Crucible
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 they are also important in ceramics for melting frits and calcining stains and other materials. Crucible manufacture is a highly developed science, with specialized processes often beyond the resources of potters and small companies. Performance depends on more than the refractory material or recipe: Material purity, mineral phases, particle grading, binders and firing schedules all influence resistance to heat, thermal shock and chemical attack by the contents. Nevertheless, potters can make useful crucibles for experiments at temperatures their kilns can reach, provided they test them for the intended application.
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The other big issue is thermal shock and thermal conductivity. 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 taken 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. aluminum oxide, zirconia and zircon are extremely refractory but may not have good thermal shock resistance). Crucibles can also be made from materials, or a mix of materials, that, when fired, transform into the ceramic needed (e.g. mullite, cordierite). 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 make your own crucibles from materials that have a high melting point. Technically, they need to be "prefired sufficiently to establish a stable ceramic bond without excessive deformation or unwanted phase changes". But a common pottery kiln cannot do that, so it is only practical to prefire them as far above the service temperature as possible. Simple fire clay is commonly employed. However, be sure the fireclay 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. Thermal shock resistance is possible by a simple mix of mullite powder and kaolin (with only enough of the latter to give adequate plasticity to form a dense product).
Super duty crucibles can be made from pure aluminum oxide or zircon; these are available at almost any ceramic supplier. They are non-plastic by nature, but when processed to sufficiently small particle size, they can be surprisingly formable and castable. Ceramic binders (e.g. bentonite, smectite) can also be added to make them plastic enough for forming (or even throwing on a potter's wheel).
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As noted, a crucible being refractory has nothing to do with its thermal expansion. Using traditional ceramic forming methods and materials (including binding with plastic clays) it seems impossible that a thermally uncrackable crucible could be made. But in non-traditional ceramics, they rely on sintering, organic binders and pressing machines. An advantage of sintering is that individual grains of low-expansion material can be engineered into a desirable bonded microstructure (e.g. as is done with tabular alumina, a material whose grains themselves have already been fired at very high temperatures). With the right particle size distribution and grain orientation, a high density can be achieved. 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 (of course, a low-expansion glass is not automatically a suitable refractory bond as it may create a preferential corrosion path or contaminate a melt).
Cordierite is refractory and has very good thermal shock properties and could be a suitable crucible solution for many types of melts. For others, it may not be as resistant to attack by the melt. However, the formation of a cordierite crystal microstructure is also beyond the temperatures achievable in most traditional 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 about what material they make them from. Of course, the quality and suitability of their crucibles is also due to how they grade, mix, form, fire and test, factors that are mostly beyond the resources of a potter.
Related Information
Making your own crucibles to make your own speckle
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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 clay, dry and break into small pieces. I then melted this batch at cone 6 in a Zircopax crucible (which I make by mixing Zircopax with 3-4% Veegum, enabling throwing them on the wheel). This speckle mixture is easy to break away from the crucible because it does not completely melt and it does not stick to the zircon. I then break the black up with a special flat metal crusher we made (the shusher), size them on sieves and add them to bodies for artificial speckle. If specks fuse too much, I lower the percentage of glaze (and vice versa). Of course, the particles are glass, jagged and sharp-edged, so care is needed in handling them (a mask is essential).
Slipcast crucibles can be made from small to large sizes
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Cast crucibles can be made using a simple one-piece plaster mold. I use L4404A refractory slip. The casting process enables more flexibility in the clay recipe and easy application of an inner layer of alumina or zircon after the initial pour.
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 ChatGPT has an opinion about making my frits: But it missed a key factor this picture demonstrates |
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| Materials |
Calcined Alumina
A high-purity Al2O3 used in technical ceramics, refractories, glazes, and bodies for its extreme hardness, high-temperature strength, and chemical stability. |
| Materials |
Zircon
Zircon (zirconium silicate) is a hard, dense, refractory ceramic material used to opacify pottery glazes and in refractory products such as kiln washes, crucibl |
| Glossary |
Fireclay
A clay that withstands fire. 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. |
| Glossary |
Cordierite Ceramics
Cordierite is a man-made refractory low thermal expansion crystalline solid that forms at very high temperatures (in the right mix of kaolin and talc). |
| URLs |
https://en.m.wikipedia.org/wiki/Hessian_crucible
Hessian Crucibles These were made in Germany from the late middle ages, fired from kaolinitic clay to produce mullite, making them resistant to heat and thermal shock. The clay, being impure, may have contained that acted as a catalyst to form mullite better than pure kaolinite. |
| URLs |
https://www.capital-refractories.com/products/crucibles/
Crucibles for many purposes made by Capital Refractories |
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