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

From: Katharine Thayer ^lt;[email protected]>
Date: 11/28/05-01:22:14 PM Z
Message-id: <482FE290-6044-11DA-87EA-001124D9AC0A@pacifier.com>

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.

Then we're talking about the same thing after all.

You do assume too much, if you assume that gum proceeds according to
the same rules as silver printing. I notice that you label the vertical
axis tentatively (density "most often") but still I think you are
*thinking* about it as if it must be density, just as with silver
printing. In fact, with gum, the response to exposure is differential
hardening of (colorless and transparent) gum. The gum does not become
darker as it's exposed longer (except that at some point you'll start
getting dichromate stain, but that can be cleared out without affecting
the print in any way, so it's just an artifact of the process, not a
"density" that means anything). The pigment, which forms the picture,
is simply along for the ride and takes no direct part in the process
itself. It also does not become darker with more exposure, as is the
case with silver printing. The reason areas that get less exposure
are lighter in tone is because there is less hardened gum in those
areas, and therefore less pigment. So tangentially, yes, the tonal
range is a function of exposure, but it's subtractive rather than
additive as in silver. At any rate the tonal range is much more a
function of the pigment and the pigment concentration, than of the
exposure per se.

> 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 think you answered your own question just now when you said you
understand that the response curve for a gum print is most likely
different than the response curve for silver.

When the tonal range of the original negative exceeds gum's ability to
reproduce in one printing, printers deal with it in one of two basic
ways. One way is to make a contact negative that is truncated in
range to match the tonal range of the gum mix you're printing with. The
other, which I prefer because most often I *want* all the tones in
the original image, is to do multiple printings to extend gum's tonal
range. One way is not necessarily better than another, they are just
different ways of approaching a problem.
Katharine
Received on Mon Nov 28 14:04:09 2005

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