Hello Charles,
exposing a paper sensitized with ammonium ferric citrate and starting developing by brushing on a solution of potassium ferrocyanide is basically the positive working Herschel process. It is described in e.g. Mike Ware´s "Cyanotype" on p. 53. Herschel gave up on this because of discoloration of the paper.
Pellet overworked the process using gum arabic (which as Eder writes prevents the chemistry to reach the paper fibres and hardens while exposure) and ferric chloride in the sensitizer and potassium ferrocyanide for developing. There are several other processes with ferric salts in the sensitizer and potassium ferrocyanide for developing ... too much material to go into this deeper here. The use of a potassium ferrocyanide formula is tempting because it is used in the food industry (e.g. for clearing wine) which makes the process even less toxic than the cyanotype with potassium ferricyanide.
The higher speed of printing with sensitizers that do not enclose ferri/ferrocyanide could be a result of the less or not occuring self shielding while exposure. Several scientists have extensively worked on the printing speed with organic acid salts (citric, tartric, oxalic, malic, ... acid).
All the best
Kai
Received on Tue Dec 13 06:55:10 2005
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