The American Carbon Manual/The Pigments Employed
THE PIGMENTS EMPLOYED.
Considerable latitude in the choice of pigments is permissible, as almost all those employed by the painter are available in preparing the tissue for printing by this process. Where especial effects, resembling artist's drawings, are required, which, in reproductions will often be valuable, it is quite possible to produce them. The effect of a drawing in lead-pencil may be imitated by using graphite as the pigment; red chalk maybe imitated by Venetian red; for sepia and bistre effects these pigments themselves may be used.
For most purposes, however, a fine black, either neutral, or inclining in tone to brown or purple, will be preferred. Fine lampblack, or good Indian-ink, will, in such case, generally form the basis of the coloring matter. If the color required be a pure neutral black, the addition of a blue pigment is necessary to neutralize the brown tint of Indian-ink; and, where necessary, coldness is corrected by the addition of some warm color. The selection of this color will be governed by the tint desired, and by the considerations of permanency. Many of the most beautiful tints are most fugitive. Carmine, for instance, is unstable; and some samples are injured by the action of the chromic salt. Crimson lake is a valuable color, but it is not strictly permanent. Indian red is a very powerful and very-permanent color. Venetian red is also permanent. For blue, ultra-marine is quite satisfactory as regards permanence.
In judging of colors for this purpose, it should be borne in mind, that the actual effect of color employed is chiefly seen in middle tint. It is difficult to distinguish much difference between a blue-black, a brown-black, a purple-black, a rosy-black, etc, in the extreme darks of a picture; but the tone is easily distinguished in middle tint, and, as, a rule, warm half-tones are the most pleasing. It should also be remembered that a weak picture will often look brilliant in a warm tone, whilst a vigorous print will look feeble in a cold color.
We have stated before, that by the addition of a large proportion of color to the gelatine, a vigorous print may be obtained from a feeble negative, and by the use of a small proportion of color, a hard and intense negative may be made to yield soft prints. As a normal proportion, however, for good negatives, two per cent, of carbon is sufficient. Of course, the proportion of pigment required is different with different pigments, and depends upon the opacity and colorific power of the color employed. Mr. Swan prefers the use of insoluble pigments, as when the tissue is prepared with soluble colors the prints are apt to lose a little of depth and force, if they are subjected to prolonged washing.