Important: Safety Warning
This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
Photography
the art of producing permanent pictures by means of the chemical action of light on sensitized surfaces. The first to produce permanent pictures by the chemical influence of light was M. Niepce at Chalais-sur-Mer, in 1814, and his idea of the development of a latent image by treatment after exposure lies at the basis of photography.
Niepce and Daguerre discovered that they were conducting experiments of a kindred character, and in 1829 entered into partnership. Daguerre, with Niepce's son Isidore, carried on this work in 1839, and invented the process known as the daguerreotype.
The introduction of collodion in 1857 was a great step in photography. The collodion process is divided into five stages: (1) The preparation of a clean glass plate by coating with collodion to which bromide of cadmium and either iodide of potassium or iodide of ammonium has been added. (2) The sensitizing of the collodion by immersion in a bath of nitrate of silver, and distilled water. (3) The production of a latent image by exposing the sensitized plate in the camera. (4) The development of latent into visible image by immersing the plate in a solution of sulphate of iron or pyrogallic acid, to either of which some acetic or citric acid is added. (5) Fixing of the permanent image by immersion of the plate in some solvent of those parts of the sensitive surface upon which the light has acted. This solvent for wet plates is cyanide of potassium, but for more modern processes hyposulphite of sodium is employed. On account of the trouble of preparing the wet collodion plates, the dry plates, made by the gelatine-bromide process, have almost universally taken their place. Dry plates made by some form of this gelatinebromide process are manufactured on a large scale. When properly made they are much more sensitive than the collodion plates and will keep good for years, and can be developed months after exposure. The positive prints are made by the action of light, passing through the developed plate, on a sensitized paper, over which the plate is placed. The silver, platinum, and ferro-prussiate papers are the most used. Silver Process. - Suitable paper is coated on one side by a thin layer of albumen, to which chloride of sodium or of ammonium has been added. The old method of dispensing with albumen and using paper which is salted only, is reviving. The paper is sensitized by float- PHOTOGRAPHY PHOTOTHERAPY ing it on a solution of silver nitrate, and coating with a gelatin-bromid emuldrying. After printing, the paper is sion. After the exposure, the plate is treated with a solution of chloride of developed by a double process that turns gold, which tones it, producing different it to a positive and the result is a beaushades of color, depending on the length of immersion and strength of solution.
After toning, the print is fixed in the same manner as the plate, by a solution of hyposulphite of sodium, which removes the undarkened silver salts. Gelatine or collodion is sometimes used in place of albumen in this silver-printing process.
All soluble substances have to be washed tiful transparency in the natural colors.