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

From: Katharine Thayer ^lt;>
Date: 11/30/05-10:50:43 AM Z
Message-id: <>

  I've been told privately that my first answer to this post was
incomprehensible, so I'm going to give it one more try before giving it
up as a lost cause.

On Nov 28, 2005, at 9:36 AM, Yves Gauvreau wrote:

> 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.

Here we run into problems already. Because gum doesn't work this way.
For gum, the response to exposure is the proportion of gum that's
crosslinked to form an insoluble colloid matrix. This matrix is
colorless and transparent throughout its range of "densities" so there
is no direct relationship between response to exposure and tonal scale,
as presumed above.

  I'm not doing any more gum instruction, I think, but if I were to
teach an indepth course in gum for serious gum printers, a master
class, one of the things I would have the students do is make gum
prints without any pigment. I don't think I really understood the gum
process until I did that. The mechanism of the gum process is purely
a reaction between the gum and the dichromate and radiation; the
pigment doesn't participate in the reaction and is not an essential
part of the emulsion. I knew that intellectually, but I didn't know it
in my bones until actually seeing it myself.

Since the gum image is colorless and transparent and is difficult to
see unless placed at an angle to the light, pigment is added to the
coating to render the print more visible. The pigment is encased in the
hardened gum like a beetle in amber; the more hardened gum there is on
a particular area, the more pigment is trapped there, and therefore the
deeper the tone. So while the pigment, which constitutes the tonal
scale, isn't directly involved in the response to radiation, it does
indirectly express the relative proportion of hardening by its relative
tonal value. But this is a relative thing, not absolute, and there is
no way to express this in terms of a graph that relates exposure to
tonal scale in the same way you can do with silver, or at least if
there were, I think it would have to be a different graph for every
possible pigment/gum/dichromate mix, which would defeat the purpose of
using a graph in the first place: to simplify the process.

The darkest tone you can get with any particular printing is very close
to the tonal value of the pigment mix that you put into the coating,
(providing you don't add more water or other stuff that would dilute
the tone). The lightest tone is determined by the number of steps
printed and the distance between those steps, which in turn is
influenced by number of factors, as discussed in the gum tonal scale
thread, including but not limited to: the characteristics of the
pigment itself, the dichromate used, the pigment concentration chosen,
the balance between exposure and development, and so forth. So the
answer to the question: what's the tonal scale that can be printed by
gum in one printing? is indeterminate.

The answer to the larger question, what's the tonal scale that can be
printed by gum? is that you can print a very long tonal scale, with
very nice subtle gradations between many steps. But you can't do it in
one printing. The tonal scale that is possible is from the darkest
tone of a dark pigment to the lightest tint of it, (or if you want to
change colors for the highest highlights, a lighter color that would
give you even lighter values in the lightest tints than some darker
colors might) with many many small steps in between the two. But
because gum can only express so many of those tones in one printing,
you have to do more than one printing to get them all, unless you are
willing to live with a print that consists of only a few widely
separated tones to express the range. Again, as I keep saying, there's
no right answer to how to approach this question. I prefer a long scale
with subtle gradations between tones; others may well prefer fewer
tones with more separation between them.

This theoretical statement above is an example of what Judy was talking
about, how coming to gum with rules and theory that apply elsewhere but
don't apply in gum, can just get in your way. It's my own personal
experience and observation that those who come to gum trying to apply
photographic formulas learned in other photographic media tend to be
much less successful at gum printing than those who come without
preconceptions but just approach it with a spirit of exploration--
let's mix up this stuff and see what happens. Judy's right, it's
simple to print gum. What's difficult and contentious is trying to come
up with rules for printing gum, or to understand in depth how the
process works.

Hope that is a clearer answer; if not I throw up my hands.....

> 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" <>
> To: <>
> 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.
> Katharine
> 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 Wed Nov 30 10:53:57 2005

This archive was generated by hypermail 2.1.8 : 12/01/05-02:04:51 PM Z CST