The cylinder is etched by spraying with solu tions of iron perchioride of successively lower strengths, which penetrate the gelatine resist image more or less quickly and attack the underlying metal to a greater or less depth, according to the thinness or thickness of the gelatine, that is to say, according to the depth of tone of the positive. The images of the trans parent lines of the screen offer an almost com plete resistance to the passage of the etching fluid, since they consist of gelatine more thoroughly hardened than any other parts of the image by reason of the two successive exposures. They thus form innumerable sep arate tiny cells, varying in depth, but each having thin partition walls uniformly level with the original surface of the metal.
On completion of etching, the cylinder is cleaned to remove the resist and is passed on to the printing machine.
The ink used for rotary gravure printing is quite different from that for other printing processes. In place of the stiff mixture of pigment and a drying oil employed for these latter, there is used a highly fluid solution of various resins and dyes in a volatile medium such as toluene or xylene. The depressions in the cylinder are charged with this liquid by means of a roller, half immersed in the ink container. The excess of ink is removed from the cylinder by a steel blade (called a doctor) which constantly bears against the cell walls, whilst jets of warm compressed air effect the drying of superfluous ink adhering to the metal in the margins. The paper passes round the cylinder under strong pressure, taking up the ink held in the cells, and then traverses steam heated cylinders by which the solvents in the ink are evaporated.
The final impression consists of different thicknesses of a coloured transparent ink. In consequence of the sideway diffusion of the the pattern of white lines formed by the parti tion walls of the cells is slightly darkened and thus the exceedingly fine structure of the image is still further improved in this respect.' 922. The " copy " for gravure reproduction is preferably the original negative ; failing this, a first-rate print on paper of quality for use as the original for a half-tone block (§ 931) should be sent.
In addition to particulars of the size or scale of the reproduction and of the amount of subject to be included, as when ordering process blocks, it is well, when giving an order, to mark also on the print details respecting the printing, viz. number of copies, colour and size of paper, colour of ink, etc., data that may also be put on the proof which finally reaches the engraver marked "Press." 923. Colour Prints. There are numerous pro cesses by which prints in colours may be made of a subject of which only an ordinary negative is available ; the colour scheme is decided by the colour printers in accordance with any written instructions or a coloured specimen photograph.
Here we will consider briefly only those pro cesses of reproduction, in three or four colours, from colour-sensation negatives.
While it is possible to make, by one or other of the processes which have been mentioned, a reproduction in three or four colours of a colour screen transparency, this method of working should only be employed for moving subjects or landscapes, of which the making of selection negatives direct presents some difficulties.
In all other cases the method to be followed is that of three-colour separation, and it is often of advantage to make, in addition, a negative on the same scale on a panchromatic plate with a deep yellow filter for use, if neces sary, as a grey or neutral key printing plate. In every case it is well to supply the colour fine-etchers with a good colour screen transpar ency of the subject, in addition to the three colour set of negatives, as a guide in their work of retouching. The transparency need not be the same size as the separation negatives.
It is always of advantage, when placing orders of this kind, to get into touch', before the work is undertaken, with the specialist who will be directly responsible for the reproductions, and to carry out his requirements.