Of great importance is the careful selection of the materials for the printing proper—i.e., the printing inks and papers, and the sometimes necessary admixtures of the former.
Printing Ink.—The most simple and suitable of all greasy or oily printing ink is the best chalk ink, as used in lithography. It is composed of finest lamp black thoroughly incorporated with boiled linseed oil (varnish). The collotypist must, in addition to different coloured ink for mixing with the black, be provided with thin boiled oil (middle varnish), and also turpentine and olive oil, the latter being used to thin or reduce the printing inks, which, as usually supplied to the trade, are too stiff for immediate use. When in printing it is necessary or intended to use an admixture of several colours, they must be first well ground in turpentine and the latter allowed to evaporate. A small addition to the chalk ink of indigo or Prussian blue greatly improves its colour and printing power, but may be considered more suitable for the reproduction of line or stipple subjects. A dash of Venetian red or carmine imparts to the chalk ink a warmer tone, resembling the colour of an ordinary silver print. Whoever ventures on colour printing must be able to prepare all the necessary combinations with the greatest exactness, and should use for each a special inking slab and roller. In the chapter dealing with the printing, further particulars will be given as to the mixing and thinning of the colours.
The Quality of Paper suitable for collotype may be next considered. Without doubt the best is that known as dull enamel, or chalk paper, which is now supplied of a pink tone as well as of the ordinary white quality. It should not be glossy or adhere too strongly to the printing surface, as the enamel will, under such circumstances, be partially or wholly torn from the paper, a difficulty equally observable whether the paper be used damp or dry. As previously observed, the adhesive power of the printing surface increases with the softness of the same. This may arise from using a small percentage of bichromate or a short exposure to light, in which cases it is always more apparent in such parts, as the densest portion of the negative may have been entirely protected during the copying in the printing frame, or it may arise from the addition of too great a proportion of isinglass. On the other hand, a gelatine film strongly chromated, proportionately longer exposed, and in which there is no isinglass, will be less likely to exhibit this annoying tendency.
Well-sized fine glazed papers generally adhere least, and are usually laid on dry. When the impressions are to be issued without glaze very thin fine-surfaced cardboard—not enamelled—is frequently used, and the margin of the printing plate covered with a suitable mask. Any well-sized paper possessing a fine surface may be used. Impressions on such paper may be passed through a weak solution of gelatine, and after drying, rolled, varnished, mounted and again rolled. Any retouching must be done immediately after the sizing, and before the application of the varnish. Enamelled papers require a special glaze, not a lacquer, or are finished without glaze at all.
CHAPTER VI.
Preparation of the Collotype Plate.
THE well-polished glass plate must be thoroughly cleaned with alcohol and ammonia, and it should be ascertained that the surface is entirely free from scratches. It is then coated with the substratum, consisting of a dilute solution of silicate of soda and albumen. The following formula is in constant use by many practical workers of experience, and answers every requirement:—