Re: Gum contrast - not that it hasn't been beaten to death

<Katharine said> Hi Chris and all,
> I think the issue raised here is a slightly different issue. I agree
> wholeheartedly with the idea that it is very difficult to print gum on a
> slick smooth surface; in fact that's why when someone suggested printing
> gum on vellum, I had to try it for myself because the material seemed
> just too slick to me; I really didn't think it would hold the gum. I was
> wrong; the 100% rag composition apparently offers enough fibers at the
> surface for the gum to hang onto, in spite of the apparent slickness.

Hi all,
     (I hope I write clearly here...I"m pooped) Agreed: vellum printed fine
for me, it was not too slick to hold the gum. However, the vellum absorbed
less quickly than the gampi, and it seems that the more absorbent the paper
the less contrast, in my experience; sizing, for instance, enables the gum
stuff to sit on top of the paper more. BTW, I print on hot pressed paper,
and Rives BFK, generally. Don't use cold pressed for gum.
> But the issue here is tooth, not contrast. While I can't speak for
> Anderson, my own experience is that hard slick surfaces won't print
> because they won't accept the coating, and the way I've always read the
> assertions about very hard surfaces not printing is not that they will
> print with high contrast but that they won't print at all; the coating
> just schlumps off in the water.

     Anderson, et al, didn't say the layer washed off, but that it produced
no tonal range. In other words, you got solid black.
     Contrast (too great), as I have always understood it, is the highlights
don't print in with detail enough before the shadows block up. In other
words, I look at both the highlights and the shadows to see if there is
detail there, and if so, the contrast is correct. Is it possible, I wonder,
that contrast in gum is *related* to tooth? Does the depth of the hardened
layer produce the darks, and does the ability of a paper to produce more
varying depths produce better tonal range in the darks? Low contrast in gum
(related to exposure, dichromate increase, flat neg, too little pigment, etc
etc whatever)--can it be related to absorbency as well? I suppose the best
way to test this would be the proverbial step wedge.

> The idea that hollows in the surface are required to accept different
> depths of hardened gum, therefore smoother papers such as hot press or
> vellum

      Unfortunately, the only author that specified a substrate was Anderson
(glass) but no one actually mentioned hot press or vellum back then...I'm
not sure what was considered "too smooth a surface" in the early part of the
century. What could be smoother than hot press, I wonder? Back then,
certainly not mylar. I can't imagine they were talking about any paper
smoother than hot press...but that's an assumption.

> won't print a full tonal range because they don't have deep
> enough hollows, is logical in a theoretical sense; the only problem
> with it is that it utterly fails to explain my own observation that I
> can print more tones and subtler tonal gradations on a smoother paper
> than on a less smooth paper, as well as the fact that many gum printers
> print fully tonal images on hot press paper.

     Agreed; I print on hot press fine, and the detail in the tones is fine.
However, in my practice I observe a *difference* in contrast between softer
and harder papers, not that hard papers are unprintable.

> But on reflection I can't even make the theory make sense to me. If the
> different tones are created by different depths of hardened gum, with a
> shallower layer of hardened gum making a lighter tone and a deeper layer
> of hardened gum making a darker tone, then a surface that has no deep
> pockets would print only lighter tones and no dark tones, in other words
> less contrast rather than more contrast. Which is more consistent with
> my own observations.
>
> But here's what I actually think: not owning a high-power microscope I
> can't prove this, but I'd be willing to wager a small amount of money,
> or better, a beer at some future APIS, that a close cross-sectional look
> at hardened gum on any of the papers that we use for gum printing,
> including vellum, (and maybe including surfaces such as etched glass or
> sanded metal as well) would reveal that they all have deep enough
> valleys to hold enough gum to print the entire tonal range. While a
> surface may look very smooth to us, it looks like mountains and canyons
> to the molecules involved.
> Katharine Thayer
>
     This certainly may be true, and perhaps the silhouette effect noticed
by the authors was a) a figment of their imagination b) a myth repeated from
plagiarist to plagiarist (a most likely case scenario) or c) not due to what
they supposed. I would agree that at the molecular level there certainly
must be enough hills and valleys to hold gum, but how much depth of gum may
be the question. And tooth, and absorbency, I realize, are two different
things, altho they may relate in their result in some way (contrast,
exposure).
Chris
Received on Wed Jan 7 22:46:04 2004