The nature of the grain depends upon the subject to be etched. If a fine grain is wanted the box must be left for a minute or so before the plate is inserted ; if this is done immediately the box comes to rest, coarser grains are obtained. Exact degrees of grain will be ascertained after a little experience ; it can be examined, on removal from the box, under a reading glass. Next the plate is laid on a copper heater or hot plate, in order to cause the grain to adhere firmly on the plate ; when a sort of bloom appears it will be a sign that a sufficient amount of heat has been applied, and the plate should be removed and allowed to dry. If left too long, the resin and bitumen will have melted and formed a solid coating on the plate ; when this happens the whole must be cleaned off with benzole, and we must begin again.
The next proceeding is to secure a negative carbon of the subject, which is generally made from a positive trans parency. Some workers can make the carbon negative direct by sensitising the tissue in a solution of ferric chloride rendered acid by the addition of citric acid ; but this is far from easy, and the positive transparency is the quickest method in the end. If a light-coloured tissue is adopted, printing is visible. The usual " safe edge" will give us a black border, and if this is not desired, a resisting border may be secured by masking the tissue after printing with a card or piece of orange paper rather smaller than the transparency, and exposing once more to light for a short time.
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 45', 40°, 37°, 33', and on a Beaune 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.
Woodburytype.—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 homo geneous 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.
Collotype.—This is the most economical process for ob taining 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.