As a matter of practice on ALL papers, Iuse a dilute citric acid
bath after the first rinse of water on my cyanotypes. Make be an
old darkroom superstition but it seems to clear the iron out both
faster AND more thoroughly. Left over ferric salts seem to me,
having examined vinatge specimens & my tests of 15 years ago, to
be the major force of deteriorating cyanotypes.
Russ Young
Santa Fe, NM, USA
elevation 7,000', 340+ days a year of brilliant UV source