PHOTO CERAMICS.—A process of producing burnt-in photographic images on porcelain, pottery, glass and other similar materials. There are two principal methods—the substitution process invented by Du Motay and Marechal, and the dusting-on process of Poitevin.
The best description of the substitution process is that of Mr. N. K. Cherrill, and from which the following information is mostly taken :* A piece of glass is first cleaned with nitric acid, well washed, dried, polished, and coated three times with the collodion.
This done, the plate is plunged into the nitrate bath before the collodion has been allowed to become too much set. The nitrate bath is a solution of thirty grains of pure silver nitrate dissolved in an ounce of pure water, and sunned all the while it is not in use, and when used rendered acid in the proportion of two drops of pure nitric acid to a half-gallon of solution. The plate is left in this solution until all the greasy marks have disappeared ; it is then taken out at once and placed in a funnel to drain for about five minutes, and is then inserted in the dark slide of the copying camera. The camera is so arranged that the light which passes through the negative to be copied comes only through one of the side lights of the studio. The exposure varies from five to twenty seconds. If the enamel to be taken is of small size it is preferable to put a mask on the negative and to block out all the light except that actually needed, as this enables one to take four or five images side by side by simply pushing the camera dark slide a little way each time. The exposure and development of the image is a matter requiring great care and attention, as on the complete success of the transparency the whole process lies. The developing solution is made up with Pyrogallic acid 12 grains .
Glacial acetic acid 4 drachms Alcohol 4 Water to fill a 12 oz. bottle.
In warm weather this may be more dilute—say as far as giving zo ounces of water to the same quantity of pyro. Then, of course, more alcohol will be needed. The developer should be made three days before required for use, as at first it is too vigorous, although it must not be kept too long, as it then deteriorates in the other direction.
The image of the positive is developed slowly and until it obtains the exact density required, and at the same time gives the right amount of detail in the high lights. The image,
when examined by reflected light, should show nearly all the drawing and shading of the subject, while, when viewed by transmitted light, it would show up with great perfection.
The plate is then well washed and fixed with a weak solution of cyanide of potassium, after the application of which it is again thoroughly well washed.
After washing, a small portion of the film at one corner of the plate is broken away, and a direct stream of water is made to run on this corner, striking the bare glass. This will gradually detach the film from the glass. The detached film is then floated on to a clean piece of glass and all unnecessary parts removed. The film is again washed while on the glass, and then lowered gently into a dish of water.
The next operation is the toning. A 16-ounce bottle, half-filled with water, is placed in a saucepan containing water. This arrangement is set on the fire or on a gas stove until the water in the pan begins to boil. Next is placed in the bottle an ounce of potassium-chloride of iridium, and it is filled up with cold water and returned to the saucepan. The water is then kept hot and the bottle shaken occasionally. After half-an-hour remove the bottle and place it where the contents will become quite cold. To make up the toning bath place twelve ounces of pure water in a bottle ; add to it 14 drachms of the iridium solution, and shake up well. Now add drop by drop, and shaking well between each addition, seven drachms of a solution of gold chloride (1 grain to i drachm). The bath is then ready for use, but improves by keeping, which it will do indefinitely. It is particular to note that the solution in the iridium bottle will have a nearly black sediment, which is undissolved chloride. When all the clear solution has been used up, more water is added, and this remainder used in the same manner as the first, but care being taken, however, that too much water is not added, as a quarter of an ounce of the chloride will not make two sixteen ounce bottles full of the saturated solution, but only about one and a half.