Separate from the stone the most apparent parts of the ultramarine, reduce them to the size of a pea, and, having brought them to a red heat in a crucible, throw them in that state into the strongest distilled vinegar. Then grind them with the vinegar, and reduce them to an impalpable powder; next take of wax, red colophonium, and lapis lazuli, an equal quantity, say 1/2 oz. of each of these three substances; melt the wax and the colophonium in a proper vessel, and add the powder to the melted matter, then pour the mass into cold water, and let it rest eight days. Next take two glass vessels filled with water, as hot as the hand can bear, knead the mass in the water, and when the purest part of the ultramarine has been extracted remove the resinous mass into the other vessels, where finish the kneading to separate the remainder; if the latter portion appears to be much inferior, and paler than the former, let it rest for 4 days, to facilitate the precipitation of the ultramarine, which extract by decantation, and wash it in fair water.
Ultramarine of four qualities may be separated by this process. The first separation gives the finest, and as the operation is repeated, the beauty of the powder decreases.
Kinckel considers immersion in vinegar as the essential part of the operation. It facilitates the division, and even the solution of the zeolitic and earthy particles soluble in that acid.
Separate the blue parts, and reduce them, on a piece of porphyry, to an impalpable powder, which besprinkle with linseed oil, then make a paste with equal parts of yellow wax, pine resin, and colophonium, say, 8 oz. of each; and add to this paste 1/2 oz. of linseed oil, 2 oz. of oil of turpentine, and as much more mastic.
Then take 4 parts of this mixture, and 1 of lapis lazuli, ground with oil on a piece of porphyry, mix the whole warm, and suffer it to digest for a month, at the end of which knead the mixture thoroughly in warm water, till the blue part separates from it, and at the end of some days decant the liquor. This ultramarine is exceedingly beautiful.
These two processes are nearly similar, if we except the preliminary preparation of Kinckel, which consists in bringing the lapis lazuli to a red heat and immersing it in vinegar. It may be readily seen, by the judicious observations of Morgraff on the nature of this coloring part, that this calcination may be hurtful to certain kinds of azure stone. This preliminary operation, however, is a test which ascertains the purity of the ultramarine.
_To Extract the Remainder of Ultramarine._
As this matter is valuable, some portions of ultramarine may be extracted from the paste which has been kneaded in water; nothing is necessary but to mix it with four times its weight of linseed oil, to pour the matter into a glass of conical form, and to expose the vessel in the balneum maria of an alembic. The water of which must be kept in a state of ebullition for several hours. The liquidity of the mixture allows the ultramarine to separate itself, and the supernatant oil is decanted. The same immersion of the coloring matter in oil is repeated, to separate the resinous parts which still adhere to it; and the operation is finished by boiling it in water to separate the oil. The deposit is ultramarine; but it is inferior to that separated by the first washing.
_To Ascertain whether Ultramarine be Adulterated._
As the price of ultramarine, which is already very high, may become more so on account of the difficulty of obtaining lapis lazuli, it is of great importance that painters should be able to detect adulteration. Ultramarine is pure if, when brought to a red heat in a crucible, it stands that trial without changing its color; as small quantities only are subjected to this test, a comparison may be made, at very little expense, with the part which has not been exposed to the fire. If adulterated, it becomes blackish or paler.
This proof, however, may not always be conclusive. When ultramarine of the lowest quality is mixed with azure, it exhibits no more body than sand ground on porphyry would do; ultramarine treated with oil assumes a brown tint.
Ultramarine is extracted from lapis lazuli, or azure stone, a kind of heavy zeolite, which is so hard as to strike fire with steel, to cut glass, and to be susceptible of a fine polish. It is of a bright blue color, variegated with white or yellow veins, enriched with small metallic glands, and even veins of a gold color, which are only sulphurets of iron (martial pyrites); it breaks irregularly. The specimens most esteemed are those charged with the greatest quantity of blue.
Several artists have exercised their ingenuity on processes capable of extracting ultramarine in its greatest purity; some, however, are contented with separating the uncolored portions of the stone, reducing the colored part to an impalpable powder, and then grinding it for a long time with oil of poppies. But it is certain that, in consequence of this ineffectual method, the beauty of the color is injured by parts which are foreign to it; and that it does not produce the whole effect which ought to be expected from pure ultramarine.
It may be readily conceived that the eminent qualities of ultramarine must have induced those first acquainted with the processes proper for increasing the merit and value of it, to keep them a profound secret. This was indeed the case; ultramarine was prepared long before any account of the method of extracting and purifying it was known.
Sulphur, 2 parts; dry carbonate of soda, 1 part. Put them into a Hessian crucible, cover it up, and apply heat until the mass fuses, then sprinkle into it gradually a mixture of silicate of soda and aluminate of soda (the first containing 72 parts of silica, the second, 70 parts of alumina); lastly, calcine for 1 hour, and wash in pure water.
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