In the first place the print should be bleached in a solution of permanganate containing hydro chloric acid, viz.- Potassium permanganate, 1% solution 2 oz. (soo c.c.) Hydrochloric acid . . i dr_ (so c.c.) Water, to make . . 20 oz. (1,000 c.c.) This destroys any coloration due to organic materials (ink stains, colouring matters, etc.). After a brief rinse, the print is redeveloped in full light in any developer and washed in several changes of water.
The same process considerably reduces the yellow fog caused by reduced silver, but in this case the treatment given for the removal of dichroic fog (§ 433) should be applied.
Small Light Spots with Sharp Circular or Oval Outlines. Air bubbles adhering to the paper during development. The risk of air bubbles is considerably reduced if the paper is wetted before immersion in the developer by leaving it for at least one minute in a deep dish of water. In the case of rough papers, however, it is advisable to pass a wad of cotton wool or a large soft brush over the surface of the emulsion during this preliminary soaking.
Black or Dark Spots. These spots often occur on a print which has been fixed face downwards, and under which air bubbles have been im prisoned. Access of the fixer is obstructed, and the developer absorbed in the gelatine coating allows development to continue in these parts.
Powdery White Deposit on the Dry Prints. The use of wash water containing much lime (in this case, finish off the washing with a little soft " water, i.e. boiled or spring water) ; or the prints have been fixed in a bath containing alum in which precipitation of alumina has been caused by neutralization of the bisulphite. This deposit of alumina will disappear if the print is left in a 5 per cent solution of anhydrous sodium carbonate for some time, and then washed in pure water.
Bloom or Bronzing of Shadows in an Old Print. Slow action of the hydrogen sulphide (which is always present in small proportions in the atmosphere of towns) on the silver of the image. The print should first be cleaned, if necessary, by gently rubbing it with a wad of cotton wool soaked in petrol, followed by alcohol. It is then washed and sulphide-toned (§§ 587 to 592), or bleached in the hydrochloric acid-permanganate solution referred to in the previous paragraph. Then wash, develop, and wash again in several changes of water. Treat ment of prints with encaustic paste renders them less liable to this disfiguring metallic appearance in the shadows.
Yellow, Brown, or Purplish Spots appearing after Some Time on the Finished Print. Such stains, which are caused by incomplete fixation, may consist of silver sulphide formed from the silver thiosulphate left in the gelatine, or may be due to coloration, by light, of the silver chloride or bromide which has not been removed in parts of the coating, being protected from the action of the fixer by air bubbles or by adherence of prints which have not been kept moving sufficiently during fixation.
Gradual Bleaching of the Image, Starting in the Light Tones. Formation of yellowish-white silver sulphide ( J . I. Crabtree, 1920), usually due to hypo left in the gelatine by incomplete washing ; sulphur deposited in the gelatine by a decomposing fixing bath (milky bath) ; presence of hypo in the pulp of the cardboard on which the print has been mounted ; action of atmo spheric hydrogen sulphide. It is sometimes possible to restore such prints as follows : Clean them with a tuft of cotton-wool soaked first in light petrol and then in alcoln 1 ; then place in a fixing-bath containing alum, and wash thoroughly. If the lights are slightly stained brown or yellow by silver sulphide as a result of the slow decomposition of the silver thiosulphate remaining after bad fixation, they should be left for some time in a 2 per cent solution of potassium or sodium cyanide and again rinsed. Next, treat the prints with the hydrochloric acid-permanganate solution mentioned in a previous part of the present paragraph, rinse briefly, and re-develop the image in full light, finally washing in several changes of water.
620. Failures in Sulphide Toning. Yellowish Tones. The print has been insufficiently devel oped, usually following over-exposure. Occa sionally such photographs can be saved by gold toning (§ 593).
Weak Prints, especially in the Light Tones. When bleaching with ferricyanide, the print still contained a little hypo (insufficient washing), thus forming a reducer ; or sulphiding has been carried out in a sulphide solution which was exhausted or oxidized and partially converted into hypo.
Blisters. The use of too strong a sulphide solution, or too prolonged treatment in a solu tion of the usual strength, the gelatine being considerably softened thereby 6r7).
Blue Spots. Formation of prussian blue by the action of the ferricyanide of the bleaching bath on particles of iron from water supplied in rusty pipes ; or, more rarely, on the iron particles existing in the paper pulp or in the baryta coating. These spots may be removed, after sulphide toning and washing, by immersion in a very weak solution of ammonia, followed by brief rinsing.