•The secret to cool bodies and glazes is alot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
Cordierite ceramics are well known for their low thermal expansion and refractory character. They get their properties from the bonded cordierite crystals that make them up. These crystals form during firing between 1300 to 1400C and the higher their percentage in the matrix the better the properties of the cordierite are. The following factors are critical in synthesizing the crystals.
-The ideal chemistry is 13.8% MgO, 34.8% Al2O3 and 51.4% SiO2 (2MgO, 2Al2O3, 5SiO2).
-Talc, kaolin and raw alumina powder can be blended to produce this chemistry (other materials are also used e.g. aluminum hydroxide, steatite and other MgO minerals). Compound materials (like kaolin and talc) react better than pure oxide materials (like MgO, Al2O3, SiO2). Materials of finer particle size react better.
-A simple firing to 1300C (about cone 11) and soaked will produce the crystals. However refinement of this for the material being used will give the best crystal development (firing may need to be as high as 1400C and soaking time may need to be an hour or more). Thermal expansion measuring equipment is needed to determine if your firing is actually developing the crystals or not.
The low thermal expansion material that the process creates has great resistance to failure induced by sudden temperature change. Alumina by itself, for example, is more refractory than cordierite, but also much more prone to cracking when subjected to thermal shock. Automotive catalytic converters constantly heat up and cool down; they are made from cordierite bodies. Cordierite is also refractory. Cordierite kiln shelves are common.
While cordierite ceramic vessels could be made, the material has such a low thermal expansion it is very difficult (or impossible) to match a glaze (without crazing). Notwithstanding this, many recipes can be found for cordierite ovenware or flameware. However these are seldom fired above cone 10 so any thermal shock resisting behavior they have is attributable to grog content or open fired matrix rather than to any cordierite development (which is of course not happening).
Interestingly, cordierite crystals shrink have a greater thermal expansion along one axis than another.
An electric kiln half shelf that costs $500!
This Advancer Nitride-bonded Silicon Carbide shelf is 26 inches wide (by 1/4 inch thick) weighs 9 lbs. These are incredible durable and strong. However there are cautions to their use. They can act as an electrical conductor so must not contact elements and should not be used in kilns with unpinned elements protruding from grooves. They must be stored in a dry place to prevent moisture penetration (which can cause a steam explosion during heatup). The company has a recommend drying schedule if shelves do absorb moisture (the application of kiln wash is not considered a prolonged exposure and is OK).
Out Bound Links
In Bound Links
Talc - Mg3Si4O6 or 3MgO.4SiO2.H2O
Magnesium Silicate, Steatite, French Chalk, Hydrated talc
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By Tony Hansen