Fig. 31.
Correct exposure and sufficient illumination of the copy are important factors in photographic reproductions of any kind, but they are of infinitely greater importance when applied to photo-process reproduction. One is, to a certain extent, dependent upon the other. The former must of necessity be controlled by the latter; yet no amount of exposure will compensate for defective illumination. Where artificial light is employed the advantage of using two lights is obviously great. Apart from the greater brilliancy and intensity of the light, the illumination of the copy is more evenly distributed. In scraper-board copies no shadows are thrown from the embossed dots or lines, and the granular texture of grained papers is almost entirely eliminated.
A bichromated, gelatine paper can be obtained by coating a hard writing paper of medium thickness with a gelatinous solution consisting of 1 oz. of gelatine and 1 oz. of water, and afterwards sensitising it with bichromate of potassium. It is advisable, however, to use the commercial varieties of coated paper, and to sensitise it as it is required.
The sensitising solution can be prepared by dissolving 1 oz. of bichromate of potassium in 20 fluid oz. of water. Add to this sufficient ammonia to give it a bright orange tinge. Keep this solution at a temperature of 60° Fahr., and float the paper on it for about one minute. Pin or clip the paper to a board or squeeze it to glass, and dry in a dark room.
Print this paper under the negative in a diffused light until the design appears in a rich golden-brown colour, when the exposure may be regarded as sufficient. The time allowed for such an exposure will, of course, vary according to the quality of the negative and the intensity of the light.
A development of the print may now be proceeded with in the following manner.