Why multiple exposure (was Re: (Gum) Tonal scale)

From: Yves Gauvreau ^lt;gauvreau-yves@sympatico.ca>
Date: 11/28/05-11:36:35 AM Z
Message-id: <01fd01c5f442$4abff480$0100a8c0@BERTHA>

Hi Katharine and all

My question is most probably very confusing I can agree with that any time.
I assume way to many things.

My understanding of "tonal scale" refers to the shape of the curve on a
graph where the vertical axis is a measure (density most often) of the
response to the relative amount of light received (horizontal axis). The
tonal range would then be the difference between the maximum and minimum
density values.

I made some test of my own and I got about 6 to 8 steps (Stouffer 21 steps)
dependent (as expected) on the gum pigment mix I used. I didn't measure the
densities but I think everyone knows each step correspond to approximately
1/2 stop of exposure which mean I got a response to 3 to 4 stop of light
exposure. If we translate these stop to density (for the negative) then we
have around 1.2 and this is well in the range of required densities to
produce a "standard" silver print. Just in case, I understand that the shape
of the response curve for a gum print is most likely different then those of
silver prints which means we have to use negatives with a "compatible" curve
to do gum prints.

So I guess my real question then is how come all the fuss about multiple
exposure that many seem to favor for gum prints???

I understand one could do multiple exposure to practically change the
response curve "ad lib" and I'm not even thinking of colors here which by
itself opens up an all new dimension to the process.

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, November 28, 2005 11:21 AM
Subject: Re: (Gum) Tonal scale

Hi All,
Yves' question brings out an issue that I think makes discussion
sometimes confusing, that people talk about tonal scale in different
ways. I couldn't understand for the longest time why people were
talking about increasing the scale when what they really meant was that
they wanted to increase DMax, until I realized that what they meant by
"tonal scale" was the difference between the darkest tone and paper
white, which is how Yves seems to be using the term here. What I mean
by tonal scale is the range between the lowest and highest tone in the
print, and the number of steps between them, not the range between the
darkest tone (which may be the only tone in the print) and the white of
the paper. Which is the right way to use this term? It would be nice
if we could agree on a convention for what tonal scale means, so we
could all be sure what we're talking about.

As to the tonal scale of a gum print, I can't help you with specific
numbers because I don't have a densitometer, but I can tell you in
general that the scale you can get, and the DMax you can get, in one
printing or in many printings varies widely and depends a great deal on
the pigment you choose (yellow, while it should be able to give you
distinct steps, as I said the other day, is never going to reach very
far into the darker tones, because it's an inherently light-valued
pigment; and a weak pigment will never give you as deep a tone as a
stronger pigment in the same color range) and on the pigment
concentration you choose to print with, as well as on the dichromate
concentration you choose. As I say somewhere on my website, most of my
darkest prints are also high-contrast and were printed in one printing,
while the very pale subtle prints I was printing 3-4 years ago were
printed in three or four printings, using almost no pigment and
building the subtle tonal gradation with repeated printings. Those
prints had many distinguishable tones but within a very short overall
tonal range.

With the right pigment and concentration, you can achieve a very dark
value in one printing, but you won't get a very long tonal scale (in my
meaning of the term) with that printing, and you won't increase the
range greatly by printing again with the same concentrated mix,
although you might increase the depth of the tone somewhat (you're
right in your supposition that a double printing won't give you twice
the DMax). For depth of tone, you want more pigment and less exposure,
  which decreases the number of steps printed; for length of tonal
scale you want less pigment and slightly more exposure, IME. That's
what I mean when I say that in any printing you can have drama or
sublety, but not both. To me, two printings, one for drama (a few dark
steps to deepen the shadows) and one for subtlety (many steps to fill
in the midtones and highlights) are sufficient to make a fully tonal
print, with saturated ammonium dichromate that is. But there are of
course more ways than mine to achieve a tonal print, and I won't even
claim that mine is the best way; it's just what works for me.

But the best way to find out what happens is to do some multiple
printing with the same pigment mix, then with different concentrations
of the same pigment, holding everything else constant, then holding
the pigment concentration constant and varying the exposure, and see
for yourself what happens.

On Nov 28, 2005, at 6:57 AM, Yves Gauvreau wrote:

> Hi everyone,
> I wonder if someone knows what kind of tonal scale (range) one
> can expect with gum printing when using a certain number of exposure
> for the same print???
> I assume density will build up, thus increasing the range between the
> white of the paper and the darkest areas. I suspect also that these
> multiple exposure are not linearly additive meaning that if I get a
> density of say 1.0 somewhere on the first exposure I wont get 2.0 with
> the next exposure.
> Thanks
> Yves
Received on Mon Nov 28 11:57:29 2005

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