| Ferric potassium oxalate | 1 | oz. or | 5 | parts[5] |
| Sodium sulphite | 1 | oz. or | 4 | parts |
| Oxalic acid | ¼ | oz. or | 1 | part |
| Hypo solution (25 in 100) | 5 | oz. or | 25 | parts |
| Water | 20 | oz. or | 100 | parts |
[5] The formula in "parts" does not strictly correspond with that in ounces, but the difference is immaterial.
The constituents must be dissolved in water in the order given. The solution can be used at once and it keeps fairly well if protected from light, in well corked bottles filled up to the neck.
INTENSIFICATION.
Intensification is a process in which the opacity of the image is increased by adding some fresh matter, metallic or otherwise, to the reduced silver that constitutes the developed image.
The usual plan is to bleach the image by means of a solution of mercuric chloride (mercury perchloride or corrosive sublimate), which converts the dark-coloured silver into a white mixture of silver chloride and mercurous chloride, and this is subsequently treated with some re-agent which will reconvert the image into a dark product of greater opacity than the original.
It is absolutely essential to successful intensification that the negative be completely fixed and completely washed after fixing, for any trace of hypo left in the film will give rise to brown stains. It is also important, in order to prevent stains of another sort and to secure uniform action, that the mercuric chloride solution be mixed with a small quantity of hydrochloric acid. Too much acid will cause frilling. If the negative has been dried it must be immersed in water for, as a rule, not less than half-an-hour, in order that it may be thoroughly and uniformly wetted.
| Mercuric Chloride Solution. | ||||
| Mercuric chloride | 1 | oz. or | 5 | parts |
| Hydrochloric acid | 1½ | drachms or | 1 | part |
| Water to make up to | 20 | oz. or | 100 | parts |
When uniform intensification is required the negative is allowed to remain in this solution until it is completely bleached. If, however, it is desired to intensify the shadows more than the high-lights, the plate should be removed from the solution as soon as the shadows have bleached, and should be rapidly washed in order to stop the action on the more opaque parts of the image.