• Materials for Ceramics

Molochite

Description: Highly Calcined Alumina Silicate

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 the 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 becomes 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 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 to 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).

Fine powdered versions of molochite are not equivalent to calcined kaolin, the latter is fired only high enough to alter mechanical properties (e.g. remove plasticity, change the character of the powder, whiten the color of the powder).

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 is no improvement. While the grog addition reduces drying shrinkage here 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. Notice another issue: The jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.

Make your own molochite porcelain body

The grogs shown here are Molochite 16/30, Christie Minerals STKO 22S and F65 Silica Sand. I wedged 150g of the molochite into 2300g of pugged porcelain (Plainsman M370). This calculates to 7.5% grog (based on 22% water content of the pugged clay). This produced the texture shown. Wedging a grog (or silica sand) into a soft clay body is easy, just sprinkle it on the table and wedge the clay over it. With each push it picks up more, the process is amazingly effective at quickly producing a homogenous material. If the clay is stiff, just moisten the aggregate. Knowing that pugged clays have about 20% water it is easy to calculate a grog addition: A 5 kg slug of clay thus contains 4 kg of powder. To add 10% grog you would add 400g. To add 10% of the total it would be 4000*10/90=444g. How much did this 7.5% grog reduce the drying shrinkage of this body? About 0.5%.

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