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Restoring Fogged Dry Plates

potassium, oz and grs

FOGGED DRY PLATES, RESTORING Plates which have been accidentally exposed to light (lightstruck is a term sometimes used), may be made almost as good as new, with the exception that their speed is reduced, by treat ment for about five minutes in either of the following restoring baths :— Chromic acid . . 15 grs. 6.25 g.

Potassium bromide . 3o „ ,, Water to . . . 5 oz. i,000 ccs.

Potassium bichromate 20 grs. 8.3 g.

Hydrochloric acid . x drm. 25 ccs.

Water to . . . 5 oz. 1,000 „ Afterwards, the plates are thoroughly washed and dried. All the operations must be carried out in the dark-room. Plates that have been exposed in the camera, but have not been deve loped, may be restored in the same way, but the immersion must be of longer duration. Plates restored in this way need from five to ten times the normal exposure. Several other methods are possible, one of which is to soak the plate in a 2 per cent. solution of ammonium persulphate ; Condy's fluid (as bought), with the addition of a few grains of potassium bromide, also answers.

Abney has recommended a bichromate bromide mixture made by dissolving to grs. of potassium bichromate in x oz. of water, to grs. of potassium bromide in another ounce of water, and adding the two together.

Fogged dry plates can be made specially suit able for transparency work by soaking for about ten minutes in— Potassium bromide . 120 grs. 12 g.

Potassium iodide . 15 „ 1.5 Hot water . . oz. 25 ccs.

When dissolved add— Hydrochloric acid f oz. 25 ccs.

Potassium bichromate 120 grs. 12 g.

Water to . . . 20 oz. i,000 ccs.

Wash and dry.

The plates are made very slow by any of the processes named, and thus they give greater contrasts ; hence their suitability for copying black-and-white work. They should also be developed with a clean-working developer, such as adurol or hydroquinone.