Re: UV Sources

Keith Schreiber (KEITH@ccp.arizona.edu)
Tue, 28 Nov 1995 13:53:21 -0700 (MST)

> there are two types of black-light tubes, coated and uncoated. the
> coated tubes are more than twice as slow as the uncoated, and cost almost
> twice as much. my own bank of florescent tubes is way faster than a bank
> of cool whites, i use the uncoateds. See my article "the platinotype" in
> the Jan. 94 issue of View Camera magazine.
> I would suggest you try my hydrogen peroxide method of contrast
> control as outlined in the July 94 issue of the same magazine, as it
> offers advantages over all other contrast control methods.
>
I have 2 fluorescent exposure units. One uses 10 24 inch AQA bulbs,
the other uses 8 48 inch BL bulbs. The BLB bulbs, which you refer to
as coated, are useless for our purposes. I suppose I should have
tried the cool whites myself before passing on such a silly notion.
Please accept my sincere apology.

Each of the contrast control methods has certain advantages and
disadvantages. How these weigh against each other may depend on
external factors such as climate (i.e. temperature and humidity),
consistency of negative density range, condition of ferric oxalate,
and even personal esthetic choices. No one method is superior to the
others in all cases and conditions. The method that I usually use
is to add 1 drop of hydrogen peroxide per ml of coating mixture (I
use syringes to measure ferric oxalate and pd/pt solutions). I am not
a chemist but I think that what this does is to restore any of the
ferric oxalate that had become ferrous to the ferric state. Ideally
this should not be necessary. I have found that more than this does
not further increase contrast. Contrast is further controlled by various
quantities of sodium dichromate added to the potassium oxalate
developer. See Dick Arentz's book or David Fokos' WWW article for
further details on this procedure.
Keith Schreiber
Center for Creative Photography
University of Arizona
Tucson, Arizona 85721
520-621-7968
520-621-9444 FAX
keith@ccp.arizona.edu