Adam Kimball (akimball@finebrand.com)
Thu, 21 Jan 1999 00:21:12 -0800
I've been thinking about how I'm going to come up with a working curve for gum
printing. This is my current idea- actually my interpretation of an article by
Charles Palmer (on the Bostick and Sullivan page) which I don't totally
understand, so humor me ;)
First, make a 100 step wedge in photoshop using opacity. Print this out and wax
it. Next, print this wedge alongside a Stouffer tablet on a particular
emulsion. Say I get five steps of good tone on the Stouffer tablet - I make
reflection readings of these steps, and find the five corresponding blocks on
the digital UberWedge{tm} which produced the same tones. Now, I know how
photoshop opacity correlates to continuous tone opacity. So, shouldn't I be
able to use this data to plot a curve for photoshop? I.E. if stouffer 20%
opacity prints X, and photoshop 10% opacity prints X, I can go into the transfer
table and set the 20% value to 10. Correct or complete stupidity?
Obviously, I am a complete beginner to digital negatives, but I'm not a complete
beginner in photoshop and have some, be it small, knowledge of sensitometry.
Any advice or comments would be appreciated,
Adam
Judy Seigel wrote:
> 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
This archive was generated by hypermail 2.0b3 on Sat Nov 06 1999 - 10:06:43