The other substratum consists of
| Gelatine | 35 parts |
| Acetic acid, No. 8 | 250 parts |
| Alcohol, 95 deg | 50 parts |
| Water | 700 parts |
| Chrome alum, 4:100 | 60 parts |
Dissolve the gelatine in the acid at a moderate heat, add afterwards the alcohol and water, and lastly mix the chrome alum by small quantities at a time.
These substrata are employed to avoid the peeling off of the image. To prevent the entire desiccation of the gelatine, which is the cause of the defect above alluded to, it is advisable to add glycerine to the washing water after the image is cleared. Some operators recommend a coating of flexible collodion, that is, prepared with castor oil, for the purpose in question. We do not think that necessary when the transparencies are not exposed to sunshine. If anything should be applied we would prefer the encaustic.
Carbon transparencies are invaluable for reproducing negatives in the original size by the same (carbon) process, or for enlarging by the collodion or gelatine process. For these purposes they should be made on the special red tissue manufactured by the Autotype Company, of London, Eng. They can, however, be made on the ordinary tissues.
Whatever be the tissue employed, the transparencies for the reproduction of negatives are seldom opaque enough, and should be intensified. This is done by treating them with a [pg 104] very dilute solution of sodium permanganate, which colors them olive green.
Transparencies for lantern slides, etc., are best colored with the couleurs à l'albumine of L. Encausse, sold by J. Reygondaud, Paris (France). They are transparent.[33]
Toning and Intensifying.—The carbon proofs can be toned and at the same time intensified by reagents acting with chromic oxide.
The dyes or coloring matters precipitated are not opaque, and, as a consequence, not objectionable for transparencies. The following processes are the most employed:
Prepare three solutions as follows: