No reproductions of drawings of scientific subjects apparently have been reproduced by this method. Examples can conveniently be examined in The Seven Lamps of Architecture by Ruskin.
MEZZOTINT. The characteristic feature of mezzotint is that the subject is translated into tones rather than lines as in the preceding intaglio methods.
The surface of a smooth metal plate—usually copper—is raised into innumerable and minute projections by going over it in all directions with a curved steel tool, known as a rocker, the edge of which is finely toothed. An impression taken of the plate in this condition will give a deep rich tone. The high lights are obtained by scraping and burnishing away the elevations so that there are no pits left to hold the ink, and, similarly, intermediate tones are produced by partly removing the pile so that the pits are made of varying degrees of shallowness and consequently will print in tones according to their depth.
Impressions are taken in the same way as in the case of etchings.
Mezzotint apparently has never been used for the reproduction of scientific subjects. Indeed, in a sense, this process is much too artistic for the purpose. At their best, illustrations reproduced by this method have mystery and depth and give the imagination much employ; in a word, they are subjective rather than objective, qualities unsuited for our purpose.
PHOTOGRAVURE. Photogravure may next be considered, for although it is a photo-mechanical process, it corresponds to mezzotint. Excellent results may be obtained by its use provided the drawings—usually executed in monochrome such as sepia—be really good, otherwise they are hardly worth reproducing by this relatively expensive method.*
*This account refers only to plates made and printed entirely by hand, not to photogravure for printing on a rotary machine.
Photogravure is particularly suitable for the reproduction of drawings showing a large amount of detail and made up of a variety of tones rather than lines or stipple.
The photographic part of the process is essentially the same as making a carbon print from a photographic negative. This consists in exposing under the negative the carbon tissue, which is a mixture of gelatine, in which is dissolved bichromate of potash, and a suitable pigment. Such a film of bichromate gelatine is, when dry, sensitive to light. If no light gains access to it, the gelatine is readily soluble in warm water; if light acts upon it, the gelatine becomes insoluble in proportion to the degree of its exposure. Obviously, the pigment will be retained in varying degrees according to the relative insolubility of the different parts of the gelatine.