=Chlorine and Steam Process.=—Chlorine does not attack iron at 150° to 250° C. The reaction is carried out in heated iron boxes 1 metre wide, 2 metres long and 65 centimetres high. The blue is spread out in a layer 2 centimetres thick on earthenware plates, which stand one above another at a distance of 5 centimetres, each supported on three feet. The plates are introduced by means of iron tongs through large openings in the top of the box; after filling iron plates are screwed on to the openings, through them pass sheet-iron tubes reaching to the bottom, in which thermometers can be lowered on wires. The iron boxes stand in heated chambers, which have openings corresponding with the openings in the boxes, and shut off from the interior of the furnace. Chlorine and steam are led into the iron boxes at both ends through lead tubes reaching to the bottom; the parts of these tubes in the furnace and the iron boxes are protected by wide sheet-iron tubes, the space between being filled with clay. The gases evolved, hydrochloric acid, sulphur chloride and steam, pass from the covers through earthenware pipes into boxes filled with limestone, upon which water drops, and thence to the chimney. After filling, the boxes are heated to 280° C. and steam introduced to remove sulphur; they are then allowed to cool to 160° C., when chlorine and steam are led in for three hours. The violet is then finished, and the residual gases are blown out by a fan. =Hydrochloric Acid and Air Process.=—The operation is conducted in the same iron boxes, upon the bottoms of which, beneath the openings, are earthenware dishes into which hydrochloric acid can be poured through earthenware pipes. The chimney draught must be strong enough to draw sufficient air through these pipes into the boxes. After the temperature has been maintained for seven hours at 220° to 230° C., hydrochloric acid being poured in from time to time, the blue is changed to a dull violet, which becomes brighter when hydrochloric acid is repeatedly added at diminishing temperatures, 210°, 200°, 180° and 160° C. More recently the iron boxes have been replaced by stone chests or by chambers above the ultramarine furnaces, the temperature in this case being kept at about 200° C. =Ammonium Chloride Process.=—A mixture of ultramarine blue with 5 per cent. of ammonium chloride is heated during fourteen days in porous crucibles placed in the upper chambers of the ultramarine furnaces, when the contents of the crucibles become a handsome violet throughout. If sodium nitrate is used together with ammonium chloride, the violet is formed in a much shorter time. After washing, the violet still contains nitrogen; on ignition or heating with strong alkalis it loses ammonia. Unfortunately, this fine violet is decomposed by the continued action of moist slaked lime. The violets made by the first two processes absorb ammonia when this gas is led over them at 180° to 200° C., and it cannot be removed by washing.