The fixing bath of necessity follows all toning operations. In this the desired action is to dissolve away all sensitive silver salts, and leave only those that have been reduced by the action of light and coloured by the toning bath. It is commonly supposed that the silver salts of P.O.P. are soluble in hypo. This is to some extent true, but it is not the whole truth, because the action is not simple but complex. When prints are immersed in a fixing bath the first action that takes place is the conversion of the silver chlorides, citrates, etc. (insoluble in water), into silver thio-sulphate—also insoluble in water, but soluble in an excess of hypo if sufficient time is given. But it is highly important that the hypo is really in excess, and this is best ensured by having a large quantity of solution, not by means of a small quantity of strong solution. A very safe practice is to have two fixing baths, and after allowing the usual ten minutes in one to transfer to the second fresh bath. It may seem an unnecessary refinement, but when lasting prints are desired it is the only safe course.
The addition of small quantities of strong ammonia to the fixing bath is advocated by some workers. To me it is an objectionable practice in that it causes swelling of the gelatine to such an extent that the prints curl and become unmanageable. I well remember one large batch of prints that I treated in this manner. In the wash water they curled to such an extent that the "guvnor" asked me if they were " pipe lights," and the time they took to mount was appalling. It is a decided advantage when the fixing bath has no acid tendency, and, after platinum toning, should be decidedly alkaline. This condition can be acquired by the addition of a few crystals of washing soda without resulting in any unpleasant curling.