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There are at least three mineral species, closely allied in chemical composition, and generally presenting a more or less marked blue colour, which contain as their essential constituents the five elements, silicon, aluminium, sodium, sulphur, and oxygen, and which owe their characteristic hue to the same compound. From one of these minerals, a variety of 'hauyne,' often called 'lapis-lazuli,' the true or native ultramarine is obtained. This stone occurs, of very varying purity and colour, at Bucherei,Transbaikal,and in many other Siberian localities; at Ditro, in Transylvania; in the Andes of Ovalla, Rio Grande; and in several regions of Persia, Tibet, and China. It is the 'sapphire' of ancient authors. Small golden specks of iron-pyrites are frequently irregularly scattered through its substance; it is also very frequently associated intimately with portions of the rocky gangue, or matrix (limestone, syenite, granite, etc.), in which it occurs. Very fine lapis-lazuli comes from Tibet. To prepare a pigment from this mineral, selected pieces of small size, as free as possible from pyrites or other impurities, are heated in a crucible and quenched (étonné) in cold water, or very weak vinegar.
The material, thus disintegrated, is washed by decantation, and then dried and carefully ground. The powder is then purified by elutriation, or 'washing over,' the several wash-waters depositing pigments of different depths of colour, and of different degrees of fineness. Some manufacturers adopt an old process, and make the powder into a soft mass with a little rosin, linseed oil and beeswax, and knead, beat, or macerate the lump, secured in a bag of coarse muslin under very weak potash, or soda-lye - the alkaline water carries off or withdraws the greater part of the pigment, and deposits it on standing. The richness of the blue product obtained depends primarily upon the original quality of the stone, but several grades are always procurable from the same raw material by means of the above-described processes, bluish-grey and grey powders, known respectively as ultramarine ash and mineral grey, being the last and the least valuable products, while the deepest and finest pigments are deposited from the earliest wash-waters.
Optically, the superb blue of native ultramarine approaches more closely than the blue of any other pigment to the pure normal blue of the solar spectrum; it shows very little violet and in this respect is unlike most specimens of artificial ultramarine. Ultramarine is somewhat harsh and granular in texture, a characteristic which may be reduced by a small admixture of Chinese white, but which becomes more marked when it is used as a light wash, or in conjunction with transparent pigments, in water-colour painting. It more nearly approaches transparency when used in oil, and is then of excellent working quality.
It is generally considered that ultramarine withstands the action of light, moisture and sulphuretted hydrogen perfectly, and that it neither affects nor is affected by any other pigments. I have, however, been informed by an English landscape-painter in oil, who has largely employed native ultramarine in the skies of his pictures, that he has lost faith in its inalterability. But the question arises, 'Was the pigment used always authentic?' It dries well in oil. It is decolourized at once by a hot solution of alum, and ultimately even by a saturated cold solution, which, however, bleaches very quickly all but the most stable varieties of the artificial pigment. All mineral acids, save carbonic, and all the common organic acids, such as acetic, oxalic and citric, discharge the colour of native ultramarine. It is only by a combination of several tests in the hands of a skilled chemist that the discrimination of genuine from spurious ultramarine may be with certainty accomplished.
It may, however, be mentioned here that when a current of hydrogen gas is passed over true ultramarine heated in a glass tube the powder retains its colour wholly or partially for an hour or more, while the best artificial pigment similarly treated becomes grey or greenish grey.
In the works - both in fresco and tempera, and in oil - of many of the old masters, and in a large number of illuminated manuscripts, the permanence of true ultramarine may be seen. If in some cases it has acquired a greenish or dull hue in oil-painting, such change is due to the yellowing of the oil and varnish, and not to any deterioration of the pigment.
The price of genuine ultramarine is very high. This is due less to the scarcity of the original lapis-lazuli from which it is derived, than to the small yield and to the elaborate and tedious operation by means of which the pigment is prepared. But when every allowance is made on account of the troublesome and lengthy process of manufacture, the cost of ultramarine is unwarrantably excessive.
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