Old-fashioned pottery clay is basically just a mixture of silica mineral particles that melt into glass, or vitrify, at different temperatures. The material used for the ceramic investment in bronze casting is quite similar.

The silica is formulated so that it will vitrify at a higher temperature than the melted bronze so that it doesn't melt when the bronze is poured in.

It also must not contract or shatter during the initial firing process when the wax is "lost," and then when the bronze is poured.

Finally, the ceramic must be brittle enough to be relatively easy to break off once the bronze is cast.