Development by Hot Bath.—The hot bath is not very widely used nowadays, except for some sepia papers or for prints that are lacking in contrast. It consists of oxalate of potash in a solution of 120 grains to the ounce of water, which is heated at time of development to about 140-160°, and the print brought face downwards over the surface of the water. Draw it rapidly backwards and forwards over the solution to prevent bubbles, and then lift it up. Development has taken place at once like a lightning-flash, and the print must be rapidly transferred to the fixing bath— Hydrochloric Acid . . . . . i oz.
Water . . . . . . . . 60 —where it must remain for ten minutes and then pass into a second fixing, or rather clearing, bath of the same compo sition, and lastly into a third. These acid baths serve to remove the superfluous iron salts, and the paper may afterwards be rinsed for a short time and dried. The developing solution may be employed for several prints in succession.
Sepia papers frequently contain a mercury salt, and are often much more sensitive than the black. They continue to be sensitive even after development until thoroughly cleared ; and they must be protected even more carefully from damp. The developer recommended by the manufacturers should be used with these papers, or a little mercury chloride added to the usual developer. Development is carried on in the hot bath at a temperature of 160°.
Cold-Bath Development.—The colder the bath, the colder the image, and conversely the warmer the bath the warmer the image. So, even with cold-bath papers, it is occasionally worth while to warm the developer. But given a negative of reasonable gradation, the best platinum prints are those developed cold. Compared with these, hot-bath prints often have a dull, flabby appearance, which is partly due to the size soaking off the paper.
The ordinary developer for the cold bath is Potassium Oxalate . c oz.
Potassium Phosphate . . . . p, Water . . . . . . . . . 10 „ Dissolve in hot water and allow to cool before using. Or for warmer blacks : Potassium Oxalate . . . . , . . c oz.
Zinc Oxalate . . . . . . . . 175 gr.
Water . . . . . . . . 10 oz.
These developers may be used repeatedly, but must be re jected when they become of a dirty greenish tint. Immerse the print from the frame face downwards in the solution draw ing 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 over exposed 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 formula is recommended by the makers (but diluted tot in 14 instead of i in to), the de veloper 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 IVIonophosphate . . . . 5o „ Cupric Sulphate . . . . . . . 2 „ Water . . . . . . . . . moo „ 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. t after each batch of prints, promoting the second bath to office as No. 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.