cyanotype question

Today I ran an exposure test with the traditional cyanotype formula
using a NuArc 21-1K mercury exposure unit. I got some unexpected
results and am looking for a possible explanation from anyone familiar
with the process.

I first coated a sheet of Crane's PS8126 ecruwhite laid kid finish and
dried it using a hair dryer w/o heat. I then cut the sheet into several
parts and exposed the sheets simultaneously using three StoufferT2115
step wedges. The NuArc was set to 250 units and one test strip removed
after a 250 unit exposure. The second and third strips were given an
additional 250 unit exposure then the second strip was removed from the
exposure unit. The third strip was then given another 250 exposure so
the three strips had total exposures of 250, 500, and 750 units,
respectively.

The sheets were then processed together given an initial 2 minute rinse
in a 2% citric acid solution followed by a 10 minute wash and final 30
second bath containing a few drops of hydrogen peroxide.

When the strips dried, the exposure scales appear to be different, not
just shifted between steps as exposure increased. (The strips did show
a shift in position towards higher steps as exposure increased.) The 250
strip had tones covering 1.63 density units, the 500 was 1.45, and the
750 was 1.31. The density figures are transmission densities of the
step wedges as measured on an X-rite 301 densitometer and the printed
densities were judged visually.

So, it looks like the exposure range is decreasing as the total exposure
increases. What's up with that?

The solutions were several months old and since they were all processed
together, the 250 and 500 unit exposures sat around while the last strip
received the final 250 exposure. Another curious thing is that the
solarized steps lost the solarized appearence as they sat in a paper
safe while the final strip was exposed. This loss of solarization was
also noticed last week when the humidity was very high. Why the loss of
solarized tones as the strips waited for processing? And, is there some
sort of intermittency effect with cyanotype?

Any other ideas?

Thanks for any info.

Joe
Received on Wed Sep 1 15:55:28 2004