>When green scales became available (as explained in photo magazines of the
>1910 to 1920 period) they were at first extremely costly, so photographers
>continued with the brown a while longer.
>
In the " Figures, Facts and Formulae of Photography" ed. H.Snowden Ward
published in 1903 green Ferric Ammonium Citrate is mentioned in
connection with Ferro-Prussiate sensitisers. So the stuff must have be
around since the beginning of the 20th Cent.
According to the formalae given for the green and brown versions in the
book, the brown FAC appears to contain some ferric hydroxide, which
might account for its colour. Perhaps the chemists might care to
comment.
Incidentally in the above book there are some details of toning methods
etc., for cyanotypes which I would be happy to post if anyone is
interested.
Alex.
Gloucester UK