This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
When using a chloro-platinite of potassium toning bath with Ilford P.O.P., print scarcely so far as for treatment with ordinary gold and sulphocyanide bath. Dissolve 15gr. of potassium chloro-platinite in 15dr. of distilled water; label this "Stock platinum solution, 1 grain in 1 dram." As it is liable to change - the platinum being precipitated - if exposed to light, it should be kept in a dull light or, preferably, in the dark room. For toning one sheet of paper, make up the following: Dissolve 50 gr. of chloride of sodium (common salt) in 10oz. of distilled water and add 100gr. of alum, and finally 2 dr. (2 gr) of stock platinum solution. The bath is ready for immediate use, but does not keep satisfactorily more than a day or so. Should any sediment be thrown down, indicating the presence of impurities, the bath should be discarded. Thoroughly wash the prints, which must not be in an acid condition. It is advisable to pass them through a 5-per-cent. solution of carbonate of soda (loz. of washing soda in 20oz. of water), and again wash before toning. It is difficult to tell when the prints are correctly toned until experience is gained, but they should not be toned longer than five minutes in winter and rather less in summer.
When toned, place in a 2',-per-cent. solution of common salt, which stops the toning. Thoroughly wash and fix for twenty minutes in hypo 2oz., water 1 pt., or a 10-per-cent. solution. Finally wash for two hours. In all the operations the prints should be kept well separate 1, hence it is advisable to tone only a few at a time and to use two dishes of hypo and transfer from one to the other. The same plan should be adopted in washing, if unprovided with a washing tank. The above is specially recommended by the Britannia Co. for their Ilford printing paper.