(16) Willis's process is founded on the action of bichromates on organic matter, the printed image being coloured by means of an aniline salt; it is extremely useful for copying plans and simple line-subjects. The operation is as follows: - Sized paper is floated in potassium bichromate containing a little phosphoric acid; it is next -exposed beneath a translucent positive, and when the image of the latter is clearly shown, it is subjected to the action of aniline vapour. The result is that the parts shielded from the light by the lines of the positive are deeply coloured (green, black, or reddish, according to the aniline salt used), while the other parts retain the weak tint of the reduced chromium oxide. In developing the print, it is exposed to the contact of the vapour from aniline dissolved in spirit of wine, the solution being placed in a basin, and heated by a spirit-lamp. The prints are fairly permanent after washing.
(17) Poitevin's Powder
A mixture of gum arabic, sugar, and glycerine, with some sensitizing solution of potassium bichromate, is poured upon an impervious surface (e.g. a glass plate), and dried by warmth. Thus prepared, the plate is immediately exposed beneath a translucent positive for a few minutes.
The parts affected by the light become hygroscopic, in proportion to the duration of the exposure, and intensity of the light, and any impalpable powder delicately brushed over the plate will adhere to the hygroscopic parts, according to their degree of moisture, thus forming a reversed copy. The developed image is coated with collodion, and transferred to paper unreversed, the soluble bichromate being washed out in the operation. Obernetter's recipe for the sensitizing solution is: - 4 parts dextrine, 5 white sugar, 2 ammonium bichromate, 2 to 8 drops glycerine for every 100 cc. of water, and 96 parts water. The glass plate is sometimes previously coated with collodion.
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 perfectly 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 perfect 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 produced (owing to the variations in the thickness of all paper), this process is capable of producing permanent images at a price but little greater than the cost of the paper and solution.
Another process founded on the insolubility of gelatine when treated with a bichromate and exposed to light, is one capable of producing pictures in printing-ink, as well as in ink adapted to transferring to zinc or stone, images being reproduced by ordinary surface-printing from the 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, and coated with transfer-ink. Washing removes the transfer-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 applied to stone or zinc, and printed from in the usual manner. The sensitizing solution is prepared as follows: - 1 to 1 1/2 oz. of gelatine (the smaller quantity if "flake ") is set to soak in sufficient water to cover it; meantime, 1 oz. of potassium bichromate 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 good for a considerable time in a cool place. To prepare the paper, some of the solution is warmed to about 100° F. (38° C), and sheets of the paper ("bank post," " positive photographic," or other fine-wove and slightly sized) are floated on it for 2 to 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-colour 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 oz. Burgundy pitch and 2 oz. bitumen are melted over a clear fire till all the water is driven off; 1 oz. white wax is also melted; the whole is then mixed together, with 1 oz. palm-oil, and run into vessels for keeping.