COWAN'S DEVELOPER A formula advocated many years ago by A. Cowan for use with negatives.
No. I .—Pyrogallic add 20 grs. 4 g.
Nitric acid . 2 drops '5 ccs.
Water to . . Io oz. 1,000 „ No. 2.—Strong liquor ammonia: . e oz. 25 ccs.
Potassium bro mide . . 374- grs. 8 g.
Water to . . io oz. i,000 ccs.
In cases of ordinary exposure mix in equal parts. For under-exposure add more of No. 2, and for over-exposure more of No. 1 and bromide.
A. Cowan also prepared a solution of pyro gallic acid which is always ready for use.
Pyrogallic acid . . I oz. 18.3 g.
Citric acid . 6o grs. 2.25 „ Water to . . . 54 oz. 1,000 ccs.
Dissolve the citric acid in water and add the pyrogallic. The solution will contain 8 grains of pyro to the ounce of water, and will keep good for many months. In using it dilute according to formula employed.
Another developer known as " Cowan's " is the following (known also as a citrate of iron developer), recommended for chloride lantern plates : No. 1. For cold tones—
Potassium citrate . too grs. 200 g.
Potassium oxalate . 3o „ 60 „ Hot distilled water to . 1 oz. 1,000 ccs.
No. 2. For warm tones— Citric acid . . 90 grs. 180 g.
Ammonium carbonate 6o „ 12o „ Cold distilled water to . I oz. 1,000 ccs.
No. 3. For extra warm tones— Citric acid . . 13o grs. 26o g.
Ammonium carbonate 40„ 80 „ Cold distilled water to . oz. 1,000 ccs.
In mixing solutions Nos. 2 and 3 it is better to put the crystals into a deep vessel, and, after adding the water, leave alone until all efferves cence ceases. It is advisable to make it over night. To 3 parts of any of the above solutions add I part of the following at the time of using : Ferric sulphate . . 120 grs. 240 g.
Sulphuric acid . . I drop 2 CCS.
Distilled water to . I oz. 1,000 „