viewed in darkness.

Filling the mould with bronze.

It is often recommended that investment moulds be filled when hot, about 1000 degrees; this is the method I use whenever possible to reduce thermal shock to bronze and mould and to keep moisture from re-entering investment material after calcining.

Derek Kerslake