|Monthly Tech-Tip |
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.
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.
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%.
Materials used in Denmark
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.
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.
Granular materials intended to impart raw and fired structural strength or thermal expansion properties to bodies or deliver special speckling effects in glazes.
Imerys molocite web site
|Co-efficient of Linear Expansion||20-1000C: 44 X 10-4|
|Pyrometric Cone Equivalent||34-35, 1750-1770C|
|Density (Specific Gravity)||2.7|