(27) Michaud's Process
A negative of the subject to be reproduced having been obtained, a print is taken on a film of bichromated gelatine supported by a metal plate. The print is developed by the usual means, and, after being dried in the open air, is placed in a box containing a hygroscopic material, the dampness of which causes the gelatine to swell slightly, and so increase the amount of relief. The secret of Michaud's process consists in pressing this gelatine film into the plastic face of a fusible metal, which is contained in suitable trays or dishes. The " forme," as it is called, is treated in much the same way as the mould and block in the process of electrotyping, being placed under a screw press, and allowed to remain until the metal becomes solid. The fusible alloy used is compounded of 50 parts bismuth, 30 tin, 20 lead, and 11 mercury. The plate produced by impressing the gelatine on the semi-fluid face of the alloy can be employed for the production of other plates; but for the purposes of typography it will be seen that several operations must still be carried out in order to obtain a raised plate in hard metal from a sunk plate in a fusible alloy. When the picture required contains half-tones, it is necessary to obtain a grained surface.
This is accomplished by means of a special film of coloured bichromated gelatine, which is exposed under a plate dusted with a rather coarse opaque powder. The film thus produced is attached to the negative by means of a solution of gum arabic, containing a little bichromate of potash, the operation being performed under water. The parts not affected by light are then removed in the usual manner, and a negative is obtained with a grained surface from which the print is taken with which the impression is made.
(28) Chromotype. The negatives for chromotypes should be thinner than those for ordinary uses, and surrounded with an opaque margin of paint or paper. The papers are sensitized by immersion in solution of potassium bichromate (1 salt: 32 water) for about 11/2 minute. The duration of immersion depends, Jiowever, on the temperature, and the nature of the paper. The more sugar the latter contains, the shorter may be the time of immersion; the paper must remain in the solution till it. spreads out quite flat. The drying must take place at low temperatures, otherwise a hard brittle paper is obtained. It is also of importance that the sensitizing liquid should contain no free acid, otherwise a covering is got which does not dissolve in water. The exposure should last but 2/3 of the time required for silver prints. A piece of silvered paper serves as a photometer; when it has turned brown, the light may be shut off. Of course the intensity of the negative has to be taken into consideration; over-exposure is better than under-exposure. The development or transference of the chro-mate layer with its invisible image to glass or metal, is effected in the usual way.
When the paper has been fastened to the plate, the latter is put into a water bath of about 86° F. (30° C), which is then slowly raised to a little higher temperature. The developed image is transferred to paper prepared with a solution consisting of 6 dr. gelatine, 12 gr. chrome-alum, 18 1/2 oz. water, 3 dr. glacial acetic acid. The paper is steeped in water at 104° F. (40° C), and then brought into contact with the developed image. It is more convenient to dispense with this second transference, and to place the image at once upon its permanent surface. In this case, special negatives are required. Chromotypes on tinted and coarse-faced papers appear very elegant. The images prepared as above described are ready for colouring with water colours, etc. If it is intended to paint with much colour, the image should be covered with a watery solution of shellac, which hardens the gelatine. When it is desired to produce an image on ivory, it is necessary to wash the image after exposure to light, in order to remove the residual chromate, otherwise a yellow tinge would be imparted to the ivory.
(29) Much has been done in the task of reproducing half-tone drawings and photographs from nature, by Woodbury, Dallas, Lenoir, and others. A manager of Goupil's named Roussillon, availing himself of the Woodbury-type process, gives a grain to the picture by the action of light, suitably regulated, and thus obtains a mould capable of giving mezzo-tints from ordinary negatives. They require some mechanical touching-up, however.
(30) Lenoir has recently made public a new process for producing engraved plates from negatives photographed from nature, which is substantially as follows. A metallic plate is lightly coated with a mixture of albumen, carmine, and potassium bichromate. The carmine (for which gamboge and various resins may be substituted with almost equal success) serves both as a dye and to assist in the lifting of the film, by its solubility in ammonia, drawing the albumen with it more or less in the stripping-off, the exposure having taken place upon the upper surface. When the film is stripped off, an image remains formed of albumen, in itself unable to resist the action of acids. It must, therefore, be rendered insoluble. There are two ways by which this may be effected; one is to cause the albumen to absorb a solution of gum lac, dissolved in hot water with borax; the other, and preferable, is to plunge the plate, once stripped, in a solution of potassium bichromate, then drying at about 120° F. (49° C.) The albumen by this means acquires the required resistance to the action of acids. The plate is next engraved, to give it a grain according to the amount of ink it should take up.
Upon the unabsorbent and stripped plate, a film is spread, consisting of a solution of bitumen and turpentine mixed with carbonate of lime. When plunged in an acid bath, carbonic acid is liberated; it forms tiny canals, through which the acid attacks the metal more or less quickly, by reason of the thickness of the albumen. The acid bath is composed of water acidulated with nitric and oxalic acids and alum. An oxalate of the metal is thus formed on the sides of the canals, and causes them to adhere to the plate. The texture of the etching is more or less fine according to the length of time the albumen is allowed to absorb the acid. In this state, the plate is finished; it requires only to be dried, and is ready to be printed from immediately. No preliminary preparation is necessary, as the whole operation may be conducted in 3 hours.