Re: Dots of gum?

On Sun, 13 Feb 2005, Etienne Garbaux wrote:

> Gum, on the other hand, is developed with its hardened layer away from the
> support and soluble gel next to the support. ...CUT... Gum,
> therefore, depends on the tooth of the support -- having substrate fibers
> more or less throughout the depth of the coating -- so that the hardened
> gel has something to grab onto to prevent being washed away. Thus, it
> tends to work best with rather woolly papers, not as well with hot press,
> and even less well on toothless substrates like glass. ...CUT...

Etienne, I think neither of us is in a position to prove the contention
above... I'm not even sure it could be done. But, for what it's worth,
Mike Ware disagreed with it, and my experience with gum printing leads me
to believe he's right. The theory of the gum layer hardening from the top
and sticking to the paper via the tooth has been accepted, and ILLUSTRATED
with the diagram that Paul Anderson (my hero -- NOT!) drew I think first
for Henney & Dudley, that was then cut & pasted on down through the
canon, hardening directly into "fact".

But Ware believed that the emulsion with the dichromate soaked into the
paper and the dichromate was active right at the paper level for...
reasons of the chemistry of the interaction. I have that somewhere in a
folder of his comments & could possibly/probably dig it out, but it jibed
with so much of my own observation that I believe it. Not to go over it
all again, as it has has been on the list years ago, once, twice, thrice
-- I simply repeat that I have printed gum satisfactorily on paper that
had virtually no tooth at all -- but was *absorbent,* and that sufficed.

To repeat: Ware declared that the wet emulsion was absorbed into the paper
and hardened there -- IN THE PAPER FIBERS. That might be considered
"internal tooth," but still does away with the top-down business.

I'll add also that I print gum with a wet, or perhaps I should call it a
*thin* emulsion -- that is, with a relatively greater portion of water
(including the sensitizer) than of gum -- so that it isn't as viscous as
some folks use, and I assume therefore may soak more into the paper.
(This does, BTW, make the coating much easier, since it doesn't dry and
get sticky before I've smoothed it from one end to the other, meaning I
can get it quite smooth and even --- and does NOT, contrary to popular
belief, make staining more likely.)

My point about the halftone negative is that the exposure would be full
(allowing for various adjacency effects) on every dot, for highlight OR
shadow, not partial as it is in a continuous tone highlight. This (in my
limited experience with halftone negatives) does make it easier to hold
the highlights -- all get (relatively) full exposure, not partial
exposure. And wherever the action in fact does occur, gum emulsion *does*
wash off if it's underexposed.

Obviously the mechanism on an impervious surface like glass or metal is
entirely other... There I believe it's common to have a good undercoating
of hardened gelatin which the dichromated material bonds with one way or
another... And/or other? ( What does Galina do?)

Judy

>
> The advantage, if there is one, of a halftone image is that wherever the
> gum is exposed, it is more or less exposed all the way through to the
> support. Even in the lighter midtones, where a continuous-tone negative
> would leave most of the area with a thick layer of soluble gum between the
> support and the hardened gum, the halftone-exposed gum should produce an
> approximately correct (if grainy) tone. The gum you are washing away isn't
> underneath the gum you want to stay, it's beside it.
>
> On the topic of chemical tooth: chemical tooth is a molecular effect.
> Very simply, good subs are molecules which have one end with an affinity
> for the support and the other with an affinity for whatever you want to
> adhere to the support. This can work excellently, and solves some thorny
> industrial problems. However, because the chemical tooth is only a
> molecule deep, I do not believe it could "reach through" the thickness of a
> gum layer to adhere the hardened gum away from the support if there were
> still soluble gum in between. So it might be a boon in conjunction with
> either of my suggestions above, but probably won't be a viable solution for
> any "support one surface and expose the other one" system.
>
> Best regards,
>
> etienne
>
Received on Mon Feb 14 01:12:45 2005