Formula No. 3, or Protonitrate of Iron, does not require any addition of Acid; but it will be advisable, in some cases, to add to it a few drops of Nitrate of Silver immediately before developing. It gives a bright metallic image, resembling that obtained by adding Nitric Acid to Protosulphate of Iron.
The following process is commonly followed for preparing Protonitrate of Iron:—
Take of Nitrate of Baryta 300 grains;—powder and dissolve by the aid of heat in three ounces of water. Then throw in by degrees, with constant stirring, crystallized Sulphate of Iron, powdered, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble Sulphate of Baryta.
In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate of Lead being an insoluble salt), but the quantity required will be different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter.
THE FIXING SOLUTION.
| Cyanide of Potassium | 2 to 12 | grains. |
| Common Water | 1 | fluid ounce. |
Cyanide of Potassium is usually preferred to Hyposulphite of Soda for fixing direct Positives; it is less liable to injure the purity of the white colour. The percentage of Carbonate of Potash in commercial Cyanide of Potassium is so variable that no exact directions can be given for the formula. It is best however to use it rather dilute—of such a strength that the plate is cleared gradually in from half a minute to a minute.
The solution of Cyanide of Potassium decomposes slowly on keeping, but it will usually retain its solvent power for several weeks. In order to escape inconvenience from the pungent odour evolved by this salt, many employ a vertical Bath to hold the solution; but in that case the plates must be carefully washed before fixing, as the Iron salts hasten the decomposition of the Cyanide.
THE WHITENING SOLUTION.