The illustrations in all except the most pretentious volumes are either
halftone or lineplate photo-engravings. In making a halftone plate, the
picture or object to be reproduced is photographed through a screen
consisting of a glass plate, diagonally ruled at right angles in two
directions with lines numbering from fifty to four hundred to the
inch. This screen is placed inside of the camera and in front of, and
very near, the chemically sensitized plate. The light reflected from
the object to be photographed, varying in intensity according to the
lights and shadows of the object, is focused on the sensitized plate
through the intervening line screen, and affects the sensitized film
more or less according to its intensity. This causes a chemical change
of such nature that the next following operations, the development
and the intensification of the picture, result in producing it in the
form of dots and stipples varying in size, and consequently in the
respective light and shade effects, according to the varying lights
and shadows of the original. Inasmuch as the lights show dark and the
darks light, the picture on the glass makes a negative of the subject.
This negative is placed in a printing frame, in close contact with a
polished copper plate prepared with a film sensitive to the light. A
few minutes' exposure to the light renders insoluble in water those
parts of the film which the light has reached through the negative, and
when the other parts of the film, which remain soluble in water, are
washed away, the picture appears clear on the surface of the plate. The
dots and the stipples forming the picture are then further treated to
enable them to resist the action of the solution of iron perchloride to
which the plate is next subjected, which etches out the spaces between
the dots, and leaves the latter in relief. As the etching on the
copper must be in reverse as regards right and left, in order that it
may appear in proper relation when printed on the paper, the negative
must be produced through a reflecting prism, or the finished negative,
properly toughened, must be stripped from the glass on which it has
been produced, and turned over. In ordinary practice, a number of such
turned negatives are placed together on a single large glass, and
exposed together on a large copper plate, to be cut apart afterwards
and mounted separately. The primary etching is usually supplemented by
further processes, such as re-etching, vignetting, hand-tooling and
routing. The finished plate is finally mounted on a wooden block to the
height of type.
Illustrations in full color are reproduced from corresponding
originals, usually paintings in oil or water-color, by means of the
three-[40] or four-color[40] process of reproduction. The plates for
this purpose are usually all halftone, but are sometimes a combination
of halftones and Benday[40] plates. Two-color halftones have either
a tint background, or secondary plate in tint, the latter forming
the underground upon which the keyplate is printed in black. In the
three-color process, the respective plates are printed in yellow, red
and blue successively over one another. In the four-color process a
fourth plate is used to emphasize the blacks of the picture, the plate
being virtually a keyplate, combining all the features of the subject,
printed on top of the other three colors, usually in black or dark gray.
It is of particular importance that the engraver who is to make the
halftone plates should be informed as to the kind of paper they are to
be printed on. A 50-line halftone plate will print on almost anything,
but is too coarse to render the details of the picture, and is usually
applied only for newspaper use. It would be entirely too coarse for the
purpose of book illustration. On the other hand, a halftone plate made
through a screen of 400 lines to the inch can be printed satisfactorily
only upon paper of the highest surface, and with correspondingly
careful presswork. For super-calendered or English-finish paper, plates
made through a 133-line screen are most advisable, while the average
coated or enameled paper will take 150-line halftones to best advantage.
Lineplates are etchings in relief on plates of zinc or copper,
reproduced from pen-and-ink-drawings, or diagrams, by photo-mechanical
process. The method in general is the same as that for halftone work,
but without the intervention of the screen. In lineplates, the light
and shade effects are produced by gradations of thick and thin lines,
in distinction from the effects of wash-drawings and photographs, which
are produced by gradations of tone. The latter require the intervention
of the screen to convert the full tone gradations into the halftone
of the dots and stipples, while the former may, as already noted, be
reproduced directly.
Other classes of engravings, of a more costly kind, and which are
therefore used only in books of more expensive character, are the
various forms of engraving in intaglio; that is to say, in effects
produced by cutting or etching the design into and below the surface
of the plate, instead of cutting or etching away the ground, and
leaving the design in relief. Examples of this order are the old-time
copperplate engraving, the more modern steel-engraving,[41] in the
form of line or mezzotint effects,[41] photogravure,[41] and the yet
more recent photo-intaglio process known as rotogravure,[41] and
photo-mezzotint.