| Take of Nitrate of Silver | 3 | drachms. |
| Hyposulphite of Soda | 4 | ounces. |
| Water | 8 | ounces. |
Dissolve the Nitrate of Silver in 2 ounces of the water, then from the total quantity of Hyposulphite of Soda, weigh out
| Hyposulphite of Soda | 2 | drachms; |
dissolve this likewise in 2 ounces of water, and the remainder of the Hyposulphite in the other 4 ounces. Then, having the three solutions in separate vessels, pour the Nitrate of Silver at once into the 2-ounce solution of Hyposulphite, agitating the precipitated Hyposulphite of Silver rapidly. In a short time it will begin to decompose, passing from white to canary-yellow, and then to orange-yellow. When the orange-yellow begins to verge towards brown, add the 4-ounce concentrated solution of Hyposulphite, which will at once complete the decomposition, a part of the precipitate dissolving and the remainder becoming perfectly black. After filtering out the black Sulphuret of Silver, the solution is ready for use.
A Bath prepared by this formula is not usually very active, but it shows clearly the process by which an ordinary fixing Bath may be converted into a toning Bath by the immersion of positives having free Nitrate of Silver upon the surface.
The following formula is more economical and gives a better result, but it cannot be used for "Ammonio-Nitrate" prints; the addition of an alkali precipitating Sulphuret of Iron.
| Strong solution of Perchloride of Iron | 6 | fluid drachms. |
| Hyposulphite of Soda | 4 | ounces. |
| Water | 8 | ounces. |
| Nitrate of Silver | 30 | grains. |
Dissolve the Hyposulphite of Soda in seven ounces of the water, the Nitrate of Silver in the remaining one ounce; then pour the Perchloride of Iron into the solution of Hyposulphite, by degrees, stirring all the time. The addition of the Iron Salt strikes a fine purple colour, but this soon disappears. When the liquid has become again colourless, which it does in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect solution will take place without any formation of black Sulphuret.
A toning Bath prepared with Chloride of Iron will be ready for use twelve hours after mixing, but it will be more active at the expiration of a week. The solution is acid to test-paper, and milky from a deposit of Sulphur, which must be filtered out.
The Perchloride of Iron should be prepared by boiling Peroxide of Iron with Hydrochloric Acid, in preference to dissolving Iron wire in Aqua-Regia.