Pigments, Paint and Painting: A practical book for practical men

by George Terry

“For the purpose of taking a sample, an iron spoon borer is introduced through the hole left in the enclosing tiles, turned round, and pulled out. The contents are laid on a clean tile, and quickly covered with another tile, on which a second quantity is placed, and allowed to remain exposed to the air. If the oven has been sufficiently heated the covered sample should appear of a bluish green, and no longer brown or yellow, while the second sample should be rather bluer. If this be the case, the oven is heated slowly for another hour, and then all communication with the outer air is cut off. It is allowed to cool and then opened, when the contents should appear as a beautiful blue mass, the lower portion of which, however, is of a greenish tinge. Both parts are now treated alike, but worked up separately, the greenish-blue portion making an inferior article. The finishing process is as follows:—

“The raw ultramarine is ground in upright mills, and then repeatedly boiled for about ten or fifteen minutes at a time in cast-iron boilers, being all the time agitated by a mechanical stirring arrangement. It is then allowed to settle, and the water is drawn off with a siphon. As soon as the powder settles into a hard compact mass, it has been sufficiently washed, and it is then dug out. The part next to the bottom of the boiler is generally coarse and of poor quality. It is carefully separated from the upper portion, which is transferred to wet mills of the ordinary description, and there ground for six to twelve hours, during which time about 150 lb. can be treated in each mill. The ground colour from these mills is then collected in a large tub, and allowed to settle for four hours, during which time the coarsest particles fall to the bottom. The liquid is then passed through a series of tubs, in each of which it is allowed to stand for a period of time, lengthening as the quality settled out becomes finer, the last settling requiring about three weeks. The various qualities are then dried and sifted, when they are ready for the market.

“The blue produced by this operation is of a good quality, but there are some objections to the process, which have given rise to another, in which the ultramarine is produced direct in crucibles similar to those used in the indirect process.

“This is conducted as follows:—The mixture of raw materials consists of about 100 parts of china clay, 90 of carbonate of soda, 110 of sulphur, 20 of charcoal, and a quantity of infusorial earth, varying according as the ultramarine produced is desired to be rich or poor in silica. These are finely ground together, in which process great care must be observed, as much depends upon its being properly carried out. The mixture is then filled loosely into crucibles provided with flat circular lids, which are fixed on with mortar containing clay. This is allowed to dry, and the crucibles are then ready for firing, which process is conducted in ovens, generally constructed so as to contain several hundred crucibles, which are arranged in rows one above another.

“The mixture undergoes a very curious change of colour while in the ovens. When put in it is greyish white, and during the process of burning it becomes successively brown, green, blue, violet, red and white, in the order named. These changes are, according to Guimet, due to oxidation. The brown appears with the blue flames due to the combustion of the sulphur, the green just after the sulphur flames have ceased, and the blue is first formed at a temperature of about 700°—i. e. a bright red heat. If, after this, heat be still applied and air freely admitted, the mixture becomes first violet, then red or rose coloured, and finally white. When this white body is heated to redness with carbon or other reducing agents, the red, violet, blue, green and brown colours (according to the amount of reducing agent employed) may sometimes be reproduced, though the reaction is by no means a certain one.

“If brown ultramarine be removed from the oven, and allowed to remain exposed to the air, it immediately takes fire and burns to an inferior blue colour. The same thing occurs with the green body. Even if the brown product be completely cooled before being exposed to the air, it will, as soon as the air is allowed to reach it, get hotter and hotter, until it is glowing, when it will burst into flame and become blue. Attempts have been made to preserve the brown colour, which is of a beautiful chocolate tint, but have always failed. In one instance, when this was tried, the colour was put immediately into water, and treated like the ordinary blue variety, and as long as it was kept moist no change was apparent. After being washed and wet ground the moist powder was put into a cask, where for some time it was allowed to remain undisturbed. At the end of about three weeks it was noticed that the mass was hot, and on being turned out of the cask and broken up it was found to be at a glowing heat in the interior.

“After the oven has been fired for several hours, it is carefully closed at every point where air might enter, and allowed to cool for four or five days. The exact length of time during which the ovens are fired, and the amount of air admitted, depend upon various circumstances, one important one being the state of the weather. Thus, on a dull, foggy day, when the draught in the chimney is not good, a longer time is required. Of course, no rule can be given for this, and it is the experience required in the management of the oven that makes the manufacture so difficult to carry out successfully, the early efforts of a manufacturer not unfrequently resulting in the loss of a whole ovenful of raw material. As soon as the oven has cooled, the crucibles are taken out, and the contents of each are turned out in a solid mass, which must be carefully cleaned with a knife of any badly burned portions, and afterwards broken up and thrown into a cask along with the contents of other crucibles.

“This forms what is known as crude raw ultramarine. It contains about 15 per cent. of sulphate of soda, which must be removed before the colour is fit for sale.

“For this purpose it is washed with hot water in large tubs, after which it is ground in wet mills to an impalpable powder, and allowed to stand for about an hour in a large tub, in order to remove the coarsest particles and dirt which are sure to be present. It is then removed to another tub, where it settles for four or five hours, and from this it passes to others, where it stands for various lengths of time, increasing, of course, as the powder to be settled becomes finer, the last settling occupying three or four weeks, and producing the strongest quality that can be obtained—that is to say, it will bear mixing with more of a reducing medium, such as mineral white, than would a former settling for the mixture in each case to be of the same depth of colour.

“The water, after the final settling, still contains about 5 per cent. of ultramarine. This would take five or six months to settle, and as this time could not generally be given to it, it is precipitated with lime water, which has a sort of coagulating influence upon the particles, which can then be removed by filtration. It is a curious thing that this last quality is quite different from the one preceding it, being very inferior in both colour and strength.