Zirconia crucibles are made of zirconium dioxide powder put into a mold, stabilized with a small percentage of calcium or yttria, pressed into shape by hot isostatic pressing and annealed. There are different grades in regards to density or porosity. Besides crucibles for the precious metal industry zirconia is also used in other technical applications e.g. artificial teeth and bone replacements as well as fuel cell membranes.
Due to the raw form as a powder many different dimensions and geometries can be produced. Zirconia enables melting and smelting at high temperatures up to 2700 °C and is very inert. Therefore unwanted reactions with the pure precious metal product do not occur (with exception to palladium). It is highly resistant to most acids. Even hydrofluoric acid and sulphuric acid only slowly affect the material.
It is suitable for melting gold, silver, platinum, rhodium, different alloys and mixed metals (recycling). Especially in melting platinum and rhodium it is widely used due to the very high temperature range and the high amount of cycles a zirconia crucible can be used for in this application. Only palladium reacts with zirconia in a way to make the finished product brittle and slightly porous.
On the downside zirconia is not suitable for use in inductive heating and has a low thermal shock resistance, so only slow heating curves can be run with this material. Slow heating and slow cooling is advisable.