Re: Adhesion

Etienne Garbaux wrote:
>
> In my previous post in this thread, I mentioned that gum bichromate, as
> traditionally practiced, leaves unhardened gum between the hardened layers
> and the support, so it is thought (apparently with some reason) that gum
> depends on the physical tooth of a support -- which I described as "having
> support fibers more or less throughout the depth of the coating" -- and
> thus might be expected to work more easily with "woolly" papers than hot
> pressed papers.

Ah, I didn't get this quite fully from your other post, but I see it now
and I like it, as it concurs with something I was thinking yesterday. I
was thinking about Judy's post you're referring to below, but what with
one thing and another I didn't get to the computer to write down what I
thought. But one of the things I thought about a lot was: is there any
reason why the degree to which the coating sinks into paper calls into
question the principles which in my experience govern the hardening of
gum and the affixing of gum to the substrate? Judy seems to believe it
does, but I don't see a necessary path, or even a compelling path, to
this conclusion.

There aren't different rules for how gum hardens and affixes itself to
the surface, depending on the substrate. The gum doesn't know the
difference, it just does its thing: The UV excites a chromium ion,
electrons are transferred between the gum and the chromium, the gum
forms crosslinks with itself and creates a matrix, which renders the gum
insoluble. If there's something on or in (depending on the depth to
which the emulsion sinks) the substrate that can get caught into the
matrix (hairs, fibers, grit, etc) then the gum is held to the
substrate, in my experience. If there's not anything there to get
incorporated into the matrix, then the gum is not held to the substrate.
This is true IME no matter how deep within the substrate the gum has
penetrated.

The one thing I found puzzling in thinking about this: 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
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. 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
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. 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"

> In a post I've misplaced, Judy noted that gum works fine for at least some
> practitioners on smooth, "un-woolly" papers. She also noted that she tends
> to use less viscous gum solutions than some gum printers.

But as I've explained every time this argument has been brought up,
paper that feels or looks very smooth and toothless to us, viewed
through a microscope (and that's how you would have to look at it to get
anywhere close to how the fibers in the paper compare to the size of a
gum molecule) will have lots of fibers not only throughout the paper but
also extending from the surface. 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. If someone doesn't believe
this, then I don't know what to do about that, but the lack of belief
doesn't change the fact that it is so, in somewhat the same way that if
a person steps out of a second-story window, the probability of his
being hurt is the same whether he believes in the principle of gravity
or not.
Katharine
Received on Mon Feb 21 12:15:45 2005