(c) Large areas to print white are cut away with a routing machine, the plate cut up as required by the various subjects, and perforated for pinning to wood of thickness to bring the surface of the metal level with that of the type in which it will be placed for printing.
913. It is out of the province of this book to deal with the making of originals for line blocks, but some hints may be given on the marking of originals to show the scale or the size of the reproduction and the use to be made of tints (stipple, hatching, or lines) over parts of the drawing.
As regards the scale of the reproduction, it is usual to mark the original with the exact size it is to be in the block, drawing a line alongside either the horizontal or vertical dimension, thus 3 in.
The points of the arrows are understood to indicate the extreme edges of the original. In the case of a number of originals to be repro duced on the same scale, it is usual to mark one only, e.g. No. i, with the size required and to attach to the others the instruction " Reduce with No. i," or "As they come." The engraver will then include as many as he can on one negative and one printing plate, and so reduce the cost to the customer.
The parts of the original to be given a tint in the block are marked on the former by cross hatching in blue pencil,' and a note is made on the original as to which pattern of tint, chosen from the photo-engraver's album of specimens. is to be used. In the event of the area for the tint not being defined in the original, it is indicated in blue pencil.
In like manner, any parts to be " scored " are indicated, preferably by attaching a cut-out mask to the original and giving on it the instruc tion for the kind of shading, e.g. cross-hatching or stipple.
914. Half-tone. The making of a half-tone block comprises numerous operations carried out by workmen in different branches of the trade.
The first part of the process is photographing the original through a screen consisting of two glasses engraved in lines and placed face to face, with the lines crossing to form a chequer pattern of from 50 to 150 lines per inch. By suitable adjustment of the size and shape of time lens diaphragm and of the distance of the ruled screen from the sensitive surface, there is an automatic rendering of the tones of the original by image dots corresponding with the trans parent apertures in the screen but larger or smaller than these latter, owing to a penumbra effect, according as a particular part of the resulting negative represents a light or a dark part of the original. The lens diaphragm most
generally employed is one with a square aper ture, placed with the diagonals parallel with the lines of the screen. This rendering of the original resembles in some degree that made by a wood-engraver or black-and-white draughts man, but the result differs from work in these styles, since it conveys an illusion of tone, although the impression itself consists of only two tones, that of the ink (which is of the same thickness in all parts) and that of the bare paper. On examining a half-tone print with a magnifying lens its dot structure is plainly seen, and is indeed perceptible to the naked eye in the case of the half-tone impressions in the daily newspapers.
915. The negative for printing a half-tone block needs to be reversed as regards right and left, and is usually so made in the camera by the aid of a prism or mirror (§ 123) ; or the negative may be reversed by stripping the film, which is usually that of a wet-collodion plate, although dry plates are also used. It is then printed on to metal, zinc being used for the coarser work, and copper for the finest work. The metal is sensitized with bichromated fish glue ; and, after printing, the excess of bi chromate is washed out, the plate dyed, and again washed, whereupon it is " burnt in," con verting the glue into a species of hard enamel which resists the action of etching baths.
Before or after burning-in, the subject is marked out with a graving tool, and, if required, a rule is put round it by aid of a drawing-pen and varnish. The back of the metal is also varnished, and the plate is then ready for etching. This latter is done in short stages with frequent inspection. Parts which the fine etcher judges to be sufficiently etched are stopped out with varnish, whilst other parts are attacked or lightened by further etching. The etched plate also often requires to be retouched with a tool for the removal of minor defects.