URANIUM TONING Uranium toning is really a modified form of intensification, and is not therefore suitable for dense prints. It is not considered to yield lasting results. It resembles the process de scribed under the heading " Uranium Intensifier." J. Weir Brown's one-solution formula is : Glacial acetic acid . I drm. 12.5 ccs.
Potass. ferricyanide . 3 grs. 7 g.
Uranium nitrate . 3 „ 7 ,, Water . . . 6 oz. 60o ccs.
This acts slowly and less water may be used in order to make it work more quickly. Many modifications have been made in the formula, and the solutions which appear to find the most favour are : A. Uranium nitrate . 45 grs. r0.5 g.
Distilled water . 10 oz. 1,000 ccs.
B. Potass. ferricyanide 45 grs. 1o•5 g.
Glacial acetic acid 2 drms. 25 ccs.
Distilled water . 10 oz. 1,000 „Uranium persulphate is said to act in precisely the same way as the nitrate above mentioned. The two solutions keep indefinitely as long as they are not mixed. For use, mix equal parts of A and B, and if this works too rapidly dilute with water. The prints should not have been developed with ferrous oxalate, as iron causes greenish spots to appear. Metol-hydroquinone developed prints tone excellently, especially if the toning is to be carried no farther than the sepias. Amidol developed prints tone to a full red. Prints if dried after washing should be immersed in water previous to toning so that the toner may work evenly. Toning begins as soon as the prints are immersed in the toner ; starting with a warm black the tones progress through the various shades of brown to red, and the image increases in density at the same time A little before the desired tone has been reached the prints are transferred to a bath of weak acetic acid (about roo drops to ro oz. of water) ;
after two or three minutes, they are washed by soaking in still water until the yellow stain has disappeared. If washed in running water colour will be washed away, leaving a patchy result.
Dilute citric acid. (io grs. to the ounce), and oxalic acid (5 grs. to the ounce), are said to give dearer tones than when acetic acid is used.
Permanent yellow stains are caused by in sufficiency of acid in the toner, the acid keeping the gelatine soft and facilitating the washing out of the stains. The last mentioned may be re moved by dabbing them with cotton-wool soaked in a very weak solution of ammonium sulpho cyanide. The toned image can be restored to its original state by soaking in a weak solution of ammonia or sodium carbonate and washing well.
Uranium may also be used for platinum prints : Uranium nitrate . 5 grs. i g.
Potass. ferricyanide . 5 „ I „ Glacial acetic acid . 5 mins. I cc.
Ammonium sulpho cyanide. . . 25 grs. 5 g•Water . . . ro oz. i,000 ccs.
Toning is carried out in the manner described for bromide prints above.
Uranium is also used in combination with gold for ordinary P.O.P. (gelatino-chloride) prints. The bath is— Sodium bicarbonate . io grs. 1 g.
Sodium chloride . 3o „ 3 ,, Sodium acetate . 6o „ 6 „ Water . . . 15 oz. 75o ccs.
Dissolve and add Uranium nitrate . 5 grs. -25 g.
Gold chloride .
• 4 If .2 „ Water . . . 20 or. 1,000 ccs.
The toner gives brownish black or pure black tones according to the quality of the negative used for printing from. The bath must not be acid. After toning, the prints are fixed in an alkaline fixing bath and washed as usual.