180 PHOTOGRAPHIC FACTS AND FORMULAS 
with the character of the print, that is to say, the area of the 
dark parts of the image. The following will give a rough 
idea of the quantity of gold to be used for every 645 qcem 
(100 sq. in.): for red, 0.00324 g¢ (0.05 gr.); for reddish- 
brown, 0.005 g (0.075 gr.) ; for brown, 0.0065 g (0.10 gr.) ; 
for warm purple-brown, 0.01 g (0.15 gr.) ; for cold purple- 
brown, 0.0125 g (0.19 gr.); for cold purple-black, 0.025 g 
(0.38 gr.). 
PLATINUM TonING.—The tones obtained with gold range 
from reds to purples and browns. Colours ranging from 
sepia to black, without any trace of purple or red, are ob- 
tained with platinum as the toning agent. A suitable bath is: 
Potassium chloroplatinite 2g 15.4 gr. 
Dilute phosphoric acid 30 ccm 230 min. 
Water 1000 ccm 16 oz. 
The phosphoric acid may be replaced with dilute lactic acid, 
or acetic, tartaric, citric, oxalic acid in like quantity; or 
sulphuric acid, 10 ccm (77 min.), may be used. The above 
baths keep well and give pure whites. The following acts 
well when freshly prepared: 
Platinum perchloride 0.2 ¢ 1.54 gr. 
Sodium formate 6.3 g 48 er. 
Formic acid 1.2 ccm 9.2 mm 
Water 1000 ccm 16 oz. 
The following is also an excellent bath (Valenta) : 
Potassium chloroplatinite O.5tolg 3.8to7.6 gr. 
Metaphenylendiamine O.5tolg 38to7.6 er. 
Water 1000 ccm 16 oz. 
For all platinum baths, the prints must be first immersed in 
a 10 per cent solution of common salt for 5 minutes, then 
rinsed; after toning, the prints should be immersed in: 
Salt 100 g 20g: 
Sodium carbonate, dry 50 g 1 oz. 
