[652] Spence, Craig, and Spence, _E._ 13260, 1911. [653] Stähler and Bachran, _Ber._ 1911, ~44~, 2912. [654] Kunheim and Co. and Stähler, _D. R. P._ 284251, June, 1912. Titanium compounds have frequently been suggested for the preparation of colouring-matters; the ferrocyanide has a fine green colour, and is used to some extent in place of arsenical pigments for the preparation of coloured wall-papers, whilst the dioxide is of some value for tinting artificial teeth, porcelain tiles, etc. Yellow and reddish-yellow pigments are produced from rutile and ilmenite by various methods. A fine covering paint is said to be obtained by a process[655] in which ilmenite is powdered and roasted to 500°C.; the cooled product is crushed with water, and after one or two washings to remove soluble compounds, yields a very finely divided orange-yellow suspension, the precise shade of which varies with the duration and temperature of the roasting. The product is at once thrown down from the suspension, by addition of a small quantity of a salt solution, and so can easily be obtained in the solid state. In another process,[656] the pulverised ilmenite is warmed with concentrated sulphuric acid, in which it dissolves with great development of heat; the excess of acid is removed by evaporation and the mass calcined to decompose the sulphates. It is stated that different shades may be obtained by carrying out the last operation in an atmosphere of sulphur dioxide or other gas. [655] Farup, _E._ 3649, 1910; _F._ 412563, May, 1910. [656] _E._ 10368, 1911. In connection with the colouring properties of the oxides of titanium, it is interesting to note that the blue colour of sapphires is probably due to the presence of compounds of trivalent titanium; Verneuil[657] has succeeded in preparing artificial sapphires in all respects identical with the natural stones by fusing alumina with small quantities of titanium dioxide and ferric oxide in the flame of the oxyhydrogen blowpipe, which effects the reduction. [657] _Compt. rend._ 1910, ~150~, 185. ~Other Uses of Titanium Compounds.~--Owing to the high price of the tin dioxide which is largely employed for the preparation of enamels and opaque glasses, innumerable suggestions have been made for the employment of the oxides of titanium and zirconium in this direction.[658] A critical examination of the question has been made by Grünwald;[659] he finds that the opacity consequent on addition of these compounds increases with the amount of clay used, within limits, and concludes that the effect is due to displacement of alumina by the oxides, with formation of silicates of titanium and zirconium, which dissolve in the melt. He states that the results obtained from the use of these oxides are not comparable with those given when stannic oxide is employed, and that therefore the former oxides are of little use for this purpose.