washing/developing cyanotypes

    I've always considered the water bath after exposing a cyanotype to be a
process of washing away the unexposed sensitizer in preparation for the
oxidation of the blue--which I always hasten with the short bath in a weak
hydrogen peroxide solution. So I have never controlled it--when the print
looks clear of yellow and stops running blue wash water I move it to the
peroxide (my prints don't run as much blue as many have complained about,
but there is some blue washoff).
    But today, trying to make two identical prints, I discovered that one
was much darker than the other. Working backward, the only likley
possibility was a shorter water bath for the darker print. Obviously I had
thought it was fully washed when I moved it to the peroxide, but, equally
obviously, it wasn't. The difference has to be less than 90 seconds and
probably less than 60 seconds.
    The solution seems to be to time the wash length. Do any of you need to
do this? Or have I overlooked some other possible cause? Needless to say,
the exposure time and the sensitizer were as close to identical as can be
controlled.
Charles Portland, OR
Received on Tue Nov 15 22:23:34 2005