[alt-photo] Re: Speedy Cyanotype Question

[Alberto Novo]

Francis, 

> Back in February the list discussed the shortening of exposure time by
> increasing the amount of FAC to Potassium Ferricyanide. I find myself in a
> bit of a crunch with a outdoor workshop tomorrow and no sun in the forecast.

Increasing FAC to PF enhances the sensitivity AND bleeding. Otherwise, you 
have to keep PF out of the formula, coating with FAC only, then developing 
in PF. But you have to be very quick pouring the PF because it is very 
likely to obtain development streaks. 

> I don't remember Richard Sullivan's recipe for cyanotype. What are the
> effects of adding ammonium dichromate and how much is added? What about the
> amounts of FAC and PF? 

The only effect of dichromate is to extend the shelf life of FAC or of the 
coated paper. As an oxidant, it prevents the reduction of Fe(III) to Fe(II) 
in the dark and converts any Fe(II) impurity to Fe(III). 

Alberto