A.

B.

C.

Potassium Bichromate .

5 gr

10 gr.

10 gr.

Hydrochloric Acid (sp. 1.160)

1 min.

5 min.

20 min.

Water......

1 oz.

1 oz.

1 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. Mr. Piper recommends amidol.

Reduction Of Negative

Many negatives, owing to overexposure, 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 10 oz., to which, just before using, a few drops of ferricyanide of potassium 1/2 oz., water 10 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.

Persulphate 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 persulphate 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 understood, 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.

The plate intended for persulphate reduction must be fully fixed, and hypo well washed out. Use a 2 1/2 per cent. solution. When first made up the solution works very slowly, increasing in power for some days; it is not well to keep the solution for more than a fortnight, nor to use the same solution for more than two plates in succession. A 5 per cent. solution of sodium sulphite must be ready to stop the action when it has been carried nearly to the required point.

Permanganate of potassium has also been suggested by Prof. Namias for reducing high lights in an under-exposed negative in the proportions 1 part permanganate, 2 parts sulphuric acid, 660 parts water; but in action it is far less reliable than the persulphate. Red stains must be removed with a weak solution of oxalic acid.

Rehalogenation

This name has been given to a method invented by Dr. Eder for improving the gradations, and softening an under-exposed but over-developed negative. The image is converted back into a bromide or chloride of silver by bleaching, and then redeveloped. A good bleaching solution is:

Potassium Bichromate . . . . . 30 gr.

Potassium Bromide . . . . . . 60 „

Water ....................... 10 oz.

After bleaching wash in several changes of water, redevelop with rodinal, or other clean-working developer, and plunge in the fixing bath. If the latter were omitted, the effect would be merely intensification; as it is, the result may rather be described as correction of the relative densities of shadows and high lights.

Stains

Developer stains which have resisted the fixing bath may be removed with:

Thiocarbamide................................ 45 gr.

Citric Acid....... 45 „

Water.............. 10 oz.

Or by a 2 per cent. solution of ammonium persulphate with a few drops of ammonia added to prevent reduction of the image. Silver. stains, especially those caused by setting off in damp weather from silver printing papers, can only be removed with cyanide of potassium in solution 10 gr. to the ounce.

Cracked Negatives

If it is desired to save a negative the glass support of which is broken, the film remaining intact, this latter must be transferred to a new glass plate. There are several methods, one of which is to begin by coating the dry film with a layer of enamel collodion, and then place the negative carefully supported on a flat surface, in a 5 per cent. solution of hydrofluoric acid. The dishes used must be of vulcanite or papier mache, glass and porcelain being attacked virulently by this agent. In a few moments the film will commence to detach itself from the glass. Have ready a new glass thinly coated with gelatine. Pour off the solution and replace with cold water; gently work the new glass under the film and draw the two together into position. The film on its new support may now be set to dry after adjustment of any wrinkles or frilling with a soft camel-hair brush.

Portrait Study.

Portrait Study.

E. O. Hoppe, F.R.P.S.

Another method is to prepare a temporary support by soaking a piece of thin paper in hot melted paraffin wax. After the negative has been soaked in the hydrofluoric acid solution the temporary support is pressed on to the film and the two are removed together in contact, and immersed for a moment or two in: