Its preparation from washed rutile or iserine is effected by the following process: - The clean ore is melted with 12 times its weight of acid potash sulphate in a Hessian crucible. After cooling, the melted mass is powdered, and digested until dissolved in hydrochloric acid diluted with 50 per cent, of water maintained at 122° F. (50° C.) The hot solution is filtered from the insoluble residue, and the filtrate is evaporated until a drop of the liquor, put upon a piece of glass or porcelain, becomes of the consistency of a magma. The whole is allowed to cool off in the porcelain dish, and the magma, composed of nearly pure titanic acid, is thrown upon a filter. The drainings are again evaporated, and furnish a new portion of titanic acid. When the magma has been sufficiently drained, it is mixed with a large volume of water holding a little ammonia, in order to prevent formation of a basic iron salt. This liquor is kept boiling for a long time, and the precipitated titanic acid, after filtration and washings, is nearly white. After several similar treatments with potash bisulphate, it may be obtained entirely free from iron. As iserine contains generally some lime carbonate, it is advisable to digest with dilute hydrochloric acid before treating with the acid potash sulphate.
A concentrated solution of sal-ammoniac is poured upon the magma, and, after a thorough mixing, filtered. The titanic acid remaining upon the filter is digested in dilute hydrochloric acid, and kept at 122° to 140° F. (50° to 60° C.) until the solution is complete. The acid liquor, after addition of potassium ferrocyanide, is rapidly brought to a boil, and forms a precipitate of titanium-green, which is washed with water holding a little hydrochloric acid. The solution of titanic acid must be very acid, because if pure water be employed, and the ferrocyanide poured upon the magma, the precipitate will be a yellowish-brown, becoming green by ebullition in dilute hydrochloric acid. The green precipitate becomes white with ammonia. The liquor, filtered from the green precipitate, still contains a certain quantity of titanic acid, which ammonia will separate in the shape of a white flocculent precipitate. The dry titanium-green, obtained either from rutile or iserine, is a dark-green powder. It is decomposed at 212° F. (100° C). Its desiccation should therefore be carefully conducted. By this method, any titaniferous iron ore will produce a green as handsome as that prepared from rutile.
Verdigris is a basic hydrated copper acetate, composed of variable proportions of bibasic and tribasic copper acetates. It is manufactured in France by oxidizing very thin pieces of old sheet copper, heated to 176° F. (80° C), with a solution of copper acetate, and then immersing them in the skins of pressed grapes, which are in a state of acetic fermentation. After a time, the copper plates are removed from the skins, dried in the air, dipped into water, and again laid in layers of grape-skins. When this has been repeated 5 to 7 times, the verdigris is scraped off, kneaded in wooden troughs, and packed in leather bags. Its desiccation is completed in the air. It is also prepared by covering copper plates with vinegar It is a pure green or bluish-green, according to the proportion of sesquibasic acetate it contains. It is highly poisonous, and not durable.