Bronze Casting -
"Investing the Model
(Creating the Shell)"

After the wax model is created and the sprue, cup, and the vents added and (if needed) mounted on a grid or framework, then the model must be coated with a ceramic shell - called "investment" of the model. Investment requires the wax to be coated with a special suspension of colloidal and suspended silica and then covered with sand of varying degrees of fineness. (Very hazardous to your health if the powder is inhaled. Dust mask are a minimum requirement, but respirator cartridges and dust collection systems are recommended.) The suspension - which is in water - must have a wetting agent - since the liquid needs to fully coat the model.

Usually the wax model is dipped into a vat of the suspension although for big pieces you can brush on the liquid. There are practical limits to the size you can work with and all large sculptures are cast in pieces and welded together.

After excess liquid is drained off of the model - but the surface is still coated with the silica suspension - the model is then coated with the sand. You start off with very fine sand - again breathing protection is a must. The sand sticks to the surface and the model is allowed to dry (3 hours is recommended). Then dipping and covering with the fine sand repeated.

The dipping, coating with sand, and drying is repeated with increasingly coarse sand until you have a nice thick shell. The picture here shows Dip #2 and Dip #6. After Dip # 5 you wrap the model with thin steel wire. This adds extra support so if the shell cracks during the bronze pour, all is not lost, and perhaps a bit more grinding and "chasing" of the bronze will (hopefully) smooth out any imperfections.