At his photo-zincographic works in Farringdon Street, the clean zinc plates are first coated with diluted albumen, and then with a solution of albumen in water, to which some potash bichromate is added. This is evenly distributed over the plate by means of the whirler, represented in Fig. 98; the zinc plate is placed face downward in the jaws of the whirler, as represented in the cut. When face downward it is less liable to be deteriorated by particles of dust settling upon its surface, and the surplus liquid is removed with more facility. That the latter may run off all the more easily, the plate does not rest in grooves in the boards holding it, but upon the ends of nails driven through from the outside of each board. The plate is whirled, centre-bit fashion, 15-50 seconds, according to the viscosity of the liquid used in any particular process. With diluted albumen, the whirling is of short duration, and a perfectly even, very thin coating is obtained. In some works a whirler with a larger handle is turned by "knack" with one hand only. The operators in such cases fancy that the eccentric motion then also given to the plate is advantageous; but this is questionable.
The wet solution of potash bichromate, albumen, and water is not sensitive to light, but when the film is dry the reverse is the case. Hence the drying over a small gas-jet is effected in non-actinic light. The effect of light is to partially deoxidise the potash bichromate; a chromium oxide insoluble in water is thrown down in the film, all the rest of which, where light has not acted through the negative, can be dissolved off in water. This process does not give half tones; it is suitable only for line drawings, or subjects in black-and-white.
The printing frames used by Swain are of a special description, and permit the application of much mere pressure than those used in ordinary photography. One of. them is represented in Fig. 99; its glass plate is 1 in. thick, and this in not in practice found to appreciably increase its liability to be cracked by the near proximity of the electric arc used in printing, perhaps because it is ' annealed with great care. In the cat the glass negative is represented upon the glass of the frame, and upon the negative is the zinc plate. Then comes the hinged back of the frame, with cross bars and strong screws for screwing all the surfaces together. When the plate is thus secured behind the negative, the frame is turned up, and the line plate is exposed to light behind the negative.
When removed from the printing frame, the plate is inked, and move the potash bichromate from the portions unacted upon by light. It is next treated with solution of gum arabic, after which the image upon the plate will take ink. It is under the gum solution for 10 minutes, and is then washed. Afterwards the plate is inked with a very greasy ink, and sent to the etching department, in which it is briefly immersed in a very weak solu-tion of nitric acid, and then sent down to the artists department, that any defects in the image, which are rarely present, may be removed by hand. It js then sent upstairs again for deeper etching. The image is inked by moans of a perfected lithographic roller, with greasy ink, dusted with rosin and heated on metal slabs, then the plate is put in baths of special Doulton ware, containing highly diluted nitric acid, and the baths are rocked by hand upon wooded cradles while the acid is eating into the metal. Every now and then the plate is taken out, inked, rosined, and heated afresh, then replaced in the etching bath. This frequent varnishing of the image is given to prevent the acid eating its way under the lines of the engraving, which under the system adopted stand up from the rest of the plate like little ranges of hills of exceedingly small elevation.
An average plate takes about 4 hours to etch.
Nitric acid is used in the etching rather than hydrochloric acid, because it works quicker; and as carboys of nitric acid are unpleasant and somewhat dangerous things to handle in workshops, the carboy in use is mounted in the wooden frame - Fig. 100 - in which it can be more conveniently manipulated.
The negativss for photo-zincography are usually taken by the old wet collodion process, as strong a contrast in them as possible of black and white being desired; but great density is of less importance than absolutely bare glass in the lines of the image. A good negative for the purpose, according to W. J. Wilkinson, an authority on the subject, should show all the lines of the image when it is laid down flat on a piece of white paper. When the lines are then quite clear, excessive density in the black is not so necessary as sometimes supposed. A few brands of the slower gelatine dry plates in the market will do for photo-zincography,but the old wet process is more economical.
The intensification of the fixed negative, to make it deep enough for printing, is effected by lead ferricyanide. Swain's exact formula is not known to us, but Dr. Eder and Captain Toth, of Vienna, who discovered the method, proceeded as follows: - The fixed and well washed negative received a washing with distilled water, then was plunged in a bath consisting of lead nitrate, 100 grm.; red prussiate potash, 5 grm.; distilled water, 5 grm. In this bath it remained until it became quite white, then it was well washed, and flooded with a 20 per cent. solution of ammonium sulphide; when the film then became blackened through, it was thoroughly washed again, and the negative was finished.
The method of intensification used at Brussels in the production of negatives for Belgian ordnance maps gives intensely black negatives, and avoids the use of ammonium sulphide. The agent used is copper bromide. By this method 120 gr. copper sulphate are dissolved in 1 oz. water, and 125 gr. potassium bromide are dissolved in as small a quantity of water as possible. The two solutions are then mixed, and the plate is flooded with the mixture. When the image becomes of a greenish-grey colour, the liquid is poured off, the plate is well washed, then washed with distilled water, after which a solution of 100 gr. to the oz. of silver nitrate is made to cover it at one sweep. These operations may be repeated if necessary, but intense blackness is usually obtained by the first treatment. The only objection to this process is the considerable amount of silver nitrate it uses up, but in all large photographic establishments the silver is finally recovered, so this process is much used; Warnerke found that the application of soda sulphantimoniate in place of silver nitrate gave a red image.