Mercury and Silver Intensifier.— A. Potassium Bromide 20 gr.
Bichloride of Mercury . . . . . 20 „ Water . . . . . . . 2 OZ.
B. Cyanide of Potassium (sticks) . . 20 gr.
Nitrate of Silver . . . . . 20 „ . . . . . . . 2 oz.
The silver and cyanide must be dissolved separately, each in an ounce of water, and then the cyanide poured in until the precipitate is not quite redissolved. Bleach the plate in solution A, and then, after washing, transfer to B. The solutions may be used over and over again, and, if intensifica tion has been carried too far, on immersion in the hypo bath the plate will revert to its original state.
Bromide of Copper—This intensifier is nearly as effec tive as chloride of mercury, and is attended by fewer inconveniences.
Dissolve separately, each in 2 ounces of hot water, Potassium Bromide . . oz.
Copper Sulphate . . . . . . . „ and then mix and if necessary filter, to get rid of the white precipitate. The resulting solution may be used to bleach the negative, which may then either be blackened with nitrate of silver or redeveloped. Only very careful rinsing is correct with a negative bleached with copper bromide, on account of the white deposit being not quite impregnable.
Chromium Intensifier—This is the simplest, and at the same time the most reliable of intensifiers, and we have used it exclusively since the introduction of Mr. C. Welborne Piper's formula, five or six years ago. The presence of hypo in the film need cause no anxiety ; just a rinse after fixing is enough ; and the operation may be repeated several times if the first application does not give enough density. Three grades may be used according to circumstances : A gives moderate density, B considerable, and C great intensification.
A. B. C.
Potassium Bichromate . . . 5 gr. to gr. to gr.
Hydrochloric Acid (sp. l• 1 6o) . i min. 5 min. 20 min.
Water . , . . . . t oz. t oz. t oz.
Bleach in one or other of these three solutions, wash until the yellow stain has fully disappeared, and then redevelop with any clean developer. Piper recommends amidol.
Reduction of Negative.—Many negatives, owing to over exposure, over-development, or surface fog, have become so dense that whole days in the printing frame, even in sunlight, would be required to get prints from them. It is then advisable to reduce the thickness of the image, and Farmer's reducer is prescribed. It consists of hypo (fresh)
2 oz., water to oz., to which, just before using, a few drops of ferricyanide of potassium i oz., water to oz., are added. The mixture will not keep for more than half an hour : the more ferricyanide is added, the more rapid the reduction. With a solution strong in ferricyanide the shadows are attacked very greedily, and it is better to proceed carefully with a weak solution, unless the shadows are very much clogged. When reduction has been carried to the extent desired, the negative may be well washed.
Another very good method of reduction, but one requiring considerable judgment and skill, is to bleach the negative in the copper bromide solution given for intensification, and then dip for a moment or two into the ordinary fixing bath. The solution should be diluted to half its strength, and the plate should only be allowed to acquire a greyish blue colour, not the pure white necessary for intensification purposes.
Where only part of a negative requires reduction, our usual plan is to mix up a quantity of Farmer's reducer, or bromide of copper with a little glycerine, and apply to those parts with a brush.
Perm1phate Reduction.—Some negatives, while printing perfectly well in the shadows, have high lights so dense that they appear on the print as masses of plain white, without the slightest relief. If an ordinary reducer, such as Farmer's were applied, the heavy thickness of silver would remain comparatively unaffected, while the shadow detail consumed away into clear glass. In old days we used what was known as Baskett's reducer, consisting of terebene 2 oz., salad oil 2 oz., and a tin of Globe Metal Polish. A few drops on a wad of cotton wool were rubbed gently and evenly over the surface of the film, and the obnoxious thick deposits gradually worn down to the required degree. Green fog was often polished away in this manner. However, within recent years, it has been discovered that ammonium per sulphate has the peculiar property of attacking the high lights while leaving the delicate detail in the shadows undisturbed. The nature of the action is not fully under stood, but is believed to be due to the oxydisation of the gelatine in those portions of the film where the silver has been most thickly deposited.