CATECHU TONING A method of toning prints on platinum paper to various shades of brown by means of a solution of catechu was introduced by J. Packham in 1895. The stock toning solution is made up as follows : Place i2o grs. of catechu in 5 oz. of water and boil for five minutes ; allow to cool, and then add r oz. of alcohol. To make up a working solution add 3o to 4o drops of the stock solution to one pint of water, and heat to a temperature between i3o° and i50° P. (54° to 66° C.). The washed black and white platinum pictures are immersed therein, and toning wil be complete in about fifteen minutes, the colour being a rich mellow brown. Within one minute of immersion the prints will probably begin to change, and thereafter pass through various shades of brown in succession ; immediately the desired brown is reached, the print is put into cold water which at once stops the toning action. A few minutes' washing completes the operation. When the water with which the toning bath is made contains a considerable amount of lime, the solution becomes pink and slightly stains the whites of the picture. This may be counteracted by adding 2 grs. of potas sium oxalate to each pint of the toning bath, the addition tending to give tones of a warmer colour. J. Packham also states that after toning with catechu and washing, the per manency and brightness of the image are aided by immersing the print for about five minutes in a solution of very weak potassium bichromate of a light straw colour. The prints must not be
allowed to remain too long in the catechu toning bath, or the whites will be degraded, as the process is really that of staining. If desired, the bath may be used cold, in which case toning is very slow indeed. As the catechu-tannic acid in the catechu combines to form an inky com pound with iron, it is important that the latter be entirely removed from the prints before toning.
Chapman Jones has stated that catechu toning is due to the action of the extract upon the iron compound left in the print, which it is difficult, if not impossible, to remove completely, and that other substances which give colours with iron salts would give similar results, though perhaps not such desirable colours. The fact that the image is toned appears to be due to the fact that platinum holds the minute residue of iron more tenaciously than the paper alone, and that the residual iron compound is therefore roughly proportional to the thickness of the platinum deposit. (See also " Platinotype Process.")