Doncaster Scientific Society
Process Work and Illustrations.
At the last meeting of this society, held in the Mansion House (by kind permission of the Mayor), a paper was read by the president (Mr. George B. Bisat) on “Process Work and Illustrations.”
The lecturer pointed out that pictorial reproductions was said to be the most pleasant way of disseminating knowledge, and by a combination of photography and process engraving this knowledge was now brought prominently before everyone. As showing this universally, it is almost impossible to peruse any illustrated daily or weekly newspaper without finding that its illustrations are due to “process” work. In fact, so popular are these photo-mechanical processes that they have practically given the death-blow to wood-engraving.

A reference to the earlier experiments of Fox-Talbot was followed by a description of the invention and perfection of the “half-tone” screen, which is so necessary a part of the process. The use of this screen is to break up the gradations of the picture into a series of small dots of varying sizes, by interposing the screen between the sensitive plate and the lens when making a negative from the original picture. This negative is exposed to light in contact with a copper plate which has been coated with a solution offish-glue, white of egg, and ammonium bichromate. This film is soluble in water before exposure to light, but the action of light upon it is to render it insoluble, so that those parts of the film which are under the high-lights of the negative, and through which the light has not been able to penetrate, will be found to be soluble, whilst other parts – those under the shadows of the negative – are insoluble, being rendered so by the light which has passed through the clear parts of the negative,. The copper plate is then washed in water, which removes the soluble portions of the film, and leaves the insoluble dots standing up on the surface of the copper. The plate is then immersed in an etching solution and etched to the required depth, and is then ready to be mounted in a wood block, ready for the printer.

The lecturer spoke also of the production of “line” blocks, and gave brief descriptions of collotype and photogravure processes, and of the “three-colour” process of printing in colours.

The paper was illustrated with examples of “line” and “half-tone” blocks, and a large selection of specimen prints executed by the principal “process” workers, which were greatly admired – the specimens of “three-colour” work exiting much interest.