Off hand, I can't think of a source that does more than make a passing
reference to this. I'll bet that someone on this list knows of one.
It is pretty much gospel though, that dichromated colloid emulsions
behave this way. My experience with them supports this (though I don't
necessarily consider that conclusive :-) to the point that it never occured
to me to even try flashing to control contrast. That's why it would
be interesting to hear about the results of a controlled test.
>
> Assuming it's true, surely it would apply to carbon and related transfer
> techniques more than gum.
Why? The only reasons I can think of are that there are many more
variables with gum that might simulate the appearence of contrast
changes. As others have mentioned, the gum (or other colloid), pigment
mixture, the nature of the pigment itself, thickness of the emulsion
layer you brush on, paper, etc. To this I could add humidity
variables. Where I first learned gum printing in SE Ohio, summer time
was a period of amazing humidity and it did affect the images, first by
changing the drying time, and second once the emulsion was dry you
could see the effect of even 15 minutes of additional exposure to a
high humidity. (Two pieces of paper, coated at the same time and
exposed sequentually to the same negative for the same 15 minute
exposure, came out differently.) Some of this isn't unique to gum, but
my point is that unless you have very tight controls over the
variables, it is possible to interpret one or a combination of these
effects as something other than what it is.
>
> I don't have a reflection densitometer, but the scale of gum
> is so short it would be hard to plot, the whole range being little more
> than a shadow step, a midtone or two and a highlight -- not exactly a
> grand sweep. However, I have folders full of gum
> tests printed under a 21-step guide, most of which show definite
> shouldering in the dark tones, that is, anywhere from one to
> four teeny tiny baby shadow steps -- discernible, but not useful.
> (Cyanotype also shows definite shouldering, quite treacherous in
> fact because of the humongous dry-down.)
>
> As for the "toe," my sense in both cyanotype and gum is that the
> highlights seem more a factor of the mechanics of printing than inherent
> sensitometric law -- variables (in addition to the negative of course)
> including paper, age and condition of emulsion, and, in gum, time
> and type of development. With both media, some papers are like dropping
> off a cliff at the high tones, others trail off daintily, delicately.
>
> As for the particular subject of flashing -- I have
> had EXCELLENT results with PRECISELY TIMED flashing of cyanotype.
> The two too-flashed gum prints I mentioned, while NG as prints, allow
> me to hope that on
> that paper at least (a rag-content typing paper, BTW), flash will work.
I wouldn't expect that what works for Cyanotypes would work equally well
(or even at all) for gum as the emulsion is iron based, not
chrome/colloid.
>
> I also note that my utmost serious attempts to lower contrast with
> lith film (silver!) by flashing failed dismally. The flash changed everything
> about the same. Slope didn't change.
Flashing is the standard technique for controlling contrast (dot size)
when making halftones with litho film. Flashing, and its opposite, dot
etching, is (was?) both an art form and a science among people making
halftones. I imagine that they are becoming a lost art nowadays with digital
manipulation.
If you are trying to flash non-screened litho film that you are developing
in litho A&B developer, I would imagine you aren't seeing much result
as the transition between clear film and black is too abrupt for such
a subtle technique to have any effect on the contrast.
I don't have enough experience with devloping litho film in less contrasty
developers than A&B to know how it might (or not) work, but remember that
this isn't how litho film was designed to be processed. If you are making
a material designed for one thing do another, that is a fairly extreme
manupulation and you might not see much happen if you add a fairly
delicate technique like flashing to an already "stressed" material.
But, I'm only speculating here.
>
> In any event, I appreciate the comment, and this discussion does
> remind me to also try a flash through the back!
You might also try reflex printing. I've only read about it (and can't
remember where - sigh...) Does any wise soul on the list have any
more information? I believe this technique also had some effect on contrast
(at least with silver materials).
Carson Graves
carson@ileaf.com