Scientific American 28, 1.4.1848
(Concluded from our last.)
To prepare ultramarine or lapis lazuli for painting, the mineral is first made red hot in the fire and then thrown into wate to make it easy to pulverise. The best way however is to heat it in a crucible to keep it clean and then quench it in vinegar and keep it therein for a few hours, when the vinegar is poured off and the lapis lazuli ground fine in a flint mortar, when it may be calcined again and treated in the same manner to make perfectly impalpable. A paste is then made of 9 ounces Burgundy pitch, 6 of white resin, 6 of Carolina or Georgia turpentine, a email quantity of wax and 2 ounces of linseed oil. This is mixed all together in a stoneware vessel and boiled therein until it will form a lump when poured into cold water. The cement thus formed may be poured out of the vessel into water and made into cakes for use. Take then an equal weight of this cement and the calcined lapis and melt all in a glazed earthen vessel adding the calcined matter by degrees, stirring with a glass rod till all is well mixed, when it is pretty well heated and thrown into a large basin of cold water. When it is cooled it is kneaded like the dough of bread and rubbed over with the hands with linseed oil till the whole are well incorporated. Then put this cake into an earthenware vessel, the bottom of which should be rubbed with oil, and pour on it water of the warmth of blood. Let this stand for a short time and as the water softens the cake, it will lose the finest part of the calcined matter, which on gently stirring the water, or separating any of the parts of the cakes, will be suspended in water, and must be poured off with it into another vessel. The quantity of water must be then renewed and the same operation repeated a second, or third time and as the mass appears slow in giving the color it must be moved or stirred in the manner of kneading with a glass spitula, but not broken into small parts and so much of the color is extracted as to render it necessary for obtaining more, the water is heated to a greater degree. The result of these washings is the ultramarine. These three washings are then mixed with a boiling hot solution of two ounces salt of tartar or pearl ashes dissolved in a pint of water and filtered through clean paper. This is cooled and when the powder has fallen to the bottom of the vessel, the clear must be poured off and the powdered must be washed until all the pearlash or tartar is carried away. The ultramarine is then dried and is duly prepared for use.
Another method of purifying the ultramarine from the cement may be used, which is by pricking the yolks of eggs and moistening the matter with what will run out and working them together in a flint mortar, after which the mixture must be put into a lixivium of the tartar, or pearlash and proceeded with as before directed.
In order to free the ultramarine from that put of the water which cannot be poured off from it without carrying away part of the powder, let it be put into a deep coffee cup, and put candlewicks so as to hung over the edge with one end in the liquor and the moisture will be removed by capillary attraction, when the matter may be dried on polished marble, or glass. Another method from the one above, is to use beeswax and white resin mixed together in equal quantities instead of the compound pitch cement, and which on its being infused in water very warm, will make the lazuli give out its color much sooner.
Ultramarine may also be prepared without any cement simply by calcining it and levigatling with pearlash, and washing and then soaking it in distilled hot vinegar. A greater quantity will be produced in this way, but lighter in the color. To make a fine ultramarine the lapis lazuli must be good, and to test this, if a small piece be made red hot and retain afterwards its hardness of color, it may be accounted good, but if it crumbles or turns brown, or dull and full of specks, it may be suspected. Ultramarine mixed with white flake and oil by the pallette knife can be compared with other parcels and judged of by its depth and clearness of color. Ultramarine from its great price is apt to be adulterated by a precipitation of copper and an alkali, and also fine smalt. Copper is a dangerous mixture, it will turn black in oils and green in enamels, as soon as fluxed. It is not so easy to adulterate with fine cobalt as it is difficult to mix on account of its hardness and is scarcely to be levigated by art to be as fine as the ultramarine rendered impalpable by the calcination it has undergone. The adulteration with smalt does not hurt it for enameling and it will stand as well for water painting, but it does not mix well with oil and it will fall from it if the mixture be very moisty, or become pasty if stiffer and never works freely. Copper adulteration may be easily detected by pouring some diluted nitric acid on a small quantity when it will soon dissolve and leave a greenish blue solution. Smalt maybe detected, by trying it with oil, or mixing in water when the coarseness of the smalt will soon be detected.
The lapis lazuli is, when perfect, a very light blue color, with a transparent effect in oil, and in some degree in water, and will stand when used in painting without fading with whatever pigment it may be mixed. For these reasons ultramarine is of the highest value in every kind of painting, being equally serviceable in all, even in enamel, and though the Prussian blue oil account of its cheapness may have lessened the use of it, yet this is to be considered as an injury to the art, as the skies of landscapes and many other parts of modern pictures shew their loss of it by their changing from a warm clear blue to a faint greenish tint.