This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Plates of glass coated with an emulsion composed of gelatin and sensitive silver salts. This emulsion is extremely sensitive to light, which makes it possible to place the dry-plate in the photographic camera and obtain an impression on the emulsion of objects situated in front of the camera by giving a very short exposure. (See Exposure.)
A sensitive plate so prepared that it is sensitive, in almost a like degree, to all colors under certain conditions; i. e., in combination with various color-screens or filters.
3 1/4 x 3 1/45
3 1/4 x 4 1/4;
4 1/4 x 6 1/2;
4 1/2 x 6 1/2;
4 3/4 x6 1/2;
4 1/4 x 6 1/2;
6 1/2 x 8 1/2;
Other plates expressed by inches.
IV. Platinum Process. One of the most permanent of all printing processes, as metallic platinum is one of the most stable substances known. It is a semi-printing-out and semi-developing process, as the image prints out slightly in the iron salts, while the perfecting of the platinum image proper is effected by means of development. W. E. Woodbury briefly describes the principles of the process as follows: Paper is coated with a mixture of potassium chloroplatinite and ferric oxalate. The ferric oxalate is sensitive to light, becoming converted into ferrous oxalate; we therefore get a faint grayish orange-colored image of ferrous oxalate. Now, ferrous oxalate possesses the power when in solution of reducing potassium chloroplatinite to metallic platinum. It, therefore, only becomes necessary to dissolve the ferrous oxalate in a suitable liquid, when the potassium chloroplatinite will be reduced to the metallic state as metallic platinum. A solution of potassium oxalate possesses this requisite power of dissolving the ferrous oxalate. The paper prepared with the chloroplatinite of potash having an image on it of ferrous oxalate is, therefore, floated on this solution, and a picture consisting of finely divided metallic platinum is the result. It then only becomes necessary to dissolve away the remaining iron salts by any suitable acid. An extremely weak solution of muriatic acid is most generally employed for this purpose.
White, malleable, ductile metal; softer than silver; grayish tinge. Soluble in aqua regia. Used extensively in photography in the platinum process. Platinum forms two chlorides as follows: