Re: Adhesion

On Sat, 19 Feb 2005, Katharine Thayer wrote:
> ...if you follow
> this line of thinking to its logical conclusion, then it seems that if
> the emulsion is sunk completely into the paper, such as when printing on

Why does the emulsion have to have sunk "completely into the paper"? Or
maybe that should be, what is "completely"? For all we know the top part
of even the shadow tones could still wash off, at least some of the time.

But these events are rarely bound by one factor. I'm merely saying that
the notion that gum printing has an intermediate unexposed layer, not
attached to the paper by exposure, but clinging to a tooth, is an unearned
extrapolation from carbon printing... unnecessary, unproved, and -- from
my observation -- unlikely. It was observed that, UNLIKE carbon printing,
a gum print did NOT require reversal. As often happens, an "explanation"
was cobbled up -- one that needs a lot of twisting and even then is weak.

> unsized paper, then almost any exposure would affix the hardened gum to
> the paper, because there's no part of the coating that's not in contact
> with paper fibers, so the coating should be affixed to the substrate
> even if only the very top part of the coating were hardened, in other
> words the gum should be affixed to the paper by much less exposure than
> would be required were the paper sized and the coating sitting higher on
> the substrate.

In the 1st place, why "less exposure"? There's always ... oh damn I
forget the term... the initiation period, as there is with all exposure
media, not to mention most other reactions.

Secondly, there is *differential hardening* according to the degree of
light. As, for instance, you could get a light suntan or a heavy suntan
(or for that matter a dark blue cyano or light blue cyano) according to
the degree of exposure.

But third,

. ... But in my considerable experience printing on unsized
> papers, compared with my lesser but not insignificant experience
> printing on sized papers, I've not noticed either of these effects. To
> wit: neither do I need to expose significantly longer on sized paper

(An aside here -- what is *significantly*? Even less than *significantly*
would spoil the theory.)

> than on unsized paper for the same printing effect, nor have I ever
> found that underexposed prints will stick on unsized paper; they wash
> off just as easily when the emulsion is soaked way into the paper than
> when the emulsion is sitting on top of a sizing, or at least relatively
> more on the surface.

My experience is that such readings need a precision tool to be meaningful
-- it is extremely difficult (at least I find it so) to make such
distinctions with a picture negative, which for these purposes can only be
called RANDOM DENSITY. My tests with a 21-step showed a distinct
difference. For Katharine's purposes the question may be moot -- but that
doesn't prove or disprove the thesis.

The tests I made with several kinds and %s and hardeners of gelatin size,
showed that gelatin sized paper had a shorter scale, that is, it LOST TOP
TONES, sometimes as much as 2 full steps, compared with unsized paper.

> The only thing that would explain this is what you
> say above, that the coating needs to have, using your excellent
> phrasing, "support fibers more or less throughout the depth of the
> coating"

> ... To a gum molecule, even a plate surface
> Bristol or a smooth vellum has gobs of tooth for crosslinked gum to
> attach to, even though our fingertips or the resolution of our retinas
> aren't fine enough to recognize this tooth.

I think here we come to a question of semantics... Katharine (and others)
may call it tooth, I call it paper fiber. I'd mention absorbency and let
it go at that.

But this entire discussion makes me glad I learned to print gum before I
got on this list... we make it sound INFINITELY more complicated than it
really is; no wonder folks are intimidated. Of course we're dealing here
with an attempt (usually doomed) to undo the errors of the past... But my
experience is that theories can be irrelevant. To not-yet gummists: just
do it and *pay attention.*

Judy
Received on Mon Feb 21 16:22:41 2005