Re: Determining SPT with gum Was: Gums a la Demachy and Puyo

>
> On Jul 12, 2006, at 7:34 AM, Christina Z. Anderson wrote:
>
>
>>
>> BTW, to all: How do you determine your standard printing time with
>> gum?
>>
>> Loris,
>> If you go to:
>> http://www.czaphotography.com/show.php?what=learning&which=1
>>
>> on my website and scroll down a tad, you will see images related
>> to this question
>>
>> David is totally right in that gum is not like the others in
>> choosing SPT, because if you think about it, when exposing, let's
>> say, pt/pd, you look for max black in Step 1, but the paper
>> absorbs the chemistry, it doesn't get thicker like gum does. So
>> what I did was quite arbitrary, and was to choose a time that
>> allowed complete development in 1 hour just letting the print sit
>> there, but also produced a nice punchy colored layer. The layer
>> is stable, does not whoosh off, and allows spray development if I
>> want to shorten the development time to 1/2 hour (spraying after a
>> 5 or 10 minute soak). Once I chose this arbitrary time (UVBL 6mn,
>> under 15 watt bulbs) then I printed my 101 step palette and
>> derived my curve from this.
>>
>
>
> Chris, what you're saying here, as well as the images themselves,
> seems to support what gum experts have been recommending for
> decades, to determine the exposure time for gum not by some
> complicated process but simply to use a step tablet to establish
> the time required to give the deepest tone possible for that
> particular emulsion, as well as a stable gum layer.
>

In fact, I don't see how it would be possible to get the deepest
tone possible with a given gum emulsion without a fully-hardened gum
layer; the two are inextricably linked. If the gum layer is hardened
throughout, then by definition you'll have the deepest color possible
with that particular emulsion. And conversely, when you've
established that you've printed the deepest tone possible, then you
know that the gum layer is fully hardened. This is the principle
that makes the step tablet a good quick means to estimate exposure
times, even if it's only an estimate.

The time it takes to harden the gum layer is of course widely
variable, depending on possibly dozens of different variables. The
other day when I didn't have a printing platform set up, I was
printing test prints by holding them firmly against a west-facing
window pane with the sun shining directly on them. I guessed at a 15-
second exposure, which gave a good print but required an extra-long
development, so probably 10 seconds would have been better. Later,
when the sun was dimmed by a thin cloud, I upped the exposure to 30
seconds, which wasn't quite enough; the color wasn't as deep as the
other one with the same emulsion. And the next day I rigged up a
temporary printing platform for my photoflood and guessed at an
exposure time of 3 minutes to start, which worked well. The print
from the 15-second exposure in the sun the day before was
indistinguishable in DMax and scale from the 3-minute exposure under
photoflood the next day; in other words I think I can assume that the
layer was hardened equally fully in the two different conditions.
But the humidity was low-ish that day, and the following day, when
it was 93%, 2 minutes worked better to produce the same result.
So what would be my standard printing time? To me, the idea of a
standard printing time doesn't make any sense; the printing time
varies depending on the pigment load, the RH, the light source of
course, and lots of other things.

Several months ago, I was trying to make the point, but couldn't seem
to make it clearly enough to be understood even after repeated
clarifications, that the only things that matter to the chemical
process underlying gum printing are the gum and the dichromate. I
was speaking specifically and exclusively of the chemical reactions
that result in hardened gum, not in terms of the printing process as
a whole. So while I was saying that the pigment isn't a factor in
the photochemical reactions that produce the hardening of the gum, I
was also agreeing with critics that the pigment is definitely a
factor in the amount of time it takes to harden the gum. Perhaps a
subtle distinction difficult to convey, I don't know. But to settle
on a "standard printing time" for all gum layers is to make the
argument that I wasn't making then and don't make now, that the
pigment and pigment load don't make any difference to printing time;
only the gum and dichromate are a factor in hardening of the gum, so
the exposure time required to harden the gum is always the same
regardless of pigment load and other variables.

I personally think that would be a hard argument to make, but the
thing about gum is that there's lots of different ways to get to the
same result. David was saying yesterday that his RH has gone up (I
hope his power isn't still off) but he was dealing with it by holding
the exposure constant and just developing longer. I deal with an
increase in RH by shortening exposure. Either works. The point
being that there are many roads to the kingdom, and maybe ultimately
there's no difference in result between holding the exposure
constant and making a different curve for every possible pigment and
pigment concentration, and holding the curve constant and exposing
differently for each emulsion. I don't know that I'd ever be willing
to take the time to test that, but maybe someone will, sometime. In
the meantime, it's a heck of a lot easier to take the second route; I
haven't yet seen a lot of difference in the prints between the two
methods, and so ... each to his own.
Katharine
Received on 07/13/06-12:43:10 PM Z