Various modifications may be brought about in the character of the image. The addition of a little potassium bichromate to the developing bath increases the contrasts of the picture, principally by stopping the development of the light tones, and so making it necessary to push development a little further. On the other hand, the addition of glycerine to the developer con siderably retards the building-up of the image, and can therefore be used to arrest development of an over-exposed picture before completely developed. This property of glycerine is parti cularly useful for the localized control of development (A. Maskell, 1892).
Instead of immersing the print in the devel oper, the operation can be performed with a brush, after the printing has been continued until the lightest tones are visible. The following solutions are placed in four small glass jars.
(a) Pure glycerine.
(b) A mixture of 4 parts of glycerine and part of cold developer.
(c) A mixture of equal volumes of glycerine and developer.
(d) Plain developer containing no glycerine.
One or more brushes are kept for each jar.
The print is fastened on a board with drawing pins, or pressed on to a glycerined sheet of glass, and is first of all uniformly coated with glycerine, the excess being removed with blotting-paper. Guided by a print from the same negative and by the rough outline of the image produced by exposure to light, the whole of the print is first covered with the developer containing the most glycerine ; the picture comes up very slowly. Development is retarded in the parts to be obtained of less depth by covering them with pure glycerine, while development is accelerated in the parts which require strengthening by covering them with developer containing 50 per cent of glycerine, or with developer contain ing no glycerine, after the liquid already im pregnating the print has been removed with blotting paper. Immediately the desired result has been obtained the print is quickly transferred to the fixing bath. 2 632. Platinum prints are fixed, without inter mediate rinsing, in several successive baths of weak hydrochloric acid (containing 21 drams of pure concentrated acid per 20 oz. of bath : 15 c.c. per litre ; the quantity of acid is reduced to about i drm. (io c.c.) in the case of sepia prints), the ironand platinum salts being removed by these baths. The prints should remain about
five minutes in the first bath, then io minutes in the second, and 15 minutes in the third. A num ber of prints may be fixed in the same baths, either simultaneously or successively. If the operations have been carried out correctly, the third bath should remain colourless (pure hydro chloric acid is perfectly colourless). If, however, the third bath does not remain colourless, a further additional bath should be employed.
As soon as the third bath becomes coloured to any extent, it should be replaced by a fresh bath, the old one being then used as the second bath, while the solution which was originally the second becomes the first bath. During fixa tion the paper loses its original yellow colour and becomes perfectly white.
After fixing, the prints should be washed for about 15 minutes, the water being renewed four or five times ; they can then be put to dry ; they should not be pressed between blotting paper, as this occasionally causes stains.
Old prints on platinum paper which have not been properly fixed often turn yellow owing to the presence of a residue of platinum and iron salts. Such prints may be restored by washing them afresh in hydrochloric acid, followed, after a short wash, by immersion in a 5 per cent or io per cent solution of ammonium oxalate. Such an eventuality can be completely avoided by treating the fixed prints before the final washing with a similar oxalate bath (R. Jacoby, 1901) ; fixation can even be done in baths of ammonium oxalate or citrate alone, i.e. without hydrochloric acid.
633. Sensitizing Papers for Platinum Printing. The preparation of platinum papers having similar characteristics to the commercial papers does not generally yield good results in the hands of an amateur. Further, such papers possess bad keeping qualities, and must therefore be prepared in very small quantities at a time, which makes the accurate weighing-out of small amounts of the materials of the sensitizing bath a delicate matter.
However, the sensitization with ferric oxalate of a paper which is to be developed with a. solution of potassium oxalate containing potas sium chloroplatinite does not present any particular difficulty. The method given below is due to W. S. Davenport (1900).