The presence of copper would point suspiciously to adulteration, and in any case should suffice to condemn the sample for use.
Naturally there is considerable variety of tint among the many kinds of terre verte, but they all belong to the pale greyish class, and are more or less translucent, consequently their covering power is small. Their value lies in their durability, and the resistance they offer to the injurious effects of strong light and impure atmosphere. They can be employed either as oil or water colours. The only preparation to which the natural pigments are submitted is fine grinding and washing.
Titanium Green.—An excellent dark green pigment, though rather costly, can be prepared from rutile or any titaniferous iron ore by the following method:—
The ore is dressed clean, and fused with twelve times its weight of acid sulphate of potash in a crucible. When cool, it is reduced to fine powder, and digested at 120° F. in dilute hydrochloric acid (half water) until solution is complete. The hot solution is filtered off from the residue and carefully evaporated down to a syrupy consistence, when the nearly pure titanic acid is allowed to cool in the dish and thrown on a filter. When sufficiently drained, it is boiled in a large volume of water containing a little ammonia, and the precipitated titanic acid is filtered and washed.
If an ore is used containing carbonate of lime, it must first be treated with dilute hydrochloric acid before the sulphate of potash is applied.
The titanic acid on the filter is next mixed with a concentrated solution of sal ammoniac, and again filtered. Then it is digested in dilute hydrochloric acid at 120° to 140° F. till the solution is complete. On adding ferrocyanide of potassium to the acid liquor, and bringing quickly to a boil, a precipitate of ferro-cyanide of titanium is thrown down. This is very carefully and slowly dried, at a temperature never exceeding 200° F.
Verdigris.—The chemical examination of verdigris shows it to be a basic hydrated acetate of copper, containing variable proportions of the bibasic and tribasic acetates.
Commercially it is prepared in districts where acetic or pyroligneous acid can be had at small cost. Thin pieces of scrap copper are subjected to the action of fermenting grape skins in mass, or cider refuse, for a fortnight or three weeks; or to the influence of pyroligneous acid for four or five days. By this means the copper surfaces are attacked by the acetic acid being generated or liberated, and become coated with acetate of copper. At intervals the pieces are removed, and surfaces are cleaned of the accumulated acetate or verdigris and this is repeated till the metallic copper has thus been completely converted. The collected verdigris is washed, and carefully dried at a very low temperature.
Its composition is subject to many irregularities, and the colour varies from green to bluish green according to the proportion of sesquibasic acetate present. It is one of the least permanent pigments, especially in the presence of water, and is exceedingly poisonous. At one time it was largely used as a pigment, but is now gradually going, if indeed it has not already gone, out of use. It can be distinguished by its solubility in acids and ammonia, the latter giving a deep azure blue solution. On being heated, it turns black, owing to its parting with acetic acid and leaving the black oxide of copper behind. This should be entirely soluble in nitric acid, the solution giving the characteristic tests for copper. The solution should give no precipitate with chloride of barium or nitrate of silver, and the original pigment should be freely soluble in any acid and in ammonia without effervescence.
Verditer.—Green verditer is another of the copper greens which has practically disappeared from the modern painter’s list of pigments. It is a yellow tinted very fugitive colour, consisting of a basic carbonate of copper, and is manufactured by treating copper solutions with carbonate of soda, or of potash.