..
>I'd been told that Van Dyke Brown simply didn't work at ICP: they'd
>resorted to salted paper. But when I taught a summer course there we had
>no problem. I thought at the time maybe the water changed in summer; later
>I figured they'd probably been heat drying.
..
Both dichromate and iron processes are sensitive to drying conditions, for
different reasons.
What I find missing in 99% of trouble reports is an *accurate* description
of working/drying conditions. This reminds me of the older literature where
one keeps reading "in the summer, use 3%; in the winter, use 5%" without
further explanation. I think we can do a little better today.
Since drying conditions can make the difference between life and death, I
suggest that workers make complete tables/charts of their experiment.
Date
Type and size of paper
Extra sizing?
RH in the room (Relative Humidity)
Temp. in the room
Drying
When? ASAP? 10, 30 seconds, etc., after coating?
How? Forced cold air? Hot? Low setting? Cold at first, then hot?
How long?
Printing
When after drying? If you hot dry a print and then wait hours to
expose your print you can assume that the paper fibers are now back
to normal RH.
The only difficult part in making an accurate report, is the RH reading...
Cheap RH meters can be inaccurate by as much as 20%! Amazingly, RH paper
strips with small patches that go from blue (dry) to pink (humid) are more
consistent, at least in my experience, and they cost anywhere from nothing
(e.g., as promotional material from chemical suppliers) to $1 or so a
piece. In my lab I have a very expensive (hair-type) Lufft Durotherm, made
in Germany. To calibrate it, which it hardly needs, I have a sling
psychrometer, by far the most accurate instrument, but inconvenient to use
and very expensive. These instruments are available through lab and
conservation material suppliers.
I don't know how the newer electronic-type RH meters work and how accurate
they are. Perhaps someone on the list is familiar with them?
Luis Nadeau
nadeaul@nbnet.nb.ca
Fredericton, New Brunswick, Canada
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