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. Derby red possesses great covering power and considerable brilliancy; but if not very carefully washed it is liable to retain a little alkali, which renders it unstable. Otherwise, it well resists damp, strong light, and impure air so long as sulphuretted hydrogen is absent. Taken altogether it is not one of the best red pigments, and its consumption is declining. INDIAN RED.--This is one of the names for the red pigments due to oxide of iron, and is described under oxide red, p. 150.