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Description: Highly Calcined Alumina Silicate

Oxide Analysis Formula
CaO 0.06% -
K2O 2.00% 0.05
MgO 0.31% 0.02
Na2O 0.10% -
TiO2 0.07% -
Al2O3 42.00% 1.00
SiO2 54.50% 2.20
Fe2O3 1.10% 0.02
Oxide Weight 243.20
Formula Weight 243.20


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

Closeups of three grogs and the texture they produce in the porcelain

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%.


Minerals Mullite
A fibrous alumina silicate mineral of very low thermal expansion. Mullite is formed within porcelain
Minerals Amorphous Silica
Non-crystalline silica
Typecodes Aggregate
Granular materials intended to impart raw and fired structural strength or thermal expansion properties to bodies or deliver special speckling effects in glazes.
Typecodes Abrasive Resistant Super Hard Material
These materials are generally available in granular form, the particles are cemented together using frits to produce abrasive products. However, powdered and slurried forms of these materials can also be formed and fired by various means to produce hard materials.
Typecodes Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
Typecodes Materials used in Denmark
Materials Grog
Materials Calcined Kaolin
Imerys molocite web site


Co-efficient of Linear Expansion 20-1000C: 44 X 10-4
Hardness (Moh) 7-8
Pyrometric Cone Equivalent 34-35, 1750-1770C
Density (Specific Gravity) 2.7
By Tony Hansen
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