Among the standard whites for artists' use, and put up in collapsible tubes, there are offered to the trade white lead under the name Cremnitz white, flake white, silver white or Venetian white; zinc oxide under the name zinc white, Chinese white, snow white and permanent white. Blanc fixe (precipitated barium sulphate) is put up in tubes under the name of permanent white, while sometimes lithopone white parades under the same name, although it should have no place on the artist's palette.
In order to produce white lead in oil for artists' use only the purest in point of whiteness should be selected, and mixed with poppyseed oil that has been bleached by settling and age, although nut oil (from the kernel of the walnut) will also work well. The mixture should be ground to impalpable fineness and of a stiffness that will not permit the oil to separate from the pigment. It will require from nine to nine and one-half pounds of oil to ninety and one-half or ninety-one pounds of white lead and a well-balanced stone mill of utmost cleanness to accomplish this. The stones of the mill must be well dressed, so that the paste white is not munched into a gummy mass during the grinding process, and it is well to see that the pigment is free from moisture before mixing it with the oil. Should the paste come out of the mill too soft it is best to let it become cool and return it again to the mill hopper, after adding more dry lead, passing it once more through the mill. If for any reason the desired stiffness of the paste white cannot be obtained in this manner and the addition of a wax emulsion is undesirable, it is best to run stiff pulp ground lead through a stone mill of small diameter, and in the resulting product the oil will not separate from the pigment. In such case the use of poppyseed or nut oil is impracticable.
When grinding Chinese or zinc white for putting up for artists' use in tubes only the very best French process zinc is selected, and whenever possible the dry zinc should be somewhat condensed in a chaser, so that it is more compact when being placed in the mixer with the poppyseed or nut oil, when, during the process of mixing and grinding it will loosen up somewhat and yet not separate from the oil after being put away in collapsible tubes. The average proportion of zinc and oil will be eighty-three parts by weight of the former to seventeen parts of the latter. The more mellow the texture of the zinc oxide the more readily and the smoother it will grind out.
As before stated, when zinc oxide in the dry state is exposed to the air for a long time it undergoes a chemical change, turning from zinc oxide into zinc carbonate, the latter pigment being practically transparent, and this fact has been made use of by some European manufacturers of artists' colors in placing this pigment ground in oil at the disposal of artists under the name of glazing white or transparent white. There are no statistics obtainable as to what extent this material has been employed.
Blanc fixe or permanent white or baryta white for artists' tubes is not made with natural barytes, but is the artificial pigment known to the trade as blanc fixe, usually made from a solution of barium chloride from which barium sulphate is precipitated by the addition of dilute sulphuric acid. Blanc fixe is distinguished from the ordinary or natural barytes by its greater fineness, greater bulk and body and by its purer whiteness, and last, but not least, its far greater absorption of oil.
While the purest natural barytes may be ground into a paste with 8 to 9 per cent of oil, i. e., ninety-one to ninety-two pounds of pigment to eight or nine pounds of oil (only inferior and off-colored grades require more oil), artificial barytes, or blanc fixe, requires anywhere from 16 to 20 per cent of oil to from 80 to 84 per cent of the pigment, according to its fineness of division.