U of S | Mailing List Archive | alt-photo-process-l | Re: Cyanotype

Re: Cyanotype

OTOH, I don't understand how those fine gram ratios are comparable since
FAC is ill defined and composition is pretty variable -> differs according
from supplier to supplier and even from batch to batch...
True. Clerc gave for the green substance the formula (NH4)3 Fe2(Oxal)3, but the suppliers prefer to give the iron content (usually in the range 14.5-16%). Clerc's formula gives 15.24%.
So, the ratios could be calculated also for the lower and higher values of that range, referred to the (presumabily) theoretical value. For example, 20:8 becomes 19:8 to 21:8 depending on if you have an iron poor or iron rich fe am citrate.

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