Toning.It seems to us somewhat absurd to tone platinum prints, destroying very much of their distinctive character. Even the sepia tints obtained by the special developer are scarcely attractive in comparison with pure black. A method of uranium toning is given by Von Hubl, but there are some doubts whether it is permanent.
A. Uranium Nitrate . . . . . . 48 gr.
Glacial Acetic Acid . . . . 48 minims.
Water . . . . . . . i oz.
B. Potassium Ferricyanide gr.
Water . . . . . . . . i oz.
C. Ammonium Sulphocyanide . . . . I oz.
Water . . . . . . . . i oz.
These solutions must be kept separately. For use take / oz. of A, B, and C and add 25 ounces of water. After fixing and thorough washing the print is placed in a dish containing the toning mixture, and rocked continuously, till the desired tone is obtained, or a little longer, as the tint will dry out rather colder than it appears while wet. A little weak acetic acid is recommended for the first washing water after toning. By substituting ammoniated iron alum for uranium nitrate the colour becomes blue instead of black, but this is rather an expensive way of producing a blue print.
Intensification.A feeble image may be intensified by the deposit upon it of a further layer of platinum.
A. Sodium Formate . . . . . . . 45 gr. Water B. Platinum Perchloride . . . . . . to gr. Water At time of using take 15 minims of A and B with two ounces of water. After intensification the print has only to be washed and dried.
Dolland's Intensifier.A neutralised solution of gold two grains to the dram, is just acidulated with hydrochloric acid and then spread over the print, which has previously been covered very evenly with a coating of glycerine applied with a sponge or brush. When intensification is adjudged com
plete the print is well washed and plunged for a moment or two in a metol or rodinal developer of moderate strength. After washing and drying the strengthened image will show itself as a rich black, about the permanence of which no doubts need be entertained.
Development with Platinum.For a home-made paper in which the platinum salts are introduced after development, and one which is both cheap to prepare and needs no special protection, the Joe iron process is to be recommended. Any good drawing paper, sized with arrowroot or agar agar, is sensitised by brushing over with the following : A. Ferric Oxalate . . . . . . . 300 gr.
Oxalic Acid . . . . . . . 30 Lead Oxalate . . . . . . . 15 Water . . . . . . . oz.
B. Mercury Bichloride . . . . 75 gr. Water Take of A 25 parts, B i part.
Dry without application of heat in the dark room and print in a good light until the picture shows all the details effectively ; then develop in Potassium Oxalate . . . . . . oz.
Potassium Phosphate . . . . . . . f Water . . . . . 5 to which immediately before use is added for every half-ounce 5 grains of potassium chloroplatinite dissolved in, say, 30 minims water. Fix in the usual hydrochloric acid bath.
Baron Van Hubl remarks on this process that it is but rarely used, but that it deserves the fullest consideration, as it exceeds the usual one in the beauty of the results and the ease with which it is manipulated.