U of S | Mailing List Archive | alt-photo-process-l | Re: Exposure times in tricolor gum

Re: Exposure times in tricolor gum

I haven't followed this entire discussion, but if the question is
about differential exposure times and curves for tricolor gum
layers, my informal experience from years of printing tricolor gum
in an intuitive rather than calibrated fashion was that the exposure
times were essentially the same for all layers, and my observations
from a recent year or more spent carefully studying the calibration
of digital negatives for gum have yielded the same conclusion, that
there's not that much difference between them. When determining
exposure times very precisely for printing PY97, PV19 and Prussian
blue on Arches bright white, gelatin-glyoxal, the exposure times were
Prussian, 3:00; PV19, 2:45; and PY97, 2:15. In other words, not that
much difference, but the yellow was shortest and the cyan longest,
not the other way around, and that's always been my experience; when
there's a difference it tends in that direction.

If anyone is interested in a somewhat longer explication of my
thoughts and observations on the subject:


The most recent tricolor print I've done is the test print I made on
masa which I showed here a few weeks ago; my purpose in the testing
was just to try to understand the requirements and peculariaties of
printing on the masa paper; I didn't care too much about the tricolor
print itself, so I just used the same exposure time for all three
without bothering to determine them separately, and it seems to have
worked just fine. (And by the way, I've decided that masa just isn't
worth my time; even though I finally got the hang of it I just don't
like the paper


As for curves, the curves I've calculated for the different layers
are so similar that they could easily be superimposed on each other;
they are all very deep, including the yellow. During the time I was
studying calibration for gum, I generated dozens of curves for all
different colors and pigment concentrations; I found differences in
curves as a result of differences in saturation of the pigment
(note, this isn't the same as pigment concentration in any absolute
sense); the less deep the pigment mix, the shallower the curve, and
vice versa, but not differences in curves simply as a result of their
hue range. And of course exposure times vary similarly; a deeper
pigment mix will take longer and a lighter pigment mix will take less
time, but this is true across hue ranges rather than sorting out
differentially for different hue ranges.

Since I have the separate curves, I use them when printing tricolor,
but when I've used the same curve for all, it doesn't make any
difference in the final print, which makes sense given how similar
the curves are.


On Nov 18, 2008, at 5:24 AM, Henry Rattle wrote:

Yes, the Y curve is quite a lot closer to the 45 degree line - less
than the cyan and magenta curves, which are close to identical.

Afraid I mix the pigment by eye - small blob, larger blob... So
can't be of
much help there.

Best wishes


On 18/11/08 12:50, "Loris Medici" <mail@loris.medici.name> wrote:

Thanks Henry,

By "...Y is much flatter..." you mean that it's closer to the
ideal 45
degrees linear transfer function or it's more drastic? I'm currently
printing using a single curve devised from M tests.

What can you say about the pigment concentrations you use?


18 Kasım 2008, Salı, 2:43 pm tarihinde, Henry Rattle yazmış:

Hi Loris,

My exposure times (50% gum/pigment, 50% saturated Potassium
7 minutes for C and M, 10 minutes for Y. These times are "standard
times" from precision digital negatives (PDN) testing, all
exposed through
R255B60 colorised negatives, with separate curves generated for each
(The C and M curves are very similar to each other, the Y is much

Best wishes


On 18/11/08 12:20, "Loris Medici" <mail@loris.medici.name> wrote:

This goes mainly to tricolor gum printers printing from digital
separately calibrated for each color layer: do you experience any
time variation? If yes, can you please tell me your exposure
times (and
dichromate strenght, only if it isn't kept constant) for each
layer? I
just want to see if there's a correlation / connection...

I balanced the pigments according to 2Y + 1M + 1C. In other
words, my Y
stock paint:gum solution contains 2x paint compared to both M and C
solutions. Using the same coating solution formulation (which is
1 part
paint:gum solution + 1 part gum solution + 2 parts 10% ammonium
solution), I find that yellow requires the most exposure whereas C
requires the less (M in between). Is this similar in your case?

Thanks in advance,