From the preceding account of the manufacture of ultramarine it will be understood that there exist only two species of ultramarine, green and blue. In commerce a large number of different shades are found, which are not to be regarded as pure ultramarine; they owe their particular shade to additions. By mixing in a white pigment, such as barytes or starch, the paler shades are obtained; by adding a small quantity of a pure red pigment a colour with a violet tinge is produced. In a similar manner any number of shades of ultramarine can be obtained.
The following account is taken from a new comprehensive work by J. Wunder, on the different ultramarines.
Preparation of Mixtures for Ultramarine.—The following points are to be observed: Chemically pure soda and ammonia soda react with difficulty. The presence of caustic soda in the soda facilitates the formation of sodium sulphide, and gives a finer product. It is also of advantage to sprinkle the soda with a strong solution of sodium sulphide. The more silica the mixture contains the more difficult is the transformation into ultramarine, but the product is deeper and better in shade, and has more resistance to alum and weak acids. The sodium sulphide must react with silica and alumina. When oxygen enters during the burning, silica is re-formed, which with soda and alumina produces slags. In order to exclude oxygen many manufacturers burn with restricted access of air; then the carbon bisulphide and sulphur gases evolved are not burnt, but, together with tarry materials from the coal, form an evil-smelling smoke, which renders the neighbourhood objectionable. The coal consumption in burning is also greater. The varieties rich in silica and stable towards alum are generally dark reddish-blue. In order to obtain pure blue shades from mixtures rich in silica they are burnt in closed crucibles to produce the green, which is powdered and heated with restricted air supply, and the admission of a little steam, to 160° to 180° C., when pure blue or greenish-blue shades are obtained as required. In this way a mixture rich in silica can be burnt to a good blue, containing 69·32 parts of silica to 30·67 parts of alumina, or 1 equivalent of Al₂O₃ to 3·84 equivalents of SiO₂; whilst ultramarine rich in silica, made in the ordinary way, contains at the most 66·7 parts of silica to 33·3 parts of alumina.
Ultramarine Violet was first introduced into commerce about the year 1859 by the Nuremberg Ultramarine Works, under the direction of Leykauf. By the reaction of moist calcium chloride on ultramarine blue in the warm chambers above the furnaces the blue was changed to violet. By the action of air a portion of the moist calcium chloride is decomposed into lime and hydrochloric acid, the latter of which, together with air, reacts on the blue. In 1872 Wunder, intending to apply to the estimation of sulphur in ultramarine blue a method which gives good results with metallic sulphides, passed chlorine over heated ultramarine. The pigment was converted into a brownish-red substance, which on washing combined with water and turned violet. The red substance is not produced at every temperature; 300° C. is the most favourable. On washing sodium chloride is dissolved; the washed violet is free from chlorine, but has taken up water, which is expelled on heating, when the violet turns to blue. A similar brownish-red body is formed when sulphur trioxide is led over heated ultramarine blue, and when sulphur chloride acts on warm ultramarine blue. When the brownish-red chlorine compound is introduced into absolute alcohol, a reddish-violet compound is produced, containing organic matter. Ethyl chloride is formed at the same time. When ammonia gas is passed over the heated chlorine compound it is absorbed; a violet ultramarine is produced, from which the ammonia is not removed by washing with water, but only by heating nearly to redness, or, better, by fusing with caustic alkalis. Aniline combines in a similar manner. In order to obtain a good violet the brownish-red chlorine compound must not be formed; the violet is produced by leading chlorine and steam over ultramarine blue at 160° to 200° C. Blue rich in silica is most suitable for transformation into violet. The violet so obtained is not decomposed by lime.
C. Mahla produced the chlorine in the blue itself by the reaction: NH₄Cl + 2NH₄NO₃ = 6H₂O + 5N + Cl. By heating a mixture of ultramarine blue with ammonium chloride and nitrate in crucibles at 200° C. a fine violet is produced. It contains ammonia, which cannot be removed by washing, but only by strongly heating or heating with concentrated alkalis. This violet is decomposed by the prolonged action of lime, in three days it is changed to a grey green. In the course of the manufacture it was noticed that less nitrate was required in the mixture according to the length of time it was exposed in porous crucibles to the action of air and heat; by heating for a sufficient length of time with access of air, a good violet is obtained with ammonium chloride alone. In the manufacture of violet by means of moist chlorine hydrochloric acid is formed, which is also the active material in its formation by means of ammonium chloride. It was therefore to be expected that chlorine gas might be replaced by hydrochloric acid, if it were accompanied by sufficient air. This has been found to be the case. The temperature must be maintained between 180° and 230° C. Below 150° C. the blue is decomposed by moist hydrochloric acid gas; at temperatures above 230° C. it is unchanged. Hydrochloric acid gas and air, without steam, give with ultramarine blue on heating a brownish red substance similar to that produced by dry chlorine; it also is changed to violet by washing with water. The violet obtained by means of moist chlorine differs from the blue from which it was formed, in that the latter has lost one-sixth of its sodium and combined with water and much oxygen. The following is the approximate formula for a blue:—
Na₆Al₄Si₆S₄O₂₁,
and for the violet obtained from it
Na₅HAl₄Si₆S₄O₂₄ + H₂O.
The violet contains much thiosulphate. If it is decomposed by nitric acid and silver nitrate added to the filtered solution a precipitate is obtained which changes in colour from white to yellow, orange and brown, just as the precipitate given by silver nitrate in solutions of thiosulphates containing nitric acid.
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.