259. Alum Bath

Alum Bath. Carbon pictures do not require fixing, but it is necessary to eliminate the bichromate, of which a certain quantity still remains in the tissue. In order to remove every trace of the bichromate of potassium, immerse the print in an alum bath, the bichromate being extremely soluble in a solution of alum. This bath has also the effect of hardening the film, thus rendering the gelatin entirely insoluble.

260. The print should be immersed in the following bath for at least ten minutes:

Water........................................

60

ozs.

Potash Alum ........................................

2

ozs.

This bath must be filtered through cotton and can be used repeatedly, but must always be filtered before using, thus avoiding any specks on the film.

261. Where you experience difficulty in producing pure whites, a sulphite-alum bath should be employed, as follows: Dissolve 3 ounces of common potash alum and 1 ounce of sulphite of soda crystals in 100 ounces of soft water. Be sure and filter before using. If you use hard water, add 1 drop of tartaric acid to clear the solution before filtering.

262. Where it is desired to dry prints quickly by artificial heat or in the sun, the following hardening formula should be used:

Powdered Alum...........................

3

ozs.

Sulphite of soda Crystals .......................

1 1/2

ozs.

Glycerin ...........................

1/2

oz.

Water (Soft)................................

32

ozs.

The bath must be filtered before use, and may be used repeatedly.

263. After this bath the prints are washed by handling in several changes of cold water for half an hour. After washing thoroughly the prints should be hung up to dry. If celluloid or any heavy support has been used for transfer, they may be placed in an ordinary negative rack and dried with a fan. They can be suspended in a room free from dust, with a current of air passing through. The prints should dry in a few hours when they will be ready for final mounting. Carbon prints on paper supports can be dried in a short time by first immersing in a weak solution of alcohol- 1/2 ounce of alcohol to 8 ounces of water. Place the print in this solution for half a minute, then hang up to dry in the ordinary way. Never use heat for drying.

264. In order that you may become familiar with the rough-surface paper support, it is recommended that you apply this material to your next experiment. In using the rough paper, either prepare the paper yourself, or use the ready prepared stock. If you prepare the paper yourself, apply a heavier coating of the substratum than for smooth papers. Also greater care must be exercised in obtaining perfect contact and expelling all air-bells when squeegeeing the support to the printed tissue.