sensitizing for carbon
Pollmeier Klaus (100561.2417@CompuServe.COM)
26 Nov 96 16:53:25 EST
I wrote the following to somebody privately, but maybe someone else on this list
is interested in some basics on sensitizing carbon tissue:
[...]
It is a good idea to prepare the tissue in the evening if you want to use it the
next morning. But without some precautions it will gain too much speed and will
fog. To avoid fogging, don't use the brayer method for sensitizing [I sometimes
use a spirit sensitizer and apply it with a foam brayer, if I don't want to wait
hours for the tissue to dry] , but soke in a ca. 2% sol. of pot or amm
dichromate to which you have added strong ammonia until the sensitizer's color
just turned from orange to yellow. You could just hang the tissue up for drying,
or squeegee it face down to a clean sheet of plexiglas with a rubber squeegee.
You can peel it off the next morning with a perfect surface. The ammonia lowers
the pH and this way gives less speed and more contrast and less fog and... This
was standard practice in earlier times. You can change the strength of the
dichromate from 0.5 to 5%, giving more and more speed and less and less
contrast.
The carbon tissue always has a strong tendancy to curl or to become brittle,
esp. if rH goes under 45%. Only after sensitizing with a sprit sensitizer or
with some glycerine in the sensitizer it may lay flat by itself. [Glycerine
gives more contrast. Addition not necessary with a paper base tissue but
sometimes needed in dry environment when the colored gelatin was coated on
plastic]. If you fear the coating might break during unrolling, leave the tissue
in a room of ca. 60-70% rH for a day or so, depending on the thickness of the
roll. I have good experience with unrolling the tissue just a few centimeters,
and then, while unrolling, pushing a sheet of plexiglas over the unrolled part
of the paper. You can then use the plexi as a ruler and cut the tissue from the
roll. It will lay flat under the plexi - and jump up when you lift it ;-) You
can then put this cigar into the sensitizer and unroll it under water. Use the
sensitizer at 15-17 C for three minutes.
Klaus Pollmeier