Pellet's—Pellet's process, which gives copies in blue lines on a white ground, is an improvement upon the white lines on blue ground, as a method of obtaining copies of drawings, inasmuch as it permits the subsequent tinting of the copies, and requires much shorter exposure. The original drawing is placed in a printing-frame, in front of a corresponding sheet of the sensitive paper (supplied by the inventor), composed of a piece of stout paper, coated with a mixture of peirebleride of iron and some easily oxidizable organic substance. The frame being exposed for a minute iu the sunshine, the permit of iron becomes reduced to a proto-salt wherever the sensitive paper is unprotected by the opaque lines of the original drawing. On removal from the frame, the exposed sheet is immersed in a strong solution of potassium ferroeyanide, and this substance, reacting with the per-salt of iron remaining on those parts of the paper protected by the opaque lines, produces Prussian blue, while the ground of the paper remains white. The print is %Noshed, soaked in dilute hydrochloric acid, and washed again to remove traees of the acid.
Woodbury-type.—Woodbury's process is intended to produce a mould of a gelatine print, from which, other prints may be obtained. A thick film of sensitive gelatine, resting on a tough stratum of collodion, is placed beneath a negative with the collodion side next the image. After sufficient exposure to a light so arranged that the rays always fall in one direction, the gelatine picture is developed as if it were an autotype print, and presents the image in considerable relief. After drying, it is laid on a perfeetly flat metallic plate, and a sheet of lead or some other soft metal is forced down upon it by a powerful press. The metallic sheet, being an exact mould of the gelatine picture, is put into a special press ; and a viscous compound of gelatine dissolved in hot water, with the addition of fine pigment or permanent dye, is poured upon this sheet. Strongly-sized paper, of even texture, is placed upon the viscous compound, and the top plate of the press is brought down upon the mould, and firmly held, thus squeezing out the superfluous gelatine. The gelatine soon sets, when the top is raised, and the paper bearing the picture is detached. The print is immersed in alum solution, to render the impression insoluble. The top plate of the press is made of thick glass, and its surface is a perfeet plane, to ensure the gelatine being squeezed out from the portions which are to be white in the picture, and to prevent a mottled and uneven appearance. Within certain limits concerning the size of white surface which can be produeed (owing to the variations in the thickness of all paper), this proeess is capable of producing per manent images at a price but little greater than the cost of the paper and solution.
Photo-lithography.—Another process founded on the insolubility of gelatine when treated with a biebromate and exposed to light, is one c,apable of producing pictures iu printing-ink, as well as in ink adapted to transferring to zinc or stone, images being reproduced by ordinary surface-printing from tbe transferred prints. The photographic negative is placed in a photographic printing- or
pressure-frame, with a piece of prepared paper face downwards upon the picture side of the glass. The back is made secure, and the glass side is exposed to the light ; in due time, it is taken to the dark-room, aud coated with transfer-ink. Washing removes the trausfer-ink from those parts which have not been affected by the light (the white parts of the paper), but leaves it where the light has acted (the lines of the picture); thus a photographic transfer is produced, and may be 5 1, applied to stone or zinc, and printed from in the usual manner. The sensitizing solution is pre pared as follows :-1-4 ez. of gelatine (the smaller quantity if " flake ") is set to soak in sufficient water to cover it; meantime, 1 ez. potassium bichremate is dissolved in 5 oz. water, and filtered ; when the gelatine has plimmed, pour on sufficient boiling water to make 11 oz., and add the bichromate solution. Sometimes a dash of glycerine is added. This solution will keep geed for a considerable time in a cool place. To prepaxe the paper, some of the solution warmed to about 38° (100° F.), and sheets of the paper ("bank post," " positive photographic," or other fine-wove and slightly sized) are floated on it for 2-3 minutes, and hung up to dry in the dark-room, then again floated, and suspended from the opposite end. The sensitized paper is exposed in the ordinary manner beneath a negative in the pressure-frame, until the lines appear of a fawn-celour on a yellow ground.
The picture is transferred to stone or zinc by coating the latter with ink, laying the former face downwards upon it, and pulling through the press. Ordinary chalk lithographic ink may be used for single prints, but a superior ink is made as follows :-16 oz. lithographic ink and 8 oz. middle linseed varnish are first mulled together ; 6 ez. Burgundy pitch and 2 ez. bitumen are melted over a clear fire till all the water is driven eff ; 1 oz. white wax is also melted; tbe whole is then mixed together, with 1 oz. palm-eil, and run into vessels for keeping. The print is developed by being floated back downwards on water at a temperature of 38°-50° (100°-122° F.), till the lines appear as depressions. It is then washed with water at about 70° (158° F.) on an inclieed slah, by which, the soluble gelatine is removed with the ink that coated it, and the image remains as ink lines on ridges of insoluble gelatine. The developed print is washed in cold water, and hung up to dry ; it is then ready for transferring te stone er zinc, being first damped till it becomes limp. The subsequent manipulation is a mere repetition of lithographic printing (see p. 1615).