The paper should be sized with a gelatine solution at 5 per 100 of water, and, when dry, kept in the dark.[40] It is sensitized by floating five minutes.
The exposure under a negative varies from fifteen to twenty minutes in the shade, and from one to three minutes, at the most, in sunshine. As a rule, it is advisable to somewhat underexpose in order that the development be regular, progressive, under control.
The image is developed by floating, or immersion in
| Silver nitrate | 2 parts |
| Distilled water | 100 parts |
| Nitrate acid, C.P. | a trace |
When the image is intense enough it is washed in several changes of water, then toned in a solution of gold at 1 per 1,000 of water acidified with traces of hydrochloride acid.[41]
The following bath develops slowly, and gives very rich purple tones without toning:
| Nitrate of silver | 3 parts |
| Nitrate of uranium | 1 part |
| Nitrate of cadmium | 1 part |
| Alcohol | 10 parts |
| Water | 100 parts |
| Nitric acid | traces |
The developing solutions should be as little acid as possible, but not neutral, for then the proofs would be veiled and grayish.
The image can also be developed in a solution of gold, or in a very weak solution of mercuric chloride at 1 per 10,000. The proof must be extremely well printed and left for from [pg 112] two to five minutes in the mercuric solution. If the time of exposure is right, the image will change but little in the solution, and will take, when treated with silver nitrate, the most splendid tones.
The proofs should be carefully washed when finished. If they were developed with silver, they must be immersed in diluted aqueous ammonia, which will perfectly clear the whites. If developed with chloride of gold, the water should be heated to 60 to 80 deg. C. (140 to 176 deg. Fahr.)