In ceramics, potters make crucibles to melt frits, stains and other materials. Crucibles are made from refractory materials that are stable against the material being melted in them.
Crucibles are most commonly used in metallurgy but are also important in ceramics. Potters and industrial manufacturers make crucibles to melt frits and calcine stains and other materials.
Crucibles must, of course, have a much higher melting point than the material being melted (or otherwise processed) inside of them. This resistance to temperature takes precedence over any other desirable property. Crucibles also must be resistant to corrosion of the material being processed (especially if it becomes highly fluid) so that it cannot dissolve its way through vessel walls. Be sure to do tests on smaller melts before attempting a new-material melt in a big crucible.
Another issue is thermal shock. Crucibles are often thick-walled and heavy, thus, by nature they are susceptible to cracking if exposed to rapid temperature changes. So, by necessity care must be used when heating and cooling them.
Ideally crucibles should be made from a material having a good resistance to thermal shock, but this is not always practical (e.g. alumina, zircon are extremely refractory but do not have good thermal shock resistance). If a sudden temperature change causes a crack while the material inside is still molten you have a definite problem! In industry, crucibles are often held at temperature for their entire lifetime of use so as to avoid cracking during cooling and heating.
You can make your own crucibles from materials that have a high melting point. They should be prefired to a higher temperature than what they are going to be used at.
Porcelain is a common crucible material. But common porcelains contain feldspar to make them vitrify (e.g. 50% clay, 25% feldspar, 25% silica for cone 10). Obviously some of feldspar should be substituted for other materials (e.g. mullite, alumina, pyrophyllite, zirconium silicate) to make the fired product more refractory. Leave enough feldspar to create a dense and durable product at the temperate you want to work at. The silica portion should be as fine as possible to avoid quartz inversion issues.
Fire clay is also commonly employed. However test to be sure the fireclay you use can withstand the temperature (some common fire clays are called such but in actual fact are not). Fire clay is often high in silica so thermal shock resistance could be low. By itself a fireclay may not fire to a dense matrix, thus it will have some porosity and could absorb some of the material being melted (if it is fluid enough). Fireclays can also contain iron particles and soluble salts that have the potential to contaminate certain melts.
Super duty crucibles can be made from pure alumina or zircon. Of course, they are non-plastic. However an addition of 3-4% Veegum will make them plastic enough for forming (or even throwing on a potters wheel).
Cordierite is very refractory and has very good thermal shock properties and could be an ideal solution for many types of melts. For others it may not be as resistant to attack by the melt. However the formation of cordierite crystals in the kiln is beyond the temperatures achievable in most production or pottery kilns.
A good way to determine a crucible body recipe is to go the websites that sell crucibles for the purpose you need. They will have information in their descriptions that we tell you what they make them from.
I mixed a cone 6 porcelain body and a cone 6 clear glaze 50:50 and added 10% Mason 6666 black stain. The material was plastic enough to slurry, dewater and wedge like a clay, so I dried a slab and broke it up into small pieces. I then melted them at cone 6 in a zircopax crucible (I make these by mixing alumina or zircopax with veegum and throwing them on the wheel). Because this black material does not completely melt it is easy to break the crucible away from it. As you can see no zircon sticks to the black. I then break this up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. As it turned out, this mix produced specks that fused too much, so a lower percentage of glaze is needed. I can thus fine tune the recipe and particle size to theoretically duplicate the appearance of reduction speckle.
In the ceramics industry, clays that are resistant to deforming and melting at high temperatures are called fireclays. Kiln bricks are often made from fireclay.
In the ceramic industry, cordierite is a man-made refractory crystalline material having extremely low thermal expansion.
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