Plate 2.—Geranium Columbinum.
A chromolithographic reproduction of a drawing by Miss O. Johnston
PHOTOLITHOGRAPHIC PROCESSES.—Of these methods of reproduction there are several, their value lying in the fact that the originals can be reduced or enlarged with the greatest of ease. The general principles are as follows.
A photographic negative is taken of the original drawing and a positive made on a film of bichromate gelatine. Wherever light reaches the film, the gelatine is rendered more or less insoluble according to the intensity of the light acting upon it; through the dark parts of the negative but little light will pass, so that the gelatine will remain soluble.
The exposure of the positive having been made, the film, which may be mounted on paper, is inked with lithographic ink in the dark room and then washed. The pigment will adhere to those parts acted on by light, but will wash away from those regions unacted upon; obviously the half-tones will retain ink in direct proportion to their density.
The developed positive is then transferred to a stone or zinc plate and impressions taken as in pure lithography for the dark parts are resistant to water and will take the ink, whilst the high lights will retain water and so will not be inked. The intermediate tones will take the pigment according to their density.
In distinction to the previous methods, corrections cannot be made except in so far as the negative can be touched up.
COLLOTYPE.—Of the various photolithographic methods which have from time to time been employed, collotype is the one in most general use at the present time, especially for the reproduction of photographs.
Collotype is a simple process which does not require so extensive a technical knowledge and ability as some of those previously described. But notwithstanding this, the results are sometimes unsatisfactory and unequal; faults due to indifferent originals and to unsatisfactory conditions obtaining in the work rooms. The great drawbacks to good collotype are cold and dampness, and it is for these reasons that continental firms, blessed with a more stable climate, often produce much the best work. Provided the workshops are properly heated, the collotypers of this country ought to be able to turn out good work at all times of the year; indeed, the best firms do.
For this and for other processes in which photographs form the originals to be reproduced, authors should send the negative to the collotyper; if this be impossible, positives of the best possible quality, printed on ordinary P.O.P. paper, toned to various shades of purple, and also on smooth bromide paper, in ordinary black tones, should be provided in order that the collotyper can choose the print he most prefers to work with. Also, it is usual to glaze the prints.
The method is as follows. A piece of british plate glass, about half-an-inch in thickness, is ground on one side with fine emery powder, and then thoroughly washed and dried. The plate is covered with a filtered mixture of the colloids sodium silicate and dextrine or albumin, and placed in a warm oven to dry. If metal plates are used, such as zinc or copper, this preliminary coating is unnecessary; glass plates, however, must have the substratum in order that the sensitised gelatine—which is next put on—may stick.
When the plate is dry, it is thoroughly washed with water in order to remove any free silicate; it is then dried and put away until required for use.
The sensitising solution is made up of gelatine and bichromate of potash dissolved in water; before use it is filtered, freed from air bubbles and heated to not more than 120° F. The plate is now placed on a stand, which is provided with levelling screws, in the oven, and, when the temperature has reached 120° F., an amount of the bichromate gelatine solution sufficient to make a thickness of film proper for the mode of printing to be employed is poured upon the plate.
The oven is kept at a constant temperature, 120° F., until the gelatine is dry, when it is allowed to cool gradually.
Whilst the gelatine is setting, precautions against vibration must be taken else the plate will be spoilt.
When dry, the collotype plate is sensitive to light and moisture; its surface shows a more or less regular series of convolutions which resemble those of the outer surface of the human brain, although, of course, very much smaller. The character of the grain is very important, for if it be too fine it will not take up a sufficiency of ink, and, on the other hand, if too coarse it will yield coarse impressions.
A reversed negative, of a quality beyond reproach, must be made of the original; if the subject is dark or has heavy shadows the negative is frequently slightly over exposed so as to soften them.
The collotype plate is then exposed under the negative and washed in cold water until the yellow bichromate no longer comes away. It is then dried.
In printing, the plate is damped and rolled up with ink as in lithographic printing; the amount of ink adhering to the film depends on the extent to which the different parts have been acted on by the light, as has already been mentioned. The moistening of the plate—mis-termed etching—is best done with dilute glycerine containing 75 per cent. of water, which when first applied should be allowed to remain on for about half-an-hour. The excess of moisture is taken up with a sponge or a ball of rag, and then the plate is inked and printed in a lithographic or a collotype press. The picture is usually masked with tin foil in order that its edges may be quite clean.
Of the faults which may occur, the following may be alluded to. A mottled appearance may obtain in the high lights; this is due to the coating of gelatine being too thick. More commonly, the reproductions may appear flat owing to the degradation of the high lights; this is a sign that the sensitive film has been acted upon by moisture during its critical existence between the drying and the washing out of the potassium bichromate, or that the temperature has been too low.
The following contain good examples of collotype.
Karsten and Schenck: Vegetationsbilder, Jena.
Oliver: Notes on Trigonocarpus and Polylophospermum. New Phytologist, Vol. 3, 1904.
Semon: Zoologische Forschungsreisen in Australien. Jena. 1912.
Thompson: The Anatomy and Relationships of the Gnetales. Annals of Botany, Vol. 26, 1912.
See also Plates 3, 4, 5 and 9 in the present work.
THE PREPARATION OF ILLUSTRATED PAGES. Of the processes dealt with, photogravure lithography and collotype are those most generally used at the present day for the printing of plates or insets. Half-tone also is employed, a process which will be considered later since it is essentially relief printing. This, therefore, is a convenient opportunity to make a few general observations on plates.
Plates should only be employed for the reproduction of subjects of such complexity that cannot be reproduced satisfactorily by figures in the text.
A plate or page made up of several illustrations should look well as a whole; in other words, it should not outrage all the canons of composition, it should have some pictorial effect. At the same time, for facility of reference, the individual figures should run in a convenient sequence. This latter point is so important that a plate composed really well is distinctly rare, for a compromise nearly always has to be made. At the same time there are, apparently, comparatively few authors who pay much attention to plate design.
Although it is not desired to write of the laws of pictorial composition, attention may be drawn to a few points which are amongst those generally neglected.
The figures should not be crowded together; a reasonable amount of margin should be left around each. They should be arranged, as far as possible, in such a way that a sense of balance is maintained. As to how this is to be accomplished will depend upon the nature of the illustrations; if they are all about equal in tone, the largest ones should preponderate towards the base of the plate, and not vice versâ. The difference which this makes will be obvious if the two accompanying illustrations (Figs. 1 and 2, Plate 3) be compared. The first is a reduced copy of the plate as it was published: it will be noticed that it has a top-heavy appearance, which is corrected in the second figure by the simple device of turning it upside down.
