The process to be described only needs the experience gained in everyday work to render it a letterpress block process of high quality. The process is simply a means, of direct electrotypy on a relief, produced on a gelatino-bromide negative, the electro being then passed on to the printer. Producing a relief on a gelatino-bromide negative dispenses with the use of collodion, with zinc or other surfaces sensitised with bichromated albumen and gelatine or bitumen, and with the difficult art of etching. It must be understood as distinctly referring to half-tone block work.
A gelatino-bromide negative is developed with alkaline pyro or hydroquinone, then fixed in strong hypo; when washed (care being taken the image does not absorb too much water), the plate is placed on a metal support and gradually heated to 212° F. This is very simple, and if tried, will result in an extraordinary production, the most probable being the shadows of the image running all over the plate. If we take a negative that has been impressed with a screen of some definite pattern, we get a very different result. Each little dot of the screen image holds an amount of reduced silver, bearing some definite proportion to the action of light and development; this is surrounded by a fine line, containing no silver, due to the opacity of the screen preventing any action. If we examine the little squares, we find the reduced silver has produced a certain amount of insolubility, the absorption or combination of the freed bromine with the gelatine a further amount, and a similar action due to the pyro. Now, on raising the temperature of such a film to the melting point of the part holding no silver, these portions melt, and are drawn by capillary action under the unmelted dots.
This capillary action is evidently proportional in some way to the amount of reduced silver, and, therefore to the image. The importance of this consists in that the image becomes the actual regulator of the sizes of the dots; such being the case, we have the remarkable fact that the image is engraving itself, and the two operations of relief and graduation are proceeding at the same time. To understand the nature of this, it is necessary to remark that, in ordinary half-tone work, the endeavour is made to produce a graduated screen negative by optical means - well known to all who have studied the matter - and then reproduce from this graduated screen negative by etching in zinc or copper. The blocks are electros taken direct off camera negatives, some of them being produced direct from a silver print, and others from glass positives. The large electro is deposited by dynamo, and may be readily printed by any printer; the other blocks are deposited by battery, and require fine printing. It remains now to produce something between - a not very difficult matter at the present stage. - (H. Sutton.)