Part 8 (2/2)

A Copying Table--Exposure--Illumination--Photo-litho Transfers--The Paper--Printing--Developing--A Direct Process.

To discuss the respective merits of the ”wet” collodion, collodion emulsion, and dry-plate processes lies beyond the province of this work, as does also a detailed description of the operations involved.

The processes are purely photographic, and have already been presented to the craft in various forms. It is, moreover, almost impossible to bring within the limits of a single chapter anything approaching to a comprehensive record of the mult.i.tudinous details upon which process photography is based. All that can be attempted is to take one or two outstanding features which suggest a few useful hints.

The original or copy for reproduction must be on the same optical plane as the sensitive plate in the camera, _i.e._ they must be absolutely parallel with each other. There are several ways of ensuring this. The most convenient method is to use a copying table and board similar to Fig. 31. With such an arrangement as this direct copying can be attained through the lens, or the camera can be turned half-way round, and the image reversed by means of a mirror or prism attachment. The chief advantage of such an arrangement is, that the position of the camera can be altered at will without affecting the relative positions of the plate and copy.

[Ill.u.s.tration: 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 sc.r.a.per-board copies no shadows are thrown from the embossed dots or lines, and the granular texture of grained papers is almost entirely eliminated.

A b.i.+.c.hromated, 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 b.i.+.c.hromate of pota.s.sium. 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 b.i.+.c.hromate of pota.s.sium 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 gla.s.s, 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.

Thin down a little transfer ink with turpentine and distribute it evenly on a composition roller. Roll up the print until it is completely covered with an exceedingly fine film of ink, after which allow the turpentine to evaporate. Immerse the transfer in tepid water for about 10 minutes, and then rub it gently with a piece of cotton lint previously soaked in water, until the superfluous ink is removed and the design stands out clean and sharp.

The transference of the print to stone can be accomplished in the usual lithographic manner. The chief points to be observed are, to allow sufficient time for printing, and to ink up the transfer with the thinnest possible film of ink.

A photo print can be made direct on the zinc or aluminium plate, and by a slight modification of the photo-engraving process it can be developed according to lithographic methods.

It is impossible to introduce an intermediate process without, in some manner at least, depreciating the quality of the work. In a direct photo print on zinc, or, in fact, on any suitable printing surface, the finer qualities of the work are much more likely to be retained than when a transfer print is made under the negative and afterwards transferred in the usual manner.

Coat a finely grained zinc plate with sensitised asphalt solution and expose it under a negative for about 4-1/2 minutes in direct sunlight, and from 12 to 15 minutes in a diffused light. The action of light on the asphalt solution is to render it insoluble in turpentine, so that if a sufficiently exposed plate is immersed in pure turpentine the lines, etc., of the design, being of course represented by clear lines in the negative, will remain intact, while the surrounding portions will be dissolved and washed away. After development wash the plate freely in water, and dry it by fanning or with a pair of bellows. Let it stand for about 10 minutes and then slightly etch it with a very weak solution of nitric acid. Cover the work with strong, fresh gum, and dry it thoroughly and quickly. Remove the gum and ”rub up” the design with black ink in the usual way. Wash, dry, and dust over with French chalk.

The plate can then be prepared in the manner described in Chap. XI. page 61.

THE END

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