Bronze Casting - Step 6
Making the Mold: Mother Mold, The First Half

One you have the rubber mold made, the shims in place, and the whole thing greased up, you start adding plaster to one side. This is a bit of a messy operation since at first liquid plaster is thin and runny. The best plaster is called molding plaster which hardens rapidly and and is stronger than much of the "craft" plaster you get in the craft stores. Such lower quality plaster hardens to a chalky crumbly consistency which alas, is the type of plaster (inadvertently) used here. But is was still adequate. Large molds need to have a strong firm mother mold.

As the plaster sets it becomes thicker and can be spread on as a smooth paste. Toward the end it sets up faster and so you have to move fairly quickly. You also have to trim away excess plaster from the base before it completely sets hard. To get a good mold requires practice and patience - and not a few discarded pieces before true skill is acquired.

Now mother molds are sometimes made with synthetic resins. They have the advantage of being less messy and the components are liquids. So you don't have to worry about breathing in fine particles. On the other hand, plaster tends to be quite heap. A current price for 100 pounds of plaster is between $30 and $40. That's about the price of 1 pint of a synthetic mother mold resin. On the other hand (you get a lot of "on the other hands"), you need less of the synthetic material to make a rigid mother mold. Take your pick.

An intermediate method between the traditional plaster and modern synthetic rigid shells, is to use plaster gauze. This is gauze with impregnated plater that when wetted can be laid across the rubber mold and allowed to set. Again cost is higher than plaster per se, but it is less wasteful.

It should also be obvious that 1) more complex mother molds might have to be made in more than two pieces and 2) as we said before, sculpture in general and mold making in particular are great ways to trash up your house. Ergo the importance of having a suitable location where said trashing is not an issue.