Baking Plaster Investment: Calcining the Mold

Baking Plaster Investment:
Calcining the Mould

Derek G. Kerslake
©February, 2001.

Dewaxing plaster 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 - 60 minutes. When the water cools, remove solidified wax from the water surface for reuse.

Calcining (baking) plaster investment.

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.
I have found a converted electric kiln an excellent device to calcine the mould and I leave the mould in the kiln during the pour, closing the lid to cool the bronze as slowly as possible after filling the mould. Unlike steel, which hardens when cooled quickly, bronze is softened by this process. The slower the bronze is cooled, the more durable the metal *casting.

Pouring the bronze.

Once the investment has been brought up to 1200 degrees Fahrenheit, the bronze can be poured. Thermal shock to the mould is minimized by pouring when the mould is still warm and there is something to be said for a warm mould preventing freezing of the bronze before the pour is complete.
I have poured in a cold and hot mould with successful results but I prefer the hot mould for delicate castings to error on the side of safety regarding premature solidifying during the pour.

Breaking out the bronze sculpture

When the mould has cooled sufficiently to be handled safely, the metal flask is struck with a rubber mallet to help shake out the investment. Take care with this procedure if the casting has delicate sections. When the casting is out of the flask, investment can be scraped off with a wooden tool and brushed with a copper or brass bristle brush. Larger castings can be cleaned with steel bristle brushes and a final sand blasting will leave the casting completely free of investment and ready to remove the vents and sprues, (chasing).

REMEMBER
Silica is not to be inhaled so wear a dust mask whenever you work with plaster investment.

Derek Kerslake