Also called Permanent white, and Barytic white, is, when well prepared, of superior body in water, but has less opacity in oil. It works in a somewhat unsatisfactory and unpleasant manner, and is considerably lower in its tone while wet than when dry, a fault which subjects even an experienced artist to great uncertainty where he uses it in compound tints. The semi-transparency of the white, while wet, prevents his judging of the true tint until his colour has dried, when he frequently finds it harsh and chalky, and out of tone with the rest of his painting. As little gum as possible should be employed with it, gum being inimical to its body, or whiteness. The best way of preparing this pigment, as well as other terrene whites, so as to preserve their opacity, is to grind them in simple water, and to add towards the end of the grinding sufficient only of clear cold jelly of gum tragacanth as will connect them into a body, and attach them to the paper in painting. Cold starch will answer the same purpose.
Constant white is a sulphate of baryta, found native and known under the name of heavy-spar, or prepared artificially by adding sulphuric acid, or a soluble sulphate, to a solution of a barytic salt. In the first mode, if the white be not well purified from free acid, it is apt to act injuriously on some pigments. Sulphate of baryta is often used for the purpose of adulterating white lead, the native salt being ground to fine powder, and washed with dilute sulphuric acid, by which its colour is improved, and a little oxide of iron probably dissolved out. Whether native or artificial, the compound is quite unaffected by impure air, and is not poisonous.
LEAD WHITES
Comprise and are known under the names of:—White lead, Flake white and Body-white, Cremnitz, or Kremnitz, Crems or Krems white, London and Nottingham white, Flemish white, Pattison's white, Blanc d'argent or Silver white, Ceruse, Dutch white, French white, Venetian white, Hamburgh white, Kremser white, Sulphate of lead, &c.
The heaviest and whitest of these are the best, and in point of colour and body, are superior to all other whites. When pure and properly applied in oil and varnish, they are comparatively safe and durable, drying well without addition; but excess of oil discolours them, and in water-painting they are changeable even to blackness. Upon all vegetable lakes, except those of madder, they have a destructive effect; and are injurious to gamboge, as well as to those almost obsolete pigments, red and orange leads, king's and patent yellow, massicot, and orpiment. With ultramarine, however, red and orange vermilions, yellow and orange chromes, yellow and orange and red cadmiums, aureolin, the ochres, viridian and other oxides of chromium, Indian red &c., they compound with little or no injury. Lead colours must not be employed in water-colour or crayon painting, distemper, or fresco. The whites of lead are carbonates of that metal, with two exceptions:—Flemish white or the sulphate, and Pattison's white or the oxychloride. In using all pigments of which lead is the basis, cleanliness is essential to health.
White lead, by which we must be understood to mean the carbonate, always contains when commercially prepared a certain proportion of hydrated oxide. The less of the latter there is present, the better does the white cover, and the less liable is it to turn brown. The products formed by precipitation have proved to be inferior in body: otherwise, pure mono-carbonate of lead-oxide, obtained by mixing solutions of carbonate of potash and a lead-salt, might be best adapted for a pigment. However, such a carbonate has been lately produced by Mr. Spence's process of passing carbonic acid gas into a caustic soda or potash solution of lead, and for this white an opacity is claimed equal to that of the ordinary compound.
Great as is the opacity of white lead, it is apt to lose that property in some measure in course of time, and become more or less transparent. If, over a series of dry oil-colour rubs of varied hues, there were brushed sufficient white lead paint to utterly obscure them, after some years those rubs would indistinctly appear, and by degrees become more and more visible, until at last their forms—if not their very colours—could be recognised. From this it would seem that white lead must slowly but surely part with some of its carbonic acid, and be at length converted into dicarbonate, a compound possessing less carbonic acid, and less coating power.
Impure air, or sulphuretted hydrogen, browns or blackens white lead, converting it partially or wholly into sulphide. It would appear from the recent investigations of Dr. D. S. Price, that white lead is less liable to be thus affected, when the pictures in which it is used are exposed to a strong light; also, that when such pictures have so suffered, a like exposure will restore them. We have ourselves noticed the rapidity with which an oil rub of white lead that has been damaged by foul gas, regains its former whiteness when submitted to air and sunshine. The action of drying oils has been likewise proved to be very powerful upon sulphide of lead, an exposure to light for a few days only being sufficient to change a surface of it, coated with a thin layer of boiled linseed oil, into a white one. Probably, these agents may have a similar effect upon other pigments injured by sulphuretted hydrogen, and many of the colours in paintings may be restored by treating them with boiled linseed oil, and submitting them to a strong light. That the result is due to oxidation, there can be no doubt. Indeed, the eminent French chemist, M. Thénard, was consulted some years back upon the means of bringing to their original whiteness the black spots which had formed upon a valuable drawing, by the changing of the white lead, and employed for that purpose oxygenated water. He had ascertained its power of converting the black sulphide of lead into the white sulphate, and, by touching the spots with a brush dipped in the fluid, soon succeeded in restoring the drawing to its primitive state. Here, again, the use of the agent might doubtless be extended to other colours, to which foul air is inimical.
In oil painting white lead is essential in the ground, in dead colouring, in the formation of tints of all colours, and in scumbling, either alone or mixed with other pigments. It is also the best local white, when neutralized with ultramarine or black; and it is the true representative of light, when warmed with Naples yellow, or orange vermilion or cadmium, or with a mixture of the yellow and either of the orange pigments, according to the light.
Ordinary white lead is often mixed with considerable quantities of heavy spar, gypsum, or chalk. These injure it in body and brightness, dispose it to dry more slowly, keep its place less firmly, and discolour the oil with which it is applied, as well as prevent it dissolving completely in boiling dilute potash-ley, a test by which pure white lead may be known.