One understands that the blue color of the ground is more or less intense according to time of insolation, for the chemical actions between the reduced and the non-reduced iron salts is so much more complete as the salts acted on are more or less deoxidized, that is, reduced to ferrous salts; and that to obtain the maximum of effect, which, therefore, depends on the allowable time of exposure, the drawing ink should be opaque and non-actinic as far as possible, because when, on testing, the lines are tinted the exposure should be discontinued.
"Photographic Reproduction Processes"
P.C. Duchochois