I’ve always been interested in all aspects of pulp production including how they reproduced line drawings in the pulps. The same process worked for stylistically very different artists like Hannes Bok, Virgil Finlay, Edd Cartier, Nick Eggenhoffer, Arthur Rodman Bowker, Dorothy Flack and John R. Flanagan. Read on and find out how they did it.
The first operation in the making of a line plate is to make a photograph of the copy on a photographic plate. This is called “making a line negative.”
The image becomes reversed when passing through the lens, so the photographic image on the line negative is backward or reversed. The negative film is therefore stripped off the glass and turned over on another glass so that it assumes the position of the copy.
A piece of metal, usually zinc, is coated with a photographic emulsion of albumen and ammonium bichromate, evenly flowed and dried. The line negative is placed in contact with this coated metal in a printing frame. Heavy pressure forces the metal into thorough contact with the line negative and the frame, negative, and coated metal are then exposed to the printing light.
The light passes through the transparent parts of the negative and affects the bichromated albumen so as to render it insoluble. The dark, opaque parts of the negative keep the light from the emulsion so it is not affected in those masked parts.
Handled in a subdued light, so as to prevent further light action on the sensitive emulsion, the printed sheet of metal is removed from the printing frame and coated with a greasy ink, called etching ink. After this ink has been evenly and smoothly distributed over the entire surface by a roller, the plate is developed by immersion in water. This slowly dissolves the soluble albumen, which washes off, taking its ink covering with it. The insoluble parts, with their ink top, adhere to the metal. They will eventually become the acid resist of the plate.
The metal plate now has on it a reproduction of the original copy and is called a “zinc print.” The line negative having been turned over, the image on the metal is now backward again, or reversed in position.
The zinc print, when dried, is covered and dusted with a resin called “topping powder,” which adheres to the greasy ink. When heated to a proper temperature, this powder melts and becomes the acid-resisting top which protects the surfaces while the exposed areas of metal are etched.
The plate is next placed in a solution of nitric acid which attacks the metal except where protected by the resist. As it eats the metal away, there is always danger that it will eat under the top, as a river undermines its banks. This is called “undercutting” and is prevented only by very careful handling by the etcher, after the completion of the first stage or “bite.”
When the plate has the proper depth for the first bite, the acid is washed off and the sides of the parts now in relief are dusted with another resin powder called “dragon’s blood.” This is melted by heat and forms a resist on the sides of the parts standing in relief, thereby protecting them against undercutting during the next bite. This is repeated for each bite until the plate has sufficient depth.
In this condition, the plate could be inked and printed, but further work is necessary for commercial use. In order to get greater depth in the larger etched areas, as well as to remove excess metal, the plate is routed. This is done with a routing machine and a milling tool, which cuts the excess metal cleanly away.
The plate is now inked with a roller and printing ink and printed on paper. The impressions from the inked plate on paper are called proofs. After proofs have been made, the plate is ready for blocking on wood or metal and is ready for the printer. This last operation of printing again reverses the position of the subject to that of the copy.
[The above description was taken from The process and practice of photo-engraving by Harry A. Groesbeck, published by Doubleday, Page and Co. in 1924]
Here’s a short video showing the process. Not exactly as they did it back in the day, but close enough.