Derby Red.—As a basic chromate of lead, often known as chrome red, Derby red is closely allied to chrome yellow, the preparation of which is described in a subsequent chapter.
It has been asserted that all the chrome reds, from the darkest cinnabar red to a lustreless minium red, are distinguishable simply by the size of the crystals composing the powder, as may be easily determined under the microscope, and that if various chrome reds of the same hue, but with different intensities of colour, are reduced by grinding to the same degree of comminution, the several powders will possess exactly the same degrees of intensity of coloration, though they lose in brightness. Therefore the conditions which give brilliancy and intensity of colour are those which favour crystallisation.
On this supposition it is recommended by Riffault to adopt a plan which dispenses with agitation, and he supports the following method:—
(1) Chrome yellow is precipitated in the usual manner, as described in a later chapter, without sulphuric acid, and is carefully washed. After draining, the mass is well stirred, and six or eight equal samples are drawn from it and put into glass vessels of equal size and thickness of structure. To each sample is added a different volume of caustic soda or potash lye, marking about 20° B. For instance, to 5 volumes of paste are added 2, 2½, 3, 3½, 4, 5, &c., volumes of lye. The different mixtures are rapidly and thoroughly agitated, but the chemical reaction is allowed to take place without any disturbance. After examination of the quality of the products, the relative proportions of pulp and lye are noted down for the best hues obtained. Too much lye will fail to deepen the red colour; in fact, Derby red is entirely soluble in an excess of lye, and forms needle-like crystals holding potash when the caustic solution has absorbed carbonic acid from the air.
On the industrial scale the operation is conducted in a large tub, which receives the mixture of pulp and caustic lye in precisely the proportions found by experiment to give the best results. The changes in colour soon manifest themselves, and the whole reaction is completed in about 12 hours. At the end of that time, the lye is drawn off, and carries with it much of the chromic acid. The precipitated pigment is carefully washed with pure water once in the tub, and the mass is gently stirred. The washing is continued in the filters by throwing water upon the pulp, and in this manner there is less friction between the crystals, which retain their deep colour. Of course a highly crystalline dark red cannot possess great covering power.
(2) Prinvalt mixes together 100 lb. of lead carbonate and 30½ lb. of potash bichromate neutralised with caustic potash, in 50 gallons of water, leaving them in contact for a couple of days under repeated agitation. About half an hour’s boiling then suffices to develop the red colour. After settling, the supernatant liquor is drawn off, and the precipitated pigment is washed twice with pure water and finally with acidulated water (1 lb. sulphuric acid in 10 gallons of water), and dried.
There are several other recipes published which differ in detail from (2), but they do not demand a lengthy description.
(3) 100 lb. of lead carbonate (white lead) made into a paste with water, then added to and boiled with a solution of 50 lb. of potash bichromate and 15 lb. of caustic soda of 77 per cent. Remainder of process as before.
(4) 4 cwt. of lead monoxide (litharge) and 60 lb. of salt dissolved in 50 gallons of water, and left with agitation for 4 or 5 days; then boiled for 2 hours with solution of 150 lb. of potash bichromate.
(5) 100 lb. of lead carbonate (white lead) made into a paste with water, then added to a solution of 30 lb. of potash bichromate and 12½ lb. of caustic soda at 77 per cent., and boiled.