A Great number of different processes have been used at one time or another for printing negatives. The earliest printing processes depended upon the fact that silver compounds darken in light, and the first printing paper to be used generally was made by soaking a sheet of paper in a solution of table salt and washing this over with a solution of silver nitrate so as to convert the salt into silver chloride. Paper so prepared was known as "salted" paper on which, after exposure to light behind a negative, a print was obtained which could be toned by the deposition of gold from a solution and then fixed with hypo. A better paper was made by using albumen obtained from the white of eggs. After adding salt to it the albumen was spread over the surface of the paper and then sensitized by treatment with a solution of silver nitrate.
After the gelatine process for negatives was discovered gelatine emulsions were applied to printing papers. Gelatine paper was made by emulsifying silver chloride in gelatine with an excess of silver nitrate and then coating it on paper just as films are coated with the sensitive negative emulsion. The typical gelatino-chloride paper of this type is Solio.
To use Solio, the negative is put in a printing frame, and the paper is put with its coated side in contact with the emulsion side of the negative and pressed into contact by closing the back of the printing frame. The frame is then exposed to daylight and the image printed on the paper, which darkens to a brownish-red colour. From time to time the depth of the printing is observed by opening the back of the frame. The image must be printed to a somewhat darker colour than will be required in the finished picture. When printed the paper is removed in subdued light and the print is toned by immersing in a solution containing gold so that the metallic gold is deposited on the print, giving it a purple colour. After toning, the print is fixed in a hypo solution and washed. A toning process is necessary with all printing-out silver papers, such as Solio, albumen-ized paper, or salted paper, because if the printed-out silver image is fixed without toning, the fixing bath changes it to an ugly yellow color and a very poor-looking print results. The gold toning produces a rich-looking, permanent image which varies in color from brown to purple; these colors, indeed, used to be regarded as the only satisfactory colors for photographs.
The chief use for printing-out papers at the present time is for the making of photographers' proofs. For this purpose the negatives are printed, but the prints are not toned or fixed, and, while they are satisfactory for examination, they cannot be kept, because they darken in the light, the photographer supplying them only as samples to show the pose and expression, and making permanent prints to order later.
Quite early in the history of photography it was discovered that many substances besides the salts of silver are sensitive to light. One process of printing, the platinum process, is founded upon the sensitiveness to light of iron salts. If paper is coated with ferric oxalate, which is a green soluble salt of iron, and this is exposed to light, the ferric oxalate is changed into another oxalate of iron, ferrous oxalate, which is insoluble, so that a sheet of paper thus prepared and printed will, after washing, give a faint image consisting of ferrous oxalate. If, to the ferric oxalate with which the paper is prepared, a solution of a platinum compound is added and then, after printing, the faintly visible image is put into a solution of a soluble oxalate, the ferrous oxalate is dissolved and attacks the platinum salt, which is not affected by the ferric oxalate, precipitating metallic platinum on the paper so that an image is obtained consisting of black metallic platinum.
Another process depends upon the fact that gelatine containing bichromate becomes insoluble in water on exposure to light, and this process is known as the "pigment" process or more commonly as the "carbon" process, the name being derived from the fact that the gelatine used in the early days of the process contained finely divided carbon or lamp black to act as a pigment. The printing paper is made by coating the paper stock with a thick gelatine solution containing finely divided pigment suspended in it. The pigment is chosen according to the color of the print required. For a black image it may be lamp black, for a red image red ochre or burnt sienna, and for images of other colors any permanent and stable pigment of the color desired which can be finely powdered. After the coated gelatine has been dried the paper is immersed in a solution of bichromate of potash or ammonia and again dried. This bichromated gelatine is quite soluble in hot water, but if it is exposed to light it becomes insoluble where the light has acted upon it. The bichromated gelatine is, therefore, printed under the negative in the same way as a Solio print. No visible image is produced, and to get the visible print it is necessary to wash away the soft gelatine. The gelatine, which has been hardened by the action of light, is on the surface of the print and the soft gelatine is at the back, so in order to develop the print it is put face down on to another sheet of paper and placed in hot water. After a short time the soluble gelatine begins to ooze out at the edges of the print and the whole of the original paper can be pulled off, leaving the image covered with a sticky mass of partly dissolved gelatine on the paper to which it has been transferred. This image is then washed in hot water until all the soluble gelatine has been washed away, leaving a clear image of the pigmented gelatine on the paper.
All these printing-out processes require a long exposure to strong daylight, and they have become more or less obsolete owing to the trouble of working them and especially the difficulty of judging the correct exposure with such a variable illuminant as daylight. They have been displaced by printing processes in which the paper used is coated with an emulsion very similar to that used for making the negative, but of considerably less sensitiveness. This paper, known as development paper, is exposed behind the negative and is then developed, in the same way as a negative, to give a visible image.