A wide range of ceramic oxide materials can be used for crucibles, the type being dependent on the metal/alloy, the basicity of the melt and the foundry practice, whether erosion resistance or thermal shock resistance takes prominence.

The most common type, historically, are the aluminosilicate range; these were based on clays but now utilise the purer synthetic materials.  At one end of the range lies fused silica, with an excellent thermal shock resistance, whilst at the other extreme is fused alumina with a very good erosion resistance.

Magnesia generally offers better chemical and erosion resistance, but is better suited to the faster cycling times.  The most resistant material is zirconia, and there are many iterations of this to suit different applications.



Fused Silica, Fused Quartz

There are just subtle distinctions between FUSED SILICA and FUSED QUARTZ products, as they are both made with amorphous silica.  Both have an excellent thermal shock resistance, by virtue of an extremely low thermal expansion limiting the stresses that cause cracks.

Both can be used for similar applications, whether as a crucible for melting precious metals or as a saggar for annealing powders.

FUSED SILICA crucibles are made by standard ceramic methods. They are less pure and contain connected porosity. There is a working temperature limit of 1120oC. as,  above this temperature the amorphous silica gradually transforms to a crystalline form, cristobalite, and the resulting expansions can cause the crucible to crack.   The minimum silica content is approximately 98.5%, with a porosity of 20%.  They perform well for gold and silver melting in short runs.

FUSED QUARTZ crucibles, however, are made using melting techniques. Purity is greater at >99.9% SiO2, and there is no open porosity; some types have no closed porosity and are transparent. Consequently, they can be used at significantly higher temperatures, such as that required for melting platinum.

The more resilient type of FUSED QUARTZ contains micropores, a natural result of one particular forming process, which act as energy sinks and so help to increase the number of cycles that can be performed at high temperature; these are available in crucibles with a minimum diameter of 100mm.

Both FUSED SILICA and FUSED QUARTZ products are available in a large range of shapes and crucible sizes.