The negative is first flooded with the uranium solution and then with the red prussiate.
Another process of intensification recommended by Prof. Eder and Toth is the so-called lead intensifier.
The somewhat under-exposed negative ought only to be developed with iron, fixed with hypo, and must then be well washed. It is finally well rinsed with distilled water, and then placed in a filtered solution of—
| Lead nitrate | 4 | parts by weight |
| Potassium ferridcyanide | 6 | parts by weight |
| Distilled water | 100 | parts by weight |
After a few minutes it becomes absolutely opaque in the covered parts. The negative is allowed to remain in the bath till it has attained the desired density. On the silver molecules of the image a pale yellow precipitate has formed by the action of the lead bath. The ferridcyanide of potash is reduced by the silver to yellow ferrocyanide of potash, and gives then, with the lead salt, an insoluble compound, ferrocyanide of lead. By the action of ammonium sulphide the unstable lead salt is converted into a stable salt.
As soon as the negative has been taken from the lead bath and well washed, till the washing water no longer gives a blue precipitate, it is flowed over with dilute ammonium sulphide 1 : 5. Sodium sulphydrate may also be used. The image becomes black instantaneously. The negative becomes generally vigorous; if, however, it was exposed too long the fine hair lines and points veil over very easily.
If the negative is not sufficiently intensified, which may happen with very much under-exposed negatives, it should after well washing, and before treating with ammonium sulphydrate, be laid in a bath of—
| Cadmium sulphate | 10 | parts |
| Distilled water | 100 | parts |
in which it becomes still whiter, and should then be treated with the ammonium sulphide. The lead, the cadmium, and the silver of the bath are converted by the ammonium sulphydrate into sulphide. The negatives treated with cadmium have a yellowish tinge, whilst those treated with lead are pure black. For {41} clearing up the white-lead image when it is too vigorous or is foggy hyposulphite of soda or a dilute solution of potassium cyanide can be used. This manipulation, however, requires great care. The negative treated with ammonium sulphide if foggy can be cleared up with hydrochloric acid, used weak and repeatedly poured on and off, with careful watching of the action. After this it must be well washed.
The lead solution will remain clear for weeks, and if it begins to work slowly can be freshened up with addition of lead nitrate and ferridcyanide of potash.