256 PHOTOGRAPHIC FACTS AND FORMULAS 
Dark red: Peach black, 2 parts; alizarin lake, 1 part; 
Indian red, 1 part. 
Black, for pencil drawings: Graphite. 
Black: Peach or blue black. 
Warm black: Ivory black. 
ARABIN GuM-BICHROMATE Process.—The best Soudan 
gum arabic should be sifted through a 40 mesh sieve. Take: 
Hydrochloric acid 7.5 ccm 127 min. 
Water 150 ccm 5Y4 oz. 
Heat to 50° C. (122° F.), sift in 100 g (3% oz.) of the gum, 
and stir until dissolved; then add methyl alcohol, 600 ccm 
(21o0z.), and stir until the arabin is precipitated. Filter 
through linen, and squeeze out as much alcohol as possible. 
Then break up the lump, put in jar, and cover with fresh 
alcohol; leave for some hours with constant stirring until 
all the water has been absorbed by the alcohol. Squeeze dry, 
spread out to thoroughly dry, and powder. To make the 
coating mixture, take: 
Arabin 20 g 300 ger. 
Magnesium carbonate 20 g 300 gr. 
Water 40 to75ccem 1% to2% oz. 
The quantity of water depends on the thickness of the solu- 
tion desired. Mix the pigment of the desired colour with the 
above until a thick cream is formed, add an equal volume of 
15 per cent solution of ammonium bichromate, and spread 
on paper. Development is effected in the usual way with 
mater at about 30° Cii(957 dae 
FORMULA FOR PurE WHITES.—Frequently pure whites are 
difficult to obtain with this process, and the following is said 
to obviate this trouble: 
Potassium bichromate 722 550: 2: 
Water 450 ccm 7 02. 
Dissolve by heat, add enough ammonia to make the solution 
