Tannin or gallic acid 4 parts
Oxalic acid 0.15 parts
Water 1,000 parts
After developing the proof should be washed rapidly—under a jet of water,
if possible—for were the iron salt and the reagent not soon removed, or
any remain in the paper, the ground would be tinted violet. And whatever
be the care taken, it very seldom occurs that the whites are pure when the
proof is dry. This for half-tone pictures has not a great importance, but
for the reproductions of plans it is sometimes objectionable. In fact it
must be acknowledged that none of the processes now at our disposal—if we
except the so-called Artigues process described further on—gives an
entirely satisfactory result. A simple and expeditious process, yielding
intense black impressions on a white ground, is yet to be found for the
reproduction of plans, maps, etc., without resorting to a negative cliché
or drawing.