INTENSIFICATION.
The toning processes are in reality methods of intensification. The prints may be intensified with either gold, silver, or platinum. If gold is chosen, the wet print, after thorough washing, is laid face up on a sheet of glass, and the excess of moisture blotted off and allowed to become almost. surface-dry. It is then covered with glycerine, as if for local de velopment. Supposing a 12 in. by 10 in. print is under treatment, 1 grain of gold is added to 30 drops of water and slightly acidified with pure hydrochloric acid. The solution is spread rapidly over the print with a brush. When the desired increase of intensity has been reached, the print may be immersed in a solution consisting of metal 10 grs., soda carbonate 100 grs., water 5 oz. As regards intensification with platinum, the process introduced by Hiibl has been found the most satisfactory. It consists of immersing the print for from 10 minutes to half an hour in a solution of sodium formate and platinum per chloride, the platinum being slowly re duced and deposited upon the metallic particles already present. The formula
advised is platinum perchloride 5 grs., water 4 drams ; sodium formate 25 grs., and water 4 drams. Add these two solutions to 30 oz. of water. Hiibl has further sug gested a method of toning with ferri cyanide of iron, which gives a variety of tones between a slate grey and a bright blue. The colour, however, is not perman ent, being easily discharged by dilute alka line solutions, as in the case of uranium toning, but with the difference that when discharged they leave the image in its original state and not reduced. So long as the prints are kept acid and free from moisture, there is no reason why the colour should not be fairly permanent. It is necessary to mount the prints with an acid mountant.