• Materials

Molochite

Notes

Molochite is a pure white made-made granular material. The granules are hard and refractory. It is made by firing raw low-iron kaolin to very high temperatures (about 1500C) to bring about maximum conversion of the clay crystal to crystalline mullite (usually 95%+). The latter has high mechanical stability and resistance to thermal shock. Any free SiO₂ present become amorphous silica glass rather than crystalline quartz. The result is a material of very low thermal expansion.

Molochite is available in a wide range of sizes (from 8 to 325 mesh) and in dedusted form. It is a very uniform. It can be used as a very white firing porcelain grog and aggregate material (but be careful with the material you choose to be sure it has no iron particles). However, its chief use is in the investment casting industry, where successive coats of increasingly coarser molochite slurry are applied onto wax models. After drying, the wax is melted out and the molten metal poured in.

Since molochite is used for mechanical purposes in most applications, its chemistry is not usually a consideration (although it will have the chemistry of a calcined kaolin of course).

Not to be confused with "malochite" or "malachite" which is a green copper mineral.

Related Information

Do grog additions always produce better drying performance?

This DFAC test for drying performance compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise there appears to be no improvement. While the grog addition reduces drying shrinkage by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Another issue is the jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.

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By Tony Hansen