An india-rubber solution, consisting of equal parts of benzole and chloroform, in which an equal part by weight of india-rubber is dissolved, is used as a substratum. When the solution has become quite clear, the unreversed and unvarnished negative is coated with it. When the film is dry, a coating of raw collodion, to which some drops of castor oil have been added, is given. When this film is quite dry, it is cut through near the edges with a sharp knife, and the glass plate and film placed in a dish filled with water, when the negative film will loosen from the glass and after some time will swim in the water. In this operation the use of force must be avoided. The film is lifted from the water by placing a card {46} underneath it, carefully dried with fibre paper, then laid flat and pressed somewhat between tissue paper. The negative prepared in this way has many disadvantages. First of all, the film is easily injured, and, therefore, the edges should be bound with strips of paper in order to protect it from tearing; further, the film is easily folded and does not remain accurately of the same size, which makes it useless for certain purposes. Moreover, this process is somewhat costly. It is now almost entirely replaced by a process in which a gelatine substratum is used.
This process is as follows:—The unvarnished negative is accurately levelled with a level and coated 2 mm. thick with a solution of—
| White gelatine | 20 | parts by weight. |
| Distilled water | 500 | parts by weight. |
| Glycerine (chemically pure) | 25 | parts by weight. |
| Glacial acetic acid | 10 | parts by weight. |
| Alcohol 36% | 50 | parts by weight. |
The above quantity is quite sufficient for at least four negatives 21 × 26 cm. Many omit the acetic acid, but I have found that it is an advantage for the slow drying, the pliability, and the keeping of its size of the negative, to use acetic acid in moderate quantities. To a certain degree the addition of glycerine also effects this, but not sufficiently enough. Before pouring on the gelatine solution, the plate must be absolutely dry. Any air bells formed whilst coating must be pushed to the edge of the plate with a piece of card. The plate must be dried spontaneously in a room as airy and as free from dust as possible. In order to protect the film from dust particles, it is advisable to give it a paper cap or cardboard cover, which allows the access of air, and obviously it ought not to touch the film.
When the gelatine film has become thoroughly hard, it should be coated with negative varnish, and, after again drying, should be cut round the edges with a sharp knife right through to the glass and stripped from the plate.
If the glass was well-cleaned and prepared, as has been previously described on page [41], any gelatine film thus prepared will well and safely strip without a substratum.
With dirty or old plates, a substratum of a 1–2% solution of india-rubber should be used.
After stripping, the gelatine films are best kept between filter-papers and under glass plates.