Re: Gum prints with an enlarger (UV loss)
SCHRAMMR@wlsvax.wvnet.edu
Thu, 11 Jan 1996 22:17:29 -0500 (EST)
All glass absorbs UV. It is more efficient at absorbing short wave UV than
long wave UV. Practical example- It's almost impossible to get a suntan
in a greenhouse. Some plastics are very, very good a absorbing UV and are
used by archivists to slow down the fading of photographic prints by UV.
(Museums also use low wattage incandescent bulbs - never halogen or
fluorescent bulbs) To sum up, glass does absorb UV but not all of it. The
thicker the glass, the more radiation it will absorb. Pure quartz, on the
other hand, transmits practically all wavelengths of UV quite well. That's
why I, tongue- in cheek, suggested quartz lenses. They really do make them
but they are very expensive. They may be found, for example in UV spectrometers.I have a number of print frames. Some contain glass and some plastic.
Frankly, even though I have wondered myself how much UV is being absorbed,
I have always been to lazy to run some tests. I would think one could do
a crude test by tapeing a neg over say paper sensitized with cyanotype
sensitizer and then fixing a glass plate over half of it. Make an exposure
and see if there is any difference. One could use something like this
to test a number of transparent substances to find out which gave the
shortest exposure for a good print. Maybe it's not worth the effort
since one can always just increase the exposure time. Nevertheless,
I think that if one could find a transparent substance one could put in
a print frame that whould cut the exposure time in half, a lot of people
on this list would be very interested. Pure quartz would work, but I
don't know if you can buy a sheet of it. There might be a variety of
glass or plastic that does a better job of transmitting UV. Maybe someone
on the list knows of something like this.
Bob Schramm