Several other formula: for making the necessary relief from the negative have been recommended, but all except one are based on the above. The exception is the use of a modern dry plate. The plate should be of the thickly coated variety it is printed from in contact with the original negative, a full exposure being given, and developed with the following solutions :—No. r : Pyrogallic acid, 3o grs. ; water, 5 oz. No. z : Sodium sulphite, f oz. ; potass. hydrate, oz. ; water, 2 oz. No. 3 : Potassium bromide, 6o grs. ; water, 1 oz. For use, take oz. of No. r and I oz. of No. 2 ; develop the plate until all details are out, then add about 20 drops of No. 3 and continue development until dense enough. Place in a io per cent. solution of chrome alum warmed to 95° F. (35° C.), and brush the surface while under the solution with a camel-hair brush, allow to remain until in good relief, then wash well, fix in " hypo," and again wash well. It is absolutely essential that the dry plate is very thickly coated, and should there be any tendency to frill, this may be avoided by coat ing the edges with india-rubber solution. The dry plate process is the least satisfactory.
The metal mould has now to be made from the relief. Woodbury in his early experiments used the electrotype process to obtain an intaglio from the gelatine relief, but found that for prac tical purposes it was impossible to obtain uniform results. Soft metal (a mixture of lead and type metal) was found by him to be better. A sheet of the soft metal is placed in contact with the gelatine relief, then both in between two per fectly true plates of steel ; a pressure of so to 200 tons is applied, according to size, 4 tons to the sq. in. being about the pressure necessary.
The gelatine film is not damaged as might be supposed, and the result is a perfectly sharp intaglio in about one minute, and the same relief will serve for several moulds. Before taking the mould, however, the film or talc containing the relief image must be stripped from the plate and used with the metal, as the enormous pressure would break the glass support, and for this reason the talc support, as advocated originally, will be found the most convenient, even if more difficult to prepare. The mould is placed in a press, and oiled slightly with a mixture of equal parts of olive and paraffin oils, and the mixture of gelatine and ink poured on. A suitable ink is made by dissolving 4 oz. of gelatine in about 25 oz. to 30 oz. of water, and then adding Indian ink, etc., the whole being kept at about 126° F. (52° C.). When the mould is covered with the ink, the paper to receive the picture is placed upon it, then a sheet of plate glass and pressure applied. The superfluous ink is forced out at the sides by the pressure, while that in the mould adheres to the paper and forms the image. After the gelatine has set, the glass plate and the paper are lifted off, and the image thereon placed in a weak solution of alum to harden.
The modified process needing no great pressure consists in making the gelatine relief in any of the ways named above ; it is then, without stripping from the glass, covered with tinfoil and passed between rubber rollers. This presses the tinfoil into the interstices of the relief, pro ducing a perfect counterpart. Copper is then electrically deposited on the tinfoil for the purpose of strengthening it.