(31). Warnerke has recently published some improvements based upon the discovery that a gelatine plate submitted to pyrogallic acid becomes insoluble in the parts exposed to light. The ordinary gelatine process requires very accurately-timed exposure; but with the pyrogallic acid, and using the emulsion on paper, no amount of over-exposure will do harm, provided the developer is sufficiently restrained. The transfer of the image from the paper to glass is very simple. The former is immersed in water, and placed in contact with a glass plate; the superfluous moisture is removed by a squeegee, and the paper is stripped off, leaving the gelatine on the glass, when the application of hot water dissolves all the gelatine not acted upon by the light, and the image is left in relief on the glass. Intensification is effected by mixing with the emulsion a non-actinic colouring matter which is not affected by silver; aniline colours answer the purpose well. Relief is said to be obtained far more easily than by the ordinary bichromatized gelatine, and the process is therefore specially applicable to Woodbury-type. It may also be adapted to engraving, enamelling, and collotype purposes.

(32) Heliotype

The most important of the many modifications of the collotype process is the "heliotype" invented by Ernest Edwards, wherein the great advantage consists in toughening the gelatine film by means of chrome-alum. His method is briefly as follows: The solution of gelatine and bichromate, with the due proportion of chrome-alum, is poured upon the previously waxed surface of a carefully levelled glass plate, and dried, when the film is readily detached. The latter resembles a piece of thick paper, and may be similarly handled. After exposure in contact with a negative, the film is placed on a plate of zinc or pewter under water, and firmly attached by passing an indiarubber "squeegee " sharply over the surface of the film. The printing film on its plate is soaked in water sufficiently long to remove the superfluous bichromate, to prevent the further action of the light, and is then ready for the press. This is preferably on the vertical principle, such as the Albion printing - press. The inking possesses peculiar features; a very stiff ink may be used to give the deepest shadows, and this may be followed by a thinner ink, even one more or less coloured, for the half-tones, thus producing a bichromatic effect in a single printing.

The time occupied in drying the film is 24 to 36 hours at 90° F. (32° C.); 1500 copies have been successfully taken from one plate; one man can print 200 to 300 copies daily; for very long numbers, it can hardly compete with lithography in price, but for moderate numbers the cost is very small.

(33) Capt. Waterhouse has introduced a modified process as follows: - The sensitive film is laid on flat copper plates, finely grained on one side. After levelling on the drying apparatus, the plates are washed with warm water, and coated on the grained side, while still wet, with a mixture of 230 gr. NeJson's opaque gelatine, and 62 gr. powdered potassium bichromate in 3 1/2 fl. oz. water, adding 1 fl. dr. formic acid when the first are dissolved. This is applied like collodion, and the excess is poured off. The coated plates are replaced in the drying apparatus, and covered over. In about 2 hours at 120° F. (50° C), the films dry with a fine, even, glossy surface perfectly free from streaks and waviness. It is best to let the plates harden for a day or two before use.

(34) Instead of using a tray filled with a compound to receive the ink, Alissoff employs sheets of " polygraphic " paper, prepared in the following manner: - Sized or unsized paper is coated, on one side, with a composition consisting of glue, or gelatine, glycerine, soap, and water, approximately in the following proportions, which have been found to give good results in practice: 80 lb. animal glue or gelatine, 20 lb. glycerine, 20 lb. soap, 200 lb. water. The paper is occasionally found too sticky for use, depending on the surrounding temperature and the quality of the materials. To obviate this objection, wash the -prepared paper with a solution of alum* the strength of which can only be determined by experiments in each case. The paper may be of different thicknesses, and rendered transparent. The ink found to give the best results for written documents is prepared by dissolving 1 lb. aniline in 1 1/4 lb. alcohol, and adding, when dissolved, as much water as is necessary to render it sufficiently fluid. It may then be bottled for use. In producing the " matrix," take a sheet of prepared paper, and lay it on a sheet of damp flannel, placed upon a zinc plate or an oil paper.

Sponge with clean water, or, in hot weather, with water containing a little alum, and place the dry original upon the prepared paper. Over that place another piece of damp flannel, zinc, or oil-paper, and put the whole pile into an ordinary copying-press. A good matrix can be obtained by mere pressure of the hands without a press, although a press is preferable. The text must be written, drawn, or printed with aniline ink, taking care that the pen be quite clean and always full of ink. The ink when dry ought to shine like a metallic surface. In taking copies from the "matrix," after having detached the original, place a sheet of ordinary paper in the place of the original, and proceed in the same way as when producing the matrix; but if copies or " matrices " are to be taken from 2, 4, 6, or 8 pages at once, place a sheet of damped polygraphic paper on each page, with damp flannels and zinc sheets between the leaves, and proceed in the way described. (35) The Asser process differs from those generally in use principally in regard to the paper employed, which is not sized or gelatined, and, consequently, though bad for transferring, nevertheless is capable of fulfilling a special role.

Take unsized paper of medium thickness and fine texture, and apply to the surface, by the aid of a clean sponge, a thin film of starch dissolved in water. After the paper, hung up on a pin, has dried, it is floated with the coated side uppermost, upon a saturated solution of bichromate of potash in distilled water, With which it is at once impregnated; consequently the paper is permitted to float upon the solution but a very short time. It is then hung in the dark to dry, and at the lowest corner is put a tiny piece of bibulous paper to absorb the superfluous liquid. As soon as the paper has completely dried, it is put into a printing-frame, the starch-coated side against the negative, which should be pretty vigorous and clear. After printing for a longer or shorter time, a clear, brown image appears upon an orange-yellow ground. When sufficiently printed, the paper is put, image uppermost, in a water bath, care being taken that no air-bubbles are formed between paper and water. It is allowed to float on the water (in the dark) until all the bichromate of potash which has not been acted upon by light is dissolved out, and the picture appears clear and of a light-brown colour; it is then taken out of the water and dried, first of all between leaves of blotting-paper, and then in the air.