Obernetter Process.
| Dextrine | 60 grains |
| White sugar | 75 grains |
| Ammonium dichromate | 30 grains |
| Glycerine | 2 to 8 minims |
| Distilled water | 3 ounces |
The gum is first dissolved and the remainder of the ingredients added. It may be necessary to warm the solution in a hot water bath to dissolve it. It is then filtered through flannel or clean muslin, and preserved for use in well-stoppered bottles. With this solution clear glass plates are coated and dried by a gentle heat over a small spirit lamp. The plate while still warm is exposed under a reversed positive[C] for from two to five minutes in sunlight, and from 10 to 20 minutes in diffused light. The image is then but slightly visible. On removing from the printing frame the plate is laid in the air (protected from light) for a few minutes to absorb a little moisture from it. The next process is the "dusting on." If the image is required to be black, fine Siberian graphite is spread over it with a soft flat brush. This will adhere to the parts unaffected by light, giving an image of the positive. Any colored fine powder maybe used, giving images in various colors. When fully developed the excess of powder is dusted off and the film coated with collodion. After this it is well washed to remove the unaltered gum and dichromate salt. The film may, if desired, be detached from the plate and used for enamels, ivory, wood, textile fabrics, opals, etc.
CHAPTER II.
The Transparency.
Regular transparency gelatine dry plates are the handiest for making positives, especially for amateurs, if one does not care if the subject is in reverse, or if one has a reversed negative to work from. There is a "special" carbon tissue, price $4.00 per roll of 2 × 12 feet, made by the Autotype Company, of London, England, with full instructions appended; by a system of double transfer, reversed negatives may be obtained with this tissue. The "special" tissue is only to be used for the transparency. A safe edge of black paper is required on the transparency, pasted up exactly to the edge of the picture, on the glass side; it comes, sold in strips, gummed, ready for use, about ¼ inch wide; this is required, as the tissue used for the negative resist on the copper plate, which is printed from the transparency, must have a safe edge, shielded from the light, or it will not attach itself to the copper plate, the tissue coming inside half way. The screw pressure printing frame should have a piece of heavy felt for backing the transparency.
The following instructions for making carbon transparencies will no doubt be found useful: