In these lists will be found yellow ocher, Roman ocher, Oxford ocher, golden ocher and probably Mars yellow, so far as earth colors are concerned. The ocher designated as yellow ocher is invariably of the French variety and the citron shade usually favored. It is best for the grinder to select only material of very finest levigation for use in this line; that is, the ocher should be entirely free of grit. In that state of fineness at least thirty-two parts by weight of poppyseed or nut oil is required to grind sixty-eight parts by weight of pigment.
Roman Ocher should have a sort of marigold tint or tone and the strong and dark French ocher, branded J. F. L. E. S., say sixty-six parts by weight and two part by weight of bright Italian burnt sienna mixed with thirty-two parts of poppyseed or nut oil ground fine in a stone mill will answer for this.
Oxford Ocher - It is optional here whether the strong dark French ocher, J. F. L. E. S., or an English or German ocher of even greater strength is used so long as the pigment is free of grit and of rich, deep yellow color. The quantity of oil in one hundred pounds paste should not exceed one-third of the total weight of the mixture.
Golden Ocher. - The artist when painting a picture will rarely, if ever, place on his palette a golden ocher that is made by mixing yellow ocher and chrome yellow, but the decorator is not so exacting, and will use that which is ready prepared to save him the trouble of doing his own mixing. There being no rule or standard as to the percentage of yellow ocher and chrome yellow, the grinder can refer back to the formulas given in the regular oil color list, with this precaution, however, that only the finest levigated materials be mixed and ground, and that in place of raw linseed oil poppyseed or nut oil be used, increasing the percentage of oil somewhat over that given in those formulas.
Mars Yellow is really an artificial ocher, but seldom met with nowadays. We are not going far astray when we say that the rich yellow oxides mentioned in the description of the yellow pigments for yellow colors in oil for the general trade are selected for this pigment, and really, when well levigated, so as to be entirely free from grit, are far better in regard to permanency of color and durability than the artificial product. Like the ochers, it must be ground in poppyseed or nut oil, fairly stout, to avoid separation of oil and pigment when filled in collapsible tubes. Thirty parts of oil to seventy parts pigment is the proper proportion for mixing and grinding.
Naples Yellow is used by artists in oil painting and favored for its great opacity. There are two shades only - pale and deep; any other, and especially that of a reddish tone, is a mixed pigment. Naples yellow, or antimony yellow, is produced by the calcination of the oxides of lead and antimony with access of air in muffled furnaces. This pigment requires for a fair paste fourteen to fifteen parts by weight of oil to eighty-five or eighty-six parts by weight of pigment, and should not come into contact with metallic iron or zinc, as it is apt to assume a grayish tinge.
Chrome Yellows in any shade, from canary to orange, are ground in poppyseed oil for artists' and decorators' purposes, and only the best and cleanest goods that are obtainable should be selected. The proportions of pigment and oil given in our description and suggestions for the regular oil color list apply here, with the exception that in place of using bleached or clarified linseed oil poppyseed or nut oil is to be employed, and the grinding so perfectly done that the oil will not readily separate from the pigment when the color is squeezed from the collapsible tube. Where the output warrants it roller mills are most practical for grinding chrome yellows for tube color.
Permanent, or Zinc Yellow, as noted above, is a chromate of zinc produced by digesting one hundred parts zinc oxide in a great quantity of water with sixty parts sulphuric acid, to which mixture is added a solution of one hundred parts bichromate of potash. However, Barium Chromate and Strontian Yellow are also sold as permanent yellow, and in some instances, as ultramarine yellow. Zinc yellow requires twenty-eight parts by weight of poppyseed oil to seventy-two parts of the dry powder for one hundred parts paste in oil. Baryta yellow (barium chromate) and strontia yellow (strontium chromate) require very close to the same proportions of oil and dry powder.
Cadmium Yellow, known, in short, as cadmium, is a compound of the metal cadmium and sulphur, and is on the market in three or four shades - pale, medium, deep and orange. The darker shades are very permanent to light and unaffected by gases, but the very pale shade is not pure cadmium, containing zinc sulphide or zinc oxide in addition. Pure cadmium yellow dissolves in a strong, warm solution of hydrochloric acid to a clear liquid while giving off sulphuretted hydrogen. If cadmium yellow is mixed with chrome yellow it will show the formation of the black sulphide of lead when treated with sulphate of soda solution. Cadmium yellows should be ground in stone mills with scrapers of bronze, as contact with iron or lead is harmful to its brilliancy, tending to produce dark streaks in the color. The proportions of oil and pigment for grinding the color for tubes are similar to that of the chrome yellows of same shade.
Gamboge is a resin and requires a special treatment before it can be mixed with oil for artistic painting, consisting in powdering the brittle material and extracting the pure resin from the material with alcohol, decanting the clear yellow alcoholic solution, evaporating the spirit and melting the residue in oil sufficient to make it viscid enough to fill in tubes. Used by artists for glazing, but not permanent to light to any degree.
Indian Yellow, or Purree, a magnesium salt of euxanthic acid, comes into the European market from East India in lumps of the size of a fist, that are of brown color on the outside, but of a bright yellow inside, and have the odor of ammonia rather strongly. When the outer crust is taken off and the yellow well washed in boiling water and ground in borate of manganese oil it is a very permanent yellow color for use of artists, while for other purposes it is too expensive. On account of its high market price it is susceptible to adulteration with inexpensive yellow lake or Dutch pink, sometimes chrome yellow. The presence of yellow lake or Dutch pink can be determined by disintegrating the pigment with hydrochloric acid and then adding ammonia in excess. If pure a bright yellow, clear solution is had; but if yellow lake or Dutch pink is present there will be a precipitate of alumina or hydroxide of tin. There is an imitation of Indian yellow on the color market that has even a better and richer tone than Purree, but, being a coal-tar derivative, it is not as permanent. However, its price is only one-fourth as much as that charged for the true article. True Indian yellow requires for one hundred parts of finished tube color in oil, fifty-six parts pigment to forty-four parts of poppy-seed or linseed oil that has been boiled with 2 per cent of manganese borate. Imitations of Indian yellow differ somewhat in the percentage of oil required, but a sample tested by the author required four parts by weight of poppyseed oil to six parts by weight of pigment.
Yellow Lakes are uncertain pigments, and the only one suited for the use of the artist is that branded as French Superior Yellow lake in drops. This is made from the extract of the quercitron bark on a base of alumina hydrate, which has a tendency to make the color liver in time, and it should be guarded against, even at the expense of brilliancy, by selecting yellow lake with a base of alumina sulphate. This is a matter that can be arranged mutually between the color maker and the color grinder. In grinding yellow lakes the utmost fineness must be attained, as the color is used for glazing, and the finer it is ground the better the transparency. Usually fifty-eight pounds pigment and forty-two pounds poppyseed oil are required for one hundred pounds of the color in oil, but these percentages will differ materially, according to the nature of the base.
Dutch Pink, Italian Pink, yellow madder and brown pink are simply very much extended yellow lakes, and are used for scene painting, the base material taking away the transparency to a great degree, also making the pigment less oil absorptive.
Aureolin, or Cobalt Yellow, is a complicated salt of the metal cobalt and has no advantages over other yellow pigments, and, as it is rather high in cost, is seldom used. Requires six parts of oil to four parts of the dry color on mixing and grinding. Usually the name Aureolin appears only on the label and in the color lists, but the color in the tube is usually another rich yellow pigment.
King's Yellow, or Orpiment, an arsenic sulphide pigment, is now practically obsolete, and when found in tubes as an artists' and decorators' oil color it will be discovered to be medium chrome yellow.