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Cordierite ceramics are well known for their low thermal expansion and refractory character. Although cordierite is available as a powder, when we use the term we are generally talking about ceramic that went into the kiln as ordinary composite of ceramic powders but emerges as a cordierite crystalline matrix (grown by the firing process tuned to its needs).
Cordierite makes the ceramic manufacture of products like catalytic converters possible. They constantly heat up and cool down quickly and must not crack because of the shock. These converters get their thermal shock resistance properties from a bonded matrix of low-expansion cordierite crystals. These crystals form during firing between 1300 to 1400C (interestingly, cordierite crystals have a lesser thermal expansion along one axis than another). Thus, the better the density of the precursor mix and controlled the firing the better the properties of the cordierite material are.
The ideal chemistry to produce the crystals 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 within the reach of many ordinary kilns) and soaked may produce some crystal development. However firing (actually sintering) at 1400C+ with significant soaking time is needed for the best results. Thermal expansion measuring equipment is needed to determine if your firing is actually developing the crystal matrix or not.
As noted, a continuous crystal matrix is the object of cordierite creation (not just the presence of the crystals in an otherwise typical ceramic). Notwithstanding this, cordierite powder is available, it is prefired and finely ground. It can be plasticized for forming and bonded during firing (at much lower than normal cordierite creation temperatures) to produce a ceramic that certainly has lower expansion.
The practicality of cordierite, as a refractory, can be better appreciated when comparing it to alumina. Although alumina by itself, is more refractory than cordierite, it is also much more prone to cracking when subjected to thermal shock. By comparison, cordierite has low expansion and much less expensive. No wonder that cordierite kiln shelves are common.
While cordierite ceramic vessels could be made (e.g. ovenware), 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).
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
Co-efficient of Thermal Expansion
A measure of the reversible volume or length change of a ceramic material with temperature. The more the expansion during heating the more contraction must occur while cooling it back down. Expansion values are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which is 0.00000065). Typ...
Refractory, as a noun, refers to a material that does not melt at normal kiln temperatures (of the industry being referenced). The term also refers to the capacity a material to withstand heat without deforming or melting. Kiln shelves and firebricks are refractory. Many natural clays and minerals a...
Cordierite at Wikipedia
Ceramic Industry article on synthesizing cordierite
How catalytic converter cordierte substrates are made
In Bound Links
Talc - Mg3Si4O6 or 3MgO.4SiO2.H2O
Magnesium Silicate, Steatite, French Chalk, Hydrated talc
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Crucibles must, of course, have a much higher melting point than the material being melted (or otherwise pr...
Flameware is ceramic that can withstand sudden temperature changes without cracking (i.e. stove top burners). Ovenware is another class of ceramics, it is not as resistant to thermal shock as flameware. There is some confusion among clay buyers and retailers about this. For example Japanese Donabe w...
By Tony Hansen