[603] Ibid. 174313, September, 1906.
By far the most important property of the oxide, from the technical point of view, is the ease with which it resists high temperatures. The natural oxide can be freed, to a very large extent, from the iron oxide which it encloses, by the prolonged action of hydrochloric acid; experiments were carried out on the material so obtained by Simonis,[604] who showed that by prolonged heating at a high temperature, the remaining impurities, chiefly ferric oxide and silica, could be volatilised, leaving the zirconia unchanged. Riecke[605] showed that whilst the oxide is very suitable for the manufacture of highly resistant crucibles, its use is restricted by the fact that it is easily reduced by carbon at high temperatures, forming the carbide.
[604] Sprechsaal, 1908, 41 (1), 210.
[605] Ibid. 214.
Weiss and Lehmann have carried out exhaustive experiments on the preparation of crucibles of zirconia.[606] They worked first with mixtures of zirconia and magnesia, with phosphoric acid as a binding material; the best results were obtained with a mixture of 90 per cent. zirconia and 10 per cent. magnesia, which gave extraordinarily resistant crucibles. Prolonged heating at temperatures over 1900°C. eliminated all the phosphoric acid by volatilisation; the crucibles could then be heated in the blowpipe flame and plunged immediately into cold water without cracking or breaking, and were not affected by fused sodium hydroxide or potassium hydrogen sulphate. Crucibles were also made with the addition of potassium and sodium salts, and were found to answer very well; platinum could be melted in them to a mobile liquid. Similar crucibles are already on the market.
[606] Zeitsch. anorg. Chem. 1910, 65, 218.
As early as 1904 the use of zirconia was suggested for coating muffles, retorts, and tubes which are required to withstand high temperatures.[607] In 1906 it was proposed[608] for the manufacture of crucibles in which rock-crystal (quartz) is fused for the preparation of quartz-glass, since zirconia is not attacked by molten silica. It promises to be of the greatest use in all cases where a very refractory material, stable towards the ordinary chemical reagents, is required.
[607] Pufahl, D. R. P. 156756.
[608] Heræus Co., D. R. P. 179570.