PYRO STAINS ON NEGATIVES These are of a yellow colour, and are caused by exposure of the film when wet with the developer to the air, or by using insufficient sodium sulphite in the developer. It is not all workers who object to them. To obviate them, use a so per cent. solution of sodium sulphite or a 2i per cent. solution of potassium meta bisulphite instead of plain water for diluting the stock developer. If a stained negative has not been dried, the use of a 2 per cent. solution of caustic soda will remove some of the stain. If it has been dried, it is rather more difficult ; but the following will be of help :— Thiocarbamide . . 3o grs. 7 g.
Citric acid . . 6o „ 14 „ Chrome alum . . 3o„ 7 ,, Water to . . . 6 oz. 60o ccs.
Soaking in the following (introduced by B. J. Edwards, in 1883), clears the stain to some extent : Alum . . . f oz. 55 g• Citric acid . . „ 55 ,, Ferric sulphate. . I „ 165 „ Water to . . . so „ x,000 ccs.
Berkeley's solution will act if the stains are not of long standing : Alum . . . I oz. IIO g.
Sulphuric acid . . 2 drms. 25 ccs.
Water . . . so oz. x,000 „ Wash thoroughly after any of the above baths.
Namias recommends dissolving 48 grs. of ammonium persulphate in 5 oz. of water, and, in order to destroy the reducing action, adding a few drops of liquor The stained negative is immersed in this, rocked until the stain disappears, and then well washed.
Chapman Jones, in 189o, stated that all clear ing solutions hitherto proposed were founded upon wrong principles, as alum (first used for the purpose by Sir W. J. Newton in '855) actually retards the washing away of stains, inasmuch as it hardens the film ; further, acids, although they lighten the colour of the stains, render them insoluble. He considers a weak solution of caustic soda to be the best clearing solution, as the staining matter is kept in a soluble condition, so that it may really be washed away, and it is also kept in its original highly coloured form, so that its removal can be noticed.