The Iron Processes 
FERROPRUSSIATE OR CYANOTYPE Process.—This process 
is much used for the preparation of engineering and 
draughtsman’s plans, giving white lines on a blue ground, 
that is to say, a blue image is formed by the action of light. 
The sensitive solutions can be made as follows: 
A. Ferric ammonium citrate (red) 250 ¢ 4 oz. 
Water — 1000 ccm 16 oz. 
Filter. 
B. Potassium ferricyanide 200 g 3 1/35.02: 
Water 1000 ccm 16 oz. 
Filter. The ferricyanide must be in perfectly clear ruby-red 
crystals, free from any adherent yellow powder; it is advis- 
able to allow a little more in weighing out, and place the 
crystals in a flask; pour in some distilled water, shake, pour 
off the water, and repeat this operation; then add the water 
for the solution, and heat until dissolved. Mix the two solu- 
tions in equal volumes and filter. Either float the paper on 
the solution for 3 minutes, or paint the solution on freely 
with a broad flat brush, and dry as quickly as possible, prefer- 
ably by heat. A much more sensitive paper is obtained by 
using the green ammonio-citrate of iron; then the solutions 
should be: 
A. Ferric ammonium citrate 
(green) 125 g 2 oz. 
Water 500 ccm 8 oz. 
Filter, and add: 
B. Potassium ferricyanide 45¢ 346 gr. 
Water 500 ccm 8 oz. 
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