THE Transactions of the Ceramic Society, vol. xvi., part i., contain an interesting article by Mr. J. A. Audley on the above title. This substance, Zr02, occurs somewhat widely distributed, chiefly in the form of the mineral zircon, in which it is combined with silica. A more abundant source has recently been found in the mineral Baddeleyite, which contains from about 8o to 94, and even 98, per cent. of zirconia. This mineral was discovered in 1892 almost simultaneously by Hussak in southern Brazil and by L. Fletcher in Ceylon, the former deposit being much the more extensive and valuable. It also occurs in North America, Australia, and the Ural Mountains. Zirconia is also a by-product of monazite sands. Baddeleyite has a hardness of 65 and a specific gravity of 44 to 6. It is insoluble in acids, except hydrofluoric acid, but is easily attacked by fused potassium hydrogen sulphate. The melting point of raw zirconia is in the neighbourhood of 2OOO° C., that of the purified material being considerably higher. It can be both melted and volatilised in the electric furnace. It is a neutral substance of the same type as alumina and is said to have a high resistance to the fluxing action of both acid and basic slags. Its heat conductivity is remarkably low, and its coefficient of expansion on heating is nearly as low as that of quartz glass, both of which are very valuable properties. It resists the action of fused cyanides and alkalies.
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Dental Materials Journal (2019)