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Photographer's Formulary Sensitizer A is a component that is used in both the Platinum and Palladium printing process.
To sensitize the paper in the Platinum or Palladium printing process, a mixture of Sensitizer A, Sensitizer B, and palladium salt is prepared in the darkroom. The paper is coated with the mixture, then dried. Print contrast is controlled by the amount of Sensitizer B that is added to the mixture.
The photographic term "ferric oxalate" is a misnomer, which has given rise to a considerable amount of confusion in the photographic literature. There are two common forms of this compound; tri-potassium ferric oxalate (K3 Fe (C2 04) 3) and trihydrogen ferric oxalate [H3Fe(C204)3]. While both forms are photosensitive, only the acidic form is sufficiently photosensitive to be useful in photography. The ferric oxalate supplied is a 20% solution of trihydrogen ferric oxalate.
This chemical is prepared by Photographers Formulary by the iron alum-oxalic acid procedure and contains a slight excess of oxalic acid. This ferric oxalate is ready for use as Sensitizer A without additional mixing. Sensitizer B does require additional mixing.