Pigments, Paint and Painting: A practical book for practical men

by George Terry

The whole group of oxide reds is of foremost importance, by reason of their good colour, covering power, and durability, besides which, being mostly bye-products of much more important manufactures, their cost is reasonable.

The methods of preparation of oxide reds vary slightly in detail according to the material from which they are made, but the general features of the processes are almost identical and eminently simple. The principal sources are impure native oxides of iron, such as the ochres, various waste liquors containing iron salts in solution, and copperas (protosulphate of iron).

(1) Native oxides. The iron present in the ochres and similar native earths exists in the form of hydrated oxide, and has a brown red colour. For many purposes this hue is satisfactory, and the preparation of such a pigment consists simply in grinding the mineral in a wet mill, subjecting it to levigation till all grit is removed, and drying.

In order to obtain a brighter red from the native oxides they must be calcined to effect dehydration. This can be accomplished in the most rudimentary forms of furnace, and many kinds are in use. The colour produced depends on the degree and duration of the heat to which the material is exposed, the shade becoming deeper as the roasting is prolonged or the temperature increased. As no two samples of ochre are just alike it is impossible to fix a precise time for the length of the operation, and therefore it is necessary to repeatedly draw samples in order to judge of the progress of the dehydration and development of the colour desired. When the requisite shade is attained, the charge is drawn and allowed to cool.

(2) Waste Products. The pyrites cinders from sulphuric acid works afford an abundance of oxide of iron. When the pyrites has contained no copper, the cinders merely require grinding and levigating, the iron being present as oxide. But when the pyriteshas been treated for the recovery of the copper, by a second roasting with salt, the liquors contain the iron as chloride and sulphate, and lime has to be added to precipitate the oxide. This last is dried and calcined in the same manner as the native oxides, and grinding and levigation can be dispensed with.

The liquors from galvanising works contain acid sulphate of iron (green copperas) in solution. To correct the acidity, more iron is added in the form of scrap. Then lime or other alkaline substance is introduced to throw down the iron as oxide, and this last is filtered out, dried, and calcined in the usual way.

(3) Copperas. Where beds of common iron pyrites occur, the iron sulphide is converted into sulphate by exposure to the oxidising influence of the air. The result is an acid sulphate of iron, which is leached out and neutralised by addition of more iron in the form of scrap. The neutral sulphate is crystallised out of the liquor, and calcined in a muffle furnace, the shade of the ultimate product being governed by the degree or duration of the roasting. The sulphurous acid liberated in the roasting is sometimes utilised for making sulphuric acid, but is more often wasted, because, to be commercially successful, the sulphuric acid manufacture must be conducted on a large scale, demanding 100l. of capital for every 1l. necessary for the copper and red oxide fabrication.

Persian Red.—A name which is used somewhat indiscriminately both for Derby red (p. 145) and for oxide red (p. 150).

Realgar.—The native mineral realgar is a yellow-red bisulphide of arsenic, often called also ruby of arsenic, or arsenic orange. It occurs native in very limited quantities in some of the older rocks, and then only requires to be ground and levigated. But for painters’ purposes it is prepared artificially by heating a mixture of sulphur and arsenic in such a way that they are melted in company and react on each other to form the arsenic sulphide. The heating takes place in crucibles, and the proportions are two parts by weight of arsenious acid (white arsenic) to one of flowers of sulphur. When the reaction has ceased, the contents of the crucible are allowed to cool, and then reduced to very fine powder.

The pigment is exceedingly poisonous and not remarkably durable, besides which, it cannot be mixed with any other pigment which is affected by sulphur.