The plate intended for persulphate reduction must be fully fixed, and hypo well washed out. Use a 2i 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 r part permanganate, 2 parts sulphuric acid, 66o 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.
Rekalogenation.—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 bromide or chloride of silver by bleaChing, and then redeveloped. A good bleaching solution is Potassium Bichromate . , 3o gr.
Potassium Bromide . , 6o „ Water . . !. -; . . io 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 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 . , . . . • 45 gr.
Citric Acid . ... • •• 'Water . .
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 io 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. Your 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.
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 Methylated Spirit Water . . . . . . . . . . I „
Glycerine . . . . . . . . . I „ when the new plate is introduced, the whole is squeegeed once more, and the temporary support may be drawn away.
Enlarging the Film.—Those who desire a larger negative than the original may try the following solution in order to remove the film : Hydrofluoric Acid . . . . . . oz.
Citric Acid . . . . . . . . . I „ Glycerine . . . . . . . . . „ Water . . . . . . . . . . 8 „ No temporary support must be used nor the enamel collodion these serving the purpose of preventing enlargement. As soon as the film is loose slip the larger glass support under it, guide into contact and transfer to clean water.
Varnishing.—The hot method of varnishing negatives is the best, but the requisite dexterity in distributing the liquid evenly over the plate is not often attained in these days when the wet-plate process no longer gives one a chance to practise. See that the film is clear of dust, then warm the back of the plate at the fire or a gas-burner until it nearly as hot as the hands can bear ; then, holding it by a corner, pour the varnish on the centre of the plate. By inclining the plate gently coax the flood up one side, down the next till, at the last corner, the surplus may be poured into an extra bottle for refiltration. Heat may be again applied as soon as any streaks across margin and back have been wiped away.
The best hot varnish for gelatine negatives is made by dissolving 2 oz. of orange shellac in a pint of methylated spirit with oz. of oil of lavender. An ounce of powdered whiting is then added, the whole shaken up and allowed to settle and finally filtered through cotton wool in a glass or paper funnel.
A good one for retouching on is Sandarac . . . . • . • . i oz.
Oil of Lavender . . . . . . . „ Methylated Alcohol . . . . 7 „ Cold Varnishes.—These may be applied by the brush in the cold state, and dry very quickly. Films may be either brushed or dipped in the preparation and then hung up to drain.
Gum Dammar . . . . . 5o gr.
Benzole . . . . . . . . . I oz.
This dries a few moments after application, but does not become hard for some hours unless moderate heat is applied. Another very effective cold varnish for plates or films is made by dissolving i oz. of celluloid in 50 oz. amyl acetate. Old film negatives stripped of their gelatine surface may be used for making this varnish.
Water varnish is prepared by pouring a quart of boiling water over half an ounce of borax, and adding by degrees a pound of orange shellac (stirring to ensure rapid melting). If the borax does not suffice to make all the shellac dissolve more is added, a few grains at a time. The solution will keep for a long time, if corked free from dust, and may ,., • — be used alone ; but it is more commonly employed as a cold varnish as follows : Water Varnish . . . . . . . . 2 oz.
Ammonia . . . . . . . . . 6o minims.
Methylated Spirit . . . . . . . 4 oz.
A practical negative varnish good enough for most purposes, for use either hot or cold, may be manufactured by diluting any good hard spirit varnish procured at oil shops with an equal volume of methylated spirit.