Yellow or Brown Stain, Local or General. General stain may be due to the staining action of products of development on the gelatine.
This occurs if the developer is old and highly coloured, or if it contains insufficient sulphite. Stains in patches may be caused, especially with films and papers, on portions of the surface which during the first few moments of fixation have not been immersed in the solution. Solu tions of alum, sometimes proposed for the removal of these stains, are quite useless, but the stains may be destroyed as follows : After washing the negative, immerse it for about 5 minutes in a solution of 5 per cent chrome alum, in order to complete the hardening of the gelatine and to avoid any softening during the further treatment, and then rinse it rapidly. The image is then bleached in a mixture of equal volumes of the following solutions (A) and (B), which should be mixed fresh when required— (A) Permanganate of potash - 45 gr.
(5 gr.) Water, to make . . 20 OZ.
(1,000.c.) (B) Hydrochloric acid I oz. (5o c.c.) Water, to make . . 20 OZ.
(I,000 c.c.) After the image has been bleached it has a general brown colouration due to manganese oxide deposited in the gelatine. The negative is rinsed and is placed in a 5 or io per cent solution of bisulphite of soda until the brown colour has disappeared. The image is then blackened by treating it in an ordinary developer in full white light until no more white silver chloride remains visible through the back. Finally, wash the negative in two or three changes of water without fixing. The above treatment may be applied to a negative even if it has been dried. 1 433. Dichroic Fog. The fog generally known as dichroic, although it does not always show two complementary colours, most commonly appears greenish-yellow by reflected light, and pink or purplish by transmitted light. It con sists of ultra-microscopic particles of silver (colloidal silver) formed when silver bromide is subjected simultaneously to the action of one of its solvents, and to that of a developer capable of reducing silver salts in situ as soon as these salts are dissolved.
The conditions necessary for the formation of dichroic fog may thus prevail during develop ment and also during fixation. The milky appearance (by reflected light) of this fog often leads, in the dim light of the dark-room, to its being mistaken for a residue of undissolved silver bromide.
I)ichroic fog hardly ever occurs during development, except in the under-exposed portions of a negative where there is no silver reduced in its ordinary black condition,' and when development is prolonged in the empty hope of bringing up detail which the light has not registered or when slow-acting developers like hydroquinone or glycin are The solvent causing its formation may be hypo sulphite of soda accidentally introduced into the developer, or it may be ammonia added as such or as an ammonium or it may be sulphite of soda used in excessive amount.'" If formed during fixation, dichroic fog may extend over the whole or part of the negative without any relation between its distribution and that of the image ; its formation is due to developer carried over into the fixing bath by the gelatine of the negatives. A neutral fixing bath favours its formation because in such a solution the developer retains its activity until it has become diffused into the bulk of the solution ; a very old fixing bath also favours its production, because of the accumulation of developer in In all cases the presence of ammonia or of ammonium salts, and the exposure of the plate to light before fixation is complete are circumstances favourable to the appearance of dichroic fog.
The occurrence of dichroic fog during fixation is almost certainly avoided by rinsing negatives between development and fixation, especially if the rinsing is done in slightly acidulated water.
The only practical reagent known to dissolve this colloidal silver" without attacking the image —that recommended by J. Hauff in 1894—was, for several years Sulpho-urea (thiocarbamitle) 2 • 35 gr.