WOODBURYTYPE PRINTING, aproc ess for obtaining by means of photography a picture or illustration for press printing. This process was invented by W. B. Woodbury, and is the only photo-mechanical one which, in the printing press, realizes the gradations of tone without grain or texture of any kind. The idea of the process emanated from the carbon print, in which the picture is formed in all its gradations by various thicknesses of pigmented gelatine. the shadows, representing the great est thickness, being in relatively high relief, and the high lights the lowest. Mr. Woodbury con ceised the idea of making an electrotype mold of a carbon picture and using the mold so ob tained as a printing surface by covering it over with warm pigmented gelatine and by flat pressure attaching a sheet of paper to the pig meat. so that when the gelatine jelly was set he could detach it from its mold and thus by repeating the operation obtain unlimited copies. This was practically the Woodburytype; but of course there were difficulties and unperiections, which the inventor quickly set to work to over come. He found that a thick film of gelatine and bichromate, when exposed under a nega tive and washed, gave a very high, sharp relief ; and he also found that when this relief was perfectly dry it possessed the property of being absolutely incompressible, in other words, it was at hard as steel and could be used as a die The relief film of gelatine was placed on a block of smooth, hardened steel, with raised edges, upon this was laid a sheet of type metal or lead about a quarter of an inch thick and the arrangement was then placed under a hy draulic press capable of exerting a pressure of about 40 hundredweight to the square inch.
The pressure forced the lead into the gelatine image with such accuracy that every shade and detail of the relief was impressed. The raised edges round the steel block prevented the metal from squeezing out and on being de tached, a mold in lead was obtained which could be used as a printing surface for thousands of copies. The seemingly delicate relief in gela tine was quite uninjured by this treatment, and would serve for any number of further pressed molds.
The printing press for these pictures is special, resembling the copperplate process in principle. It is formed of a cast-iron base, on which is fitted a movable table to hold the mold, which is bedded down with gutta percha; over the table is a hinged lid, faced with plate-glass, with a lever attachment for giving the pressure. The printing ink is a solution of gelatine in a hot state to which a pigment is added to give any desired tint. In printing, the lead mold is first oiled to prevent the gelatine sticking, and a pool of the warm ink is poured on to the middle of the mold; over this pool is laid a sheet of paper waterproofed with shellac, and the lid of the press is brought down over the whole, and pressure applied. The pressure squeezes out over the edges of the mold all superfluous ink, and all that is left is that re tamed in the graduated hollows and depres sions of the lead mold; the warm ink sets in a few minutes, and on opening the press the paper support is removed with its gelatine copy of the molded picture firmly attached to it.