POSITIVE PRINTING, in photography. This term is used to designate that process by which impressions from a negative (q.v.) are produced upon suitably prepared paper. The term, however, does not belong exclusively to positives produced on paper, and intended to be viewed by reflected light, since transparent positives for examination by transmitted light are produced on glass. The means by which this kind of positives is obtained are so exactly similar to the dry negative collodion process, that a detailed notice thereof is hardly necessary in the present article, which will be confined exclu sively to the means of Obtaining positive proofs on paper.
Regarding, then, the negative, not so much a picture as the means of producing one, the first thing which presents itself for notice is the paper. This may be either German or French, known in the markets under the respective names of saxe and rive. They are used in the simply salted condition, or more generally in the salted and albumin ized state, the purpose of the albuminizing being to prevent the chemicals used in the pro cess front sinking into the paper, whereby the delicate details of the negative would become defective on the surface. The process is briefly as follows: float the paper on the salting bath from one to five minutes; drain for one minute; hang up to dry. Float the paper on the exciting bath from five to ten minutes, according to its strength; drain, and hang np to dry. Expose in a pressure-frame under a negative. The neces sary depth of impression being obtained (a point only to be determined by experience), wash the print in common water. Some operators at this stage immerse the print in a bath containing one per cent. of ammonia for two or three minutes. This is by no means absolutely necessary; should it, however, be done. it should be afterward washed in water for five or ten minutes; after which it is immersed in the toning bath from cne to ten minutes, or until the desired tone be obtained; it is then washed in several changes of water, preparatory to immersion in the fixing bath. This last operation occupies from fifteen to thirty minutes, according to the strength of the fixing solution, and the depth to which the printing has been carried. The print is then copiously washed in many changes of water, and hung up to dry.
The baths referred to above are composed as follows: salting bath, water, one ounce; albumen, 4 ounces; good common salt, 48 grains. Exciting bath, nitrate of silver, 240 grains; water, 4 ounces; glacial acetic acid, half a dram. Toning bath, chloride of gold, 4 grains; water, 24 ounces; carbonate soda,100 grains. Fixing bath, hyposul ph ite of soda, 4 ounces; water, 1 pint.
Printing in carbon, lamp-black, or other impalpable powder, which doubts as to the stability of silver prints had long made a desideratum, has recently been brought to a considerable degree of perfection. It is based on the discovery by Mungo Ponton, that a mixture of gelatine and bichromate of potass is rendered insoluble by the action of light, and on the experiments of Poitevin. Fargier and Blair. There are three typical processes —the autotype, Woodburytype, and //thin/ruck. The autotype consists in coating a sheet of prepared paper with a mixture of gelatine, bichromate of potass, and carbon, and when dry, exposing it under a negative. On removal from the printing frame, the pig ment is moistened with water, and laid, prepared side down, on a support of glass, zinc, or shellac-coated paper, to which a gentle pressure makes it adhere. The paper is then removed, and the print is developed by immersion in warm water, which dissolves the unaltered gelatine, but ctunot touch the parts rendered insoluble by the light which has passed through the negative. Tne developed print is again transferred to paper, when the high lights arc found to consist of those parts where the gelatine has been completely dissolved, the middle tints of the parts less soluble, and the shadows of the parts quite insoluble.—In the TVoodburytype, a bichromated gelatine film, but without carbon, is pre pared, exposed, and developed in a somewhat similar way, but dried without being trans ferred to paper, the result being a sheet of gelatine with a picture in relief. This is laid on a plate of soft. metal, and covered with a plate of steel, and the whole subjected to the action of a hydraulic press, by which the soft metal takes the impression of the gelatine film. In printing from the plate so produced, an ink, consisting of carbon and gelatine, is pound on the center, the paper is laid on the ink, and the pressure of a suitable press applied, whereby the ink is squeezed into the shadows and half-tones, and the highlights are left clean, the result being a really fine print. Glass may be used instead of paper, with very fine results.-11,01druck is a method of printing photographs in an ordinary! lithographic press, with printer's ink, from gelatine films prepared on the same princi ple as the Woodbury tissue, except that the soluble gelatine is not washed away. The film is attached to a thick plate of glass fixed in the press, and when sponged over, the soluble parts absorb water, and so are prevented from taking on ink, while the insoluble portions remain dry, and the ink adheres to them.