The ordinary developer for the cold bath is:

Potassium Oxalate.......1 oz.

Potassium Phosphate ..... 1/2 „

Water......... 10 „

Dissolve in hot water and allow to cool before using. Or for warmer blacks:

Potassium Oxalate ....... 1 oz.

Zinc Oxalate........175 gr.

Water.........10 oz.

These developers may be used repeatedly, but must be rejected when they become of a dirty greenish tint. Immerse the print from the frame face downwards in the solution drawing it backwards and forwards, and then examining to see whether the image is appearing. Development is slower than with the hot bath and allows time to save an overexposed print by quick transfer to the acid bath. For most papers the developer should be neutral, but with Ilford Platona, for which the first of the two formulae is recommended by the makers (but diluted to 1 in 14 instead of 1 in 10), the developer works better if rendered acid by the addition of, say, 30 grains oxalic acid. Damp paper develops somewhat brown and "mealy."

According to Dr. Jacoby a developer compounded as follows will give sepia tones on an ordinary cold-bath paper. Development will occupy about five minutes.

Potassium Oxalate...... 200 parts.

Ammonium Monophosphate . . . 50 „

Cupric Sulphate....... 2 „

Water......... 1000 ,,

Owing to the slower and steadier action of the cold bath there are time and opportunity for local development. The paper may be laid upon a sheet of glass and the parts which it is desired to intensify brushed over with a stronger solution of oxalate in glycerine. Other portions may be retarded by painting with plain glycerine.

When development is completed the print is transferred direct to the clearing solution, and the subsequent procedure the same as the hot bath.

Under-printed pictures may sometimes be coaxed into appearing by raising the temperature of the developer; the time of development is shortened thereby, and the print loses its pure black effect, becoming brownish by slow degrees. On the contrary, if a print has by any accident been allowed to remain in the printing frame so long that it has practically "printed out," all that can be done is to place it directly into the fixing bath without development. Of course it will never be a very creditable print, but may serve to file as a proof of a negative.

Acid Baths

These must be replaced from time to time, especially the third, which ought never to be otherwise than quite clean. A good way is to throw away No. 1 after each batch of prints, promoting the second bath to office as No. I and making up a new solution for No. 3. A little soda is sometimes added to the subsequent washing water in order to neutralise the acid in the paper, but hydrochloric acid is very soluble and will soon disappear. Hard washing-water sometimes leaves a deposit on the paper, which must be wiped off with a soft rag or cotton-wool.

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 ........ 1 oz.

B. Potassium Ferricyanide . . . 48 gr. Water........ 1 oz.

C. Ammonium Sulphocyanide .... 1/2 oz. Water........ 1 oz.

These solutions must be kept separately. For use take 1/4 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........ 1 oz.

B. Platinum Perchloride...... 10 gr.

Water........ 1 oz.

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 complete 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......... 3 1/2 oz.

B. Mercury Bichloride...... 75 gr.

Water ........ 3 1/2 oz.

Take of A 25 parts, B 1 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.......1/2 oz.

Potassium Phosphate.......1/4 ".

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.