Ordinary tap water, which is generally more or less hard, is used for the preliminary washing of P.O.P. The chlorides, etc., combine with the free silver, which is thus removed. Trouble may arise with extremely hard water, particularly with gelatine papers. The chlorides and sulphates have been found to form compounds in the film that are not readily soluble and are not removed in the hypo bath or in the final washing if gold has been deposited on them. In such cases the whites of the picture usually turn yellow. M. Scholzig has suggested the following experiments. Wash two pieces of unexposed paper in four changes of tap water for a total period of ten minutes. Let one piece soak for another ten minutes in a new toning bath. Place both pieces in the hypo bath; wash and dry. Next soak them both in water containing a few drops of ammonium sulphide. The untoned piece of paper will be found to have kept its colour, but the toned piece will have yellowed considerably owing to the formation of sulphide of silver. Thus the theory has been propounded that the gold forms a sort of casing around the injurious compounds and keeps them from being dissolved out.
As the preliminary washing is dispensed with when the combined bath is used, toning by this method offers a possible way out of the difficulty where the water is very hard; but with the combined bath the results are not permanent.