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acid is repeatedly added at diminishing temperatures, 210°, 200°, 180° and 160° C. More recently the iron boxes have been replaced by stone chests or by chambers above the ultramarine furnaces, the temperature in this case being kept at about 200° C.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.

Pale Blue Ultramarine.—If the violet is heated in hydrogen at 280° to 290° C., it is converted into a pure, bright, pale blue. This has an absorption spectrum in which the red is not absorbed, but appears more brilliant than in the spectrum of ultramarine rich in alumina. Pale blue is turned violet blue by heating at 300° C., and at a red heat a dull blue. It is not yet made on a commercial scale, but on account of its great purity of shade it appears to be valuable for many purposes; perhaps it may replace alumina cobalt blue. The composition of pale blue is:—

Calculated for Found.(by difference).
Na₅12·4per cent.11·9per cent.
H₅ 0·54” 0·62”
Al₄11·7”13·1”
Si₆18·2”19·7”
S₄13·9”12·7”
O₂₅43·3”42·0”

By a comparison of the composition of the violet and pale blue ultramarines, it is seen that the chief difference is an increase of hydrogen in the latter.

Ultramarine Red.—Since ultramarine violet increases in brightness and redness of shade in the air, Wunder erroneously believed that this was due to oxidation, and that, consequently, the violet could be converted into a red by oxidising agents. Nitric acid vapours led over ultramarine violet at 170° to 200° C. do not act upon it, but where drops of nitric acid are spirted over, the violet is changed to red. Wunder then reduced the temperature to 135° to 145° C. and obtained the first ultramarine red. Iron is attacked by nitric acid at lower temperatures, but not at 135° C.; the iron boxes previously described could therefore be used. It was afterwards found that at a sufficiently low temperature hydrochloric acid gas converts ultramarine violet into red. The iron boxes cannot be used for this operation, as they are attacked at the temperature; the stone chests are used instead. Other acids also act on ultramarine violet; boric acid gives a reddish-violet.

The violet is spread out on the dishes standing on three feet mentioned before, and heated to 128° to 132° C. At higher temperatures the violet is unaltered, whilst below 100° C. it is decomposed. The hydrochloric acid is poured in from time to time through earthenware tubes into dishes in which it evaporates.

A mixture of red and blue would appear violet, but would behave towards reagents in a different manner to real ultramarine violet. Ultramarine blue is decomposed at 128° to 132° C. by hydrochloric acid to a gelatinous mass, whilst at this temperature ultramarine violet is changed into a bright red. From the blue no violet can be obtained by nitric acid, but the violet gives a red at 135° to 145° C. Analysis would also indicate the difference. Ultramarine red has the following composition:—

Calculated for Found.(by difference).
Na₃ 7·9per cent. 8·1per cent.
H₅ 0·57” 0·72”
Al₄12·3”13·3”
Si₁₆19·1”19·3”
S₄14·6”15·2”
O₂₅45·6”43·4”

It appears that in the red two more equivalents of sodium have been replaced by hydrogen. The violet is apparently a sodium salt of which the red is the acid. The violet made by means of ammonium chloride is also converted by hydrochloric acid gas at 128° to 132° C. into a handsome red containing nitrogen, and the red is changed by hydrogen at 280° to 290° C. into a lighter pale blue.