FERROUS OXALATE DEVELOPER One of the oldest developers for plates and bromide papers, announced in 1877 simultan eously by Carey Lea and Willis, of America and England respectively, now almost entirely re placed by the newer organic developers. It has the great advantage of giving an image in pure metallic silver—that is, without any oxidised stain which is so often the defect of the newer developers ; but, on the other hand, it oxidises rapidly, and gives in hard water a precipitate of oxalate of lime. Also, it raises the inertia of the plate, or, in other words, it does not bring so much out of a plate as do the newer developers. In the case of bromide papers, it is necessary to use an acid bath after development in order to prevent the deposition of basic iron salts in the fibres of the paper ; and this disadvantage has led to its disuse. It can be prepared most con veniently by double decomposition between potassium oxalate and ferrous sulphate. The
following may be considered a standard formula : I.—Neutral potassium oxalate . . 5 oz. 250 g.
Distilled water to 20 „ ccs.
2.—Ferrous sulphate. 61- oz. 33o g.
Distilled water to 20 „ ',coo ccs.
Pure sulphuric acid io mins. cc.
For use, add i part of No. 2 to 4 parts of No. 1. The iron must be added to the oxalate, and never vice versa, so that the oxalate is always in excess, because ferrous oxalate is insoluble in water and soluble only in excess of an alkaline oxalate. The developer is a deep orange solu tion which does not keep well. Another method of making it is by heating the oxalate solution to boiling point and adding dry ferrous oxalate until saturated, and then cooling and bottling ; but the first method is preferable. Bromides can, of course, be used as with any other deve loper, and the addition of a very small quantity of " hypo," •o6 per cent., acts as an accelerator.