12½ × 9. VIEW FROM THE TOWER BRIDGE WORKS.
Bitumen process. Sky revised by hand-work.
The procedure in making a heliogravure is in this wise:—A copper plate, similar to the kind used by etchers, receives a ground of bichromatized bitumen. A photograph is taken of the drawing to be reproduced, and from the negative thus obtained a positive is made. The positive, in reverse, is placed upon the grounded plate and printed upon it. The bitumen which has been printed upon by the action of light is thus rendered wholly insoluble, and the image of the drawing remains the only soluble portion of the ground. The plate is then treated with turpentine, and the soluble lines thus dissolved. Follows then the ordinary etching procedure. This is a more simple and ready process than the making of a relief block. It is, however, more expensive to commission, but then expense never is any criterion of original cost. The printing, though, is a heavy item, because, equally with etchings or mezzotints, it must be printed upon a copper-plate press, and this involves the cleaning and the re-inking of the plate with every impression.
The subject which the present plate bears does not show the utmost capabilities of the heliogravure. It was chosen as a fair example to show the difference between two methods without straining the limitations of the relief block. But if the drawing had been most carefully graduated in intensity from the deepest black to the palest brown, the copper plate would have shown everything with perfect ease. Large editions of these plates are not to be printed without injury, because the constant wiping of the soft copper wears down the surface. But to obviate this defect a process of acierage has been invented, by which a coating of iron is electrically deposited upon the surface of the plate, rendering it, practically, as durable as a steel engraving.
