The silvered copper plates upon which the gelatine images are to be developed are prepared by dipping clean polished plates in a solution of cyanide of silver, made by adding cyanide of potassium solution to a solution of nitrate of silver until there is a slight permanent precipitate. The copper plate remains in this for three or four minutes; it is then removed, washed under the tap, and dried.
The gelatine tissue compound is had by first of all making a jelly of
| Gelatine (soft) | 8 | ounces. |
| Sugar | 5 | ounces. |
| Water | 25 | ounces. |
Soak the gelatine until soft, then melt it at as low a temperature as possible, add the sugar and churn all for fifteen minutes; then add to one-half of the jelly one ounce of finely powdered graphite, and to the other half one ounce of a coarser powdered graphite. Churn these well together.
Paper is coated with the above mixtures, floating it first upon that containing the coarser graphite, then after drying, coat again by floating the paper upon the gelatine containing the fine graphite, and again drying.
The paper is then sensitized by being immersed in a three per cent. solution of bichromate of potash. Dry it in the dark. After exposure to light under {128} a negative, it is developed upon the silvered copper plate, after which the electro is made.
Another plan is to expose a piece of ordinary carbon tissue under a negative in the usual manner, then develop it upon the silvered copper plate, and sift over the wet gelatine image some finely powdered glass, sand, or emery which has been previously soaked in paraffine. The gelatine is then allowed to dry, when the grit is brushed away, leaving the gelatine image all marked and pitted. This pitted image is then black-leaded, and the electrotype is taken from it.
Another method is to place the newly developed carbon print on the copper plate, inside the aqua-tint box, Chapter III. (having previously shaken up the powder); when it is well dusted over, allow it to dry, and then remove the powder by dissolving it out with either alcohol, turpentine, or benzole, after which the electrotype is made.