Now, the carbon is transferred direct on to the copper plate and developed in the usual manner. All the soluble gelatine is thoroughly removed by a final washing in warm water, and the plate, with the clean negative image upon it, is allowed to dry, but not under heat sufficient to alter the character of the graining. Sometimes the plate is flooded with methylated spirit to facilitate drying, but it is better to allow it to dry by natural means. When dry, the back and edges are carefully protected with engraver's varnish, made by dissolving orange shellac in methylated spirit, and all is ready for etching.
The usual etching fluid is a solution of perchloride of iron in five varying strengths, prepared by boiling with distilled water, and then adding distilled water in five portions till they respectively register 450, 400, 370, 330, and 27° on a Beaume hydrometer. In the first solution, which attacks only the deep shadows, the plate may remain one minute. Then this is poured away, and No. 2 solution substituted for two or three minutes; then No. 3 for three minutes, and so on. Meanwhile the progress of the etching may be watched by the decoloration of the metal, as it turns black under the action of the mordant. It is important that each bath should be poured away when it ceases to spread, and the higher lights will probably not be affected until the fifth bath has remained on for a minute or two. Within a minute of the darkening in the higher lights the last bath must be poured off and the plate dipped in a 5 per cent. solution of caustic potash or soda. Lastly, the carbon, varnish, and grain are cleared away with the aid of paraffin or turpentine, followed by hot water and soda; and then a final rinse in warm water before rubbing dry with a soft rag. Photogravure plates are, of course, of "intaglio" character, with the ink in the hollows, and therefore can only be printed in a copperplate press. A fuller account of various methods of photogravure will be found in the works on the subject, by Herbert Denison and Thomas Huson.
A very beautiful method of intaglio printing, invented by the late Mr. W. B. Woodbury, but now seldom adopted. Well-executed prints were often mistaken for silver prints, so soft and delicate were the gradations; but it was better suited for the old-fashioned wet-plate negatives, or for landscape negatives taken with a very small stop in the lens. A film of bichromated gelatine, on some homogeneous support, was exposed in the printing frame in the usual way, after which the superfluous gelatine was dissolved off in hot water, leaving the picture in relief. This relief was laid on a perfectly true steel plate, a sheet of lead placed upon it, and subjected to a pressure of at least four tons to the square inch of surface. An intaglio plate was left, forming a faithful copy of the relief, while, strange to say, the delicate gelatine was left uninjured and was capable of serving as the matrix for several more lead plates. A special liquid gelatine solution containing pigment was employed for the ink, and copies were obtained in a copperplate press with fair rapidity.
This is the most economical process for obtaining reproductions of photographs in printing inks, with the tones represented in the same gradations as in an ordinary silver print. So far as the process itself is concerned there are no great complications, but, in practice, better results are likely to be secured by a commercial firm, than by the ordinary photographer. Collotype is by far the best method for multiplication of postcards when the numbers required do not exceed a thousand or two, and the cost is not much greater than the ordinary "half-tone" block.
Negatives for Collotype must be reversed, unless they are on celluloid film. These films also are far preferable for the purpose, than those made on ordinary dry plates, the latter being far from flat, and therefore liable either to break under pressure or to give an imperfect image, from insufficiently close contact with the collotype plate.
Sheets of best plate glass, at least 3/8 in. thick, are first ground truly level, and then coated with a sizing consisting of -
Stale Beer ........ 10 parts
Syrupy Silicate of Soda..... 1 part and dried in an oven at 120° Fahr., after which they are allowed to cool, rinsed under the tap, and left in a rack to drain. The sensitive film is composed as follows:
Hard Gelatine........ 3 oz.
Potassium Bichromate ...... 300 gr.
Ammonia (strong) ....... 10 min.
Water......... 15 oz.
The gelatine is soaked for an hour in the water, and then dissolved gradually under heat, after which the other ingredients are stirred in. Immediately before coating, about an ounce of methylated spirit is added for every ounce of gelatine solution about to be used, and the whole is strained through fine muslin. The glass plates must be scrupulously level and warm to the touch. Thirty to fifty minims may be allowed for every square inch of glass. After coating, the plate will dry in about a quarter of an hour in a drying oven, in which the temperature must not exceed 1150. The films will keep for about ten days if protected from moisture and light.
Previous to exposure in a pressure-frame it is usual to mask off the edges, either with tinfoil or black paper. Progress of exposure maybe judged, either by means of an actinometer or by examination of the image through the glass at the back of the printing-frame. When complete, most workers advise that the plate should be laid face downwards on a piece of black paper, with the back exposed to the light, for a minute or two. This will harden the back of the film, and so insure its adherence to the glass during the subsequent operations.
Development takes place in several changes of water, until all the bichromate salt has been removed, when the film may be dried and put away in store for months, if necessary. When copies are to be taken, the collotype plate is soaked in cold water and then "etched" in a solution composed either of:
Glycerine ........ 20 parts
Water . . . . . . . . . 20 ,,
Ammonia ......... 1 part
Common Salt . . . . . . 1 ,,
Water . .......2 parts
With a few drops of oxgall.
After being allowed to act for about half an hour, by which time the film will have been thoroughly penetrated, the superfluous glycerine is dabbed off, the picture inked in with lithographic ink, and proofs taken on an Albion or other press. A lithographic machine is often adaptable for collotype, but an extra set of rollers and a mask-frame for the margins are essential; a special collotype machine is the best in the long run. Printing requires care and experience, and the plates have to be damped with fresh etching solutions after each 70 to 100 impressions, according to the quality of paper used. If the prints are glazed with white water varnish or a label varnish, and then dried in moderate heat, they will closely resemble ordinary P.O.P., or albumen paper, according to the colour of the varnish employed. Only a limited number of copies can be obtained from each film before it wears out.
The above may serve as an outline of collotype methods, but in the hands of individual firms the process has been brought to the very highest perfection. A simplified process for the use of amateurs, substituting gelatine ready coated on parchment sheets for the heavy glass plates, was introduced a few years ago. We have not heard any news about this lately. Collotype prints of good quality are worthy of all praise; inferior work is not remunerative, as it cannot compete with photo-lithography, produced at a quarter the trouble and expense.
Among the leading works on photo-engraving, etc., may be mentioned The Half-tone Process, by Julius Verfusser, and Photo-Mechanical Processes, by W. T. Wilkinson.