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. CHAPTER XXII THE INDUSTRIAL APPLICATIONS OF TITANIUM AND ITS COMPOUNDS Though probably at least as plentiful in nature as most of the common metals, titanium has always, until quite recently, been regarded as one of the rare elements. Of its chemistry, very little indeed was known, and it is improbable, even now, that the pure element has been isolated. It had no technical value; indeed, its commonest ore, ilmenite or titaniferous iron ore, was sedulously avoided by manufacturers, who considered that even very small percentages of the element rendered an iron ore valueless because unsuitable for working in blast furnaces. Towards the end of the last century, one or two metallurgists had demonstrated that ilmenite, under the proper working conditions, would yield a pig iron of very good quality when smelted in the blast furnace, but it was left for the long and arduous researches of Kossi to show that the element is possessed of properties which render it very valuable for metallurgical purposes. Since the successful culmination of his work in the first few years of the present century, titanium has attained considerable importance in the treatment of special steels for rails, car wheels, crushing machinery, etc. At present, titaniferous iron ores are being worked on a large scale, and many titanium compounds are coming into use for technical purposes.