Judy Seigel (jseigel@panix.com)
Mon, 18 Jan 1999 15:51:00 -0500 (EST)
On Mon, 18 Jan 1999, Adam Kimball wrote:
> >
> > As for the "curve" of gum, generally speaking it's quite straight. If
> > you're getting the bottom blocked up, or a big "hump," odds are you're
> > either over-exposing,
>
> I'm a little unclear here. Wouldn't over-exposing just push the range of
> steps down (numerically speaking) but not effect the tonal range? Does it
> react nonlinearly to the light source? Or am I malinformed?
No, gum seems to be quite linear under "right" conditions. I understood
the comment was about a number of blocked steps at the bottom. Ideally
there should be NO blocked steps at the bottom. If there are, expose less
& they'll unblock. Or, as noted, they can be caused by too much pigment,
which also blocks up the bottom steps (explained, as I recall, in the same
article in Issue #1, also illustrated in Issue #2, bottom of page 46.)
Tonal range is affected by the length of the SOAK, as also noted, guess
where? The longer soak opens up those bottom steps, but will probably
also, depending on the strength of the emulsion, wash off some of the
highlights, though usually still for a net gain in steps.
Strength of the emulsion relies on many factors, primarily, in my
experience, the ratio of dichromate -- more dichromate makes stronger
emulsion. Also of course, the amount of exposure. If you expose hard,
emulsion gets harder -- but then development will be less delicate. As
with so much in life, it's a matter of checks & balances (except in the US
Congress, of course).
Let me suggest that at this point you should do at least one run of the
"First Step into Gum (Famous Gum-Intro Exercise)" on page 18 of
Post-Factory, #1: expose 3 identically coated strips under step tablets
for the same time, then develop for 1/2 hour, 1 hour, and at least 3 hours
"or up to 24 hours."
This will show the trade offs for your particular mix, paper, size
combination better than I can guess them by e-mail
Of course for an actual print, rather than the step test, you've got to
key the negative to the step print to get those results, as explained
starting on page 33, Issue #1.
> > Try thalo blue, which is quite transparent, about 1/2 gram to 3 cc
> gum, 2 > cc water & 3 cc am di. Well, that's from memory, I'll have to
> check
>
> Why the water? I haven't begun playing with extra water - you mean
> yet another variable... oh no!
Page 15, Issue #1: "You can also add water, up to 30% of the volume of
gum, for easier spreading, less density and less shine in the coat."
I learned this by the way from a student, who got it who knows where --
her mother I like to think. Anyway, in my experience, the business about
thicker gum is better is an error. I nearly always add water if I'm using
only half the emulsion as sensitizer. A higher ratio of gum is hard to
coat nicely, dries tacky before you want it to -- at least when printing
larger than 8x10, which takes longer to coat. The water helps.
cheers,
Judy
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