Development, as a general rule, should be conducted in feeble daylight or artificial light.
Development need not take place immediately, but at some subsequent time, provided the prints be meanwhile stored in a calcium tube and in every way rigorously protected from damp.
The proportions which I have given for the oxalate of potash bath represent the standard developer as given by the makers of the paper for the hot-bath papers, and they recommend that this be diluted to about half strength for cold-bath papers. Personally, I use it at full strength for the cold process, and see no reason for diluting it.
It may be said that such a course is calculated to give strong, vigorous prints, for generally speaking, the stronger the bath, the stronger the contrasts of the print. The difference, however, produced by altering the strength of the bath is not very great.
There are two alternatives to the oxalate of potash developer, both possessing certain, if not very strongly marked characteristics. The first of these is known as the "D" salts. These are sold in tins by the Platinotype Company, and consist of a loose admixture of certain salts, and hence it is essential that the entire contents of a half-pound tin be dissolved at once and kept thus as a stock solution.
The proportions to be used are as follows: Dissolve ½ lb. of D salts in 50 ozs. water, and then take equal portions of this solution and water, in other words, dilute it to half-strength.
The "D" salts are said to give colder colours and more half-tone, but the colour derived from development on the first-named oxalate bath may be made colder by adding to 20 parts of developer 1 part of a saturated solution of oxalic acid, in like manner slightly warmer colour may be obtained if the oxalate bath be made alkaline by the addition of carbonate of potash, but only just enough should be added to turn a red litmus test paper blue.
If prints developed on D salts should appear mealy or granulated, the bath should be strengthened or used at the full strength of the stock solution (salts ½ lb. to water 50 oz.).
Another developer, the effect of which is to minimize half-tone and increase the vigour of the contrasts, and so give very brilliant and even hard blacks and whites, is as follows:
| Oxalate of potash | 16 | ozs. |
| Phosphate of potash | 4 | ozs. |
| Sulphate of potash | ½ | oz. |
| Water | 120 | ozs. |