|Monthly Tech-Tip |
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.
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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. The refractory character also imparts physical rigidity. Crucibles also must be resistant to corrosion of the material being processed (especially if it becomes highly fluid) so that they cannot dissolve their way through vessel walls. Be sure to do tests on smaller melts before attempting a new-material melt in a big crucible.
Another huge 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 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). In industry, crucibles are often held at temperature for their entire lifetime of use so as to avoid cracking during cooling and heating. Smelting crucibles are designed to avoid asymmetrical temperature gradients, and come with preheat treatment and filling recommendations.
You can easily make your own crucibles from materials that have a high melting point, prefiring them as far above the service temperature as possible. Simple fire clay is commonly employed. However, be sure the fireclay you use can withstand the temperature (just because something is called "a fireclay" does not necessarily mean it actually is). Fireclays are often high in quartz so thermal shock resistance will be poor. Fireclays may not fire as dense as needed, and thus will have some porosity. 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 oxide or zircon, these are available at any ceramic supplier. They are non-plastic by nature, but when processed to sufficiently small particle size they can be surprisingly formable and castable (zircon opacifier is very fine, and so are common grades of alumina). Binders (e.g. bentonite, smectite) can also be added to make them plastic enough for forming (or even throwing on a potter's wheel).
Kaolin, ball clay and silica are the most commonly available ceramic materials and they are all very refractory (because of their purity). Just about any mix of them if more refractory than almost any fireclay. Actually, most fireclays are just coarsely ground ball clays. An equal-part mix of the three produces a body of great workability and is highly refractory. But, such a body does not have good thermal shock resistance. Porcelain is a common crucible material, it employs these three materials and adds enough feldspar to produce some vitrification. Other materials can be incorporated also (e.g. mullite, alumina, pyrophyllite, zirconium silicate). In all of these, the silica portion should be as fine as possible to aid in the formation of silicates (minimizing the amount of residual unreacted quartz particles).
As noted, a crucible being refractory has nothing to do with its thermal expansion. True, thermal shock resistance in a crucible is had in ware that is sintered, not vitrified or melted. And it is the thermal expansion characteristics of the individual grains that determine that of the whole vessel. For low expansion, the material of choice is tabular alumina. Its particle size distribution will determine the density and collective particle bonding. Binders are needed to form it but are kept to a minimum. Of course, the vast majority of companies and potters do not have kilns that can fire high enough to sinter-bond pieces, so a compromise is often in order: Fire-bonding the particles with a low expansion flux, perhaps small amounts of borax or a frit (e.g. Ferro 3249).
Cordierite is very refractory and has very good thermal shock properties and could be an ideal crucible 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 also beyond the temperatures achievable in most production or pottery kilns.
A good way to determine a crucible body recipe is to go to 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.
|By Tony Hansen|
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