Photogravure.

Photogravure. By this process the finest possible results are obtained, but the expense of producing pictures by its aid, which is akin to the operation of copperplate printing, limits its use to high-class book-work. It is also used by Boussod, Valadon & Co., by Durand of Paris, by the Autotype Company, by Messrs Annan & Swan of Glasgow, and others for the production of large pictures which rival the finest steel engravings in their delicacy and finish. Photographs can be reproduced in this form, but the process seems to be more largely employed for obtaining engraving-like copies of celebrated pictures. The process is so perfect that every touch of the painter's brush is clearly seen in the copy, and even the upstanding ridges of paint in the bolder touches are rigidly reproduced. There are naturally different ways by which printing plates for use in this process are made, and a brief description of two methods only must here suffice to give an indication of the line of operations. (1) A gelatine relief is obtained by exposing bichromated gelatine to the action of light beneath a negative. But the gelatine employed is mingled with a certain quantity of graphite (black lead) in a more or less granular form. This addition causes the resulting relief to have a surface which is granular in character, and which is also a conducting one to electricity. If therefore the relief be placed in an electrolyte bath it will speedily become covered with a deposit of copper. From the copper plate so formed copies on paper can be obtained by the usual copperplate printing process. (2) A bichromated gelatine print—negative in character—is developed upon the specially prepared surface of a copper plate, which is then subjected to the action of a solution of perchloride of iron. This penetrates the gelatine more or less quickly according to its varying thickness, and then attacks the copper, which is eaten away by the chemical action that ensues. Thus in the end the copper plate bears on its surface an etched image, penetrating more or less in depth according to the shadows and lights of the gelatine image previously affixed to it. The plate is next 'steel'-faced and printed in the copperplate press. (It is often necessary to resort to hand-finishing in order to get the finest results.)