Re: Could someone summarize that gum up or down discussion?

from M. Ware:

> Observations seem to suggest that dichromate ions (actually the photoactive
> ionic Cr(VI) species is likely to be [HCrO4]- ) are quite strongly absorbed
> onto cellulose (hydrogen-bonding would be a possible mechanism). Depending
> on rates of ionic diffusion through the liquid phase, probably controlled by
> its viscosity, this absorption will tend to set up a concentration gradient
> of the Cr(VI) species, increasing with depth, to replace the initially
> uniform distribution.

Etc. I haven't been able, due to restrictions of both time and mental
capacity, to fully follow the argument of top or bottom down hardening,
tho I think I get the gist or maybe just the possibilities of the above.

But I will add (or repeat) some practical observations.

First, however, as a devoted reader of the NY Times Science Section I have
come to understand that the first principal of secular science (that is,
learning by other than direct revelation) is that it's a long long road,
if ever, before all wrinkles are understood, & hardly a week goes by that
some sacred principle isn't overturned, or at least modified in a journal
with a title like, Coronary Thrombosis in Aging Fat People of the Pacific
Northwest, among an astonishing number & variety of specialty journals
followed by Times reporters.

Here are some observations of my own that militate (IMO of course) against
the GottaBeTopDown theory....

First is watching actual development, seeing top layers of tone wash off,
while bottom layers remain set. Nor do I understand how we could have any
delicate graduated highlights in continuous tone negatives at all if
hardening was only from the top.

Someone wrote a description of photon behavior I am too pressed to
pursue, besides which we have no photons in New York City, they've all
been destroyed by pollution from New Jersey. But since "dark hardening"
apparently occurs in all dichromate processes where, I would suppose,
photons are absent or hardly robust, that's again subject to
qualification. Do we know the cause of dark hardening?

As for explaining one-coat "full range" prints by, for instance Demachy,
who did what we would indeed consider full range, gentle abrasion by water
generally cleared highlight and midtone detail.

As for scale of gum, I imagine it was Sandy saying the scale of gum
bichromate is too short for (I think it was) a normal 1-coat print, so I
repeat a point from my 1-coat-gum article: If you EXPOSE ENOUGH, unless
it's absolutely bullet-proof, and then DEVELOP LONG ENOUGH, the highlights
will start to peel back. Whether that hardening came from top or bottom is
not this explanation, but if it "develops" from the top, I would assume
that means it had to be hardened from the bottom -- no? (And that would
apply to midtones & shadows, too, no?)

I've been meaning anyway to say that if I said specifically that hardening
could only be entirely "bottom up" that may have been too sweeping. My
feeling, however, is that major action takes place at and/or in the paper
and my observations make "strictly" bottom-down doubtful.

As for why gum printers switched to carbon printing, if one reads through
the Dictionary of Photography year by year, the story suggests itself.
That's NOT uppermost in what remains of my mind now supposedly elsewhere,
but one explanation (among many) is that carbon tissue became available
commercially. But, as noted, the dictionary makes other points which some
day if I have time to re-read P-F #8 on Fresson, I may recall (I do seem
to recall, BTW, that they ultimately began calling any direct carbon
"fresson" -- which for a long time was sold as a sheet you could print
yourself.

It may be, again, failing memory, but I don't remember gum printers saying
gum couldn't do gradation as has been repeated here.... Who said that?
When? Anyone besides, for instance Photographers Formulary selling kits?
I know gum went into a period of corruption when the old techniques were
forgotten, but much of that (to put not too fine a point on it) can be
laid at the foot of the grand -- OK, you knew this was coming -- Paul
Anderson "stain" test, which caused so many unnecessarily weak multiple
coats that made the prints look pastel and re-register problems made them
look soft.

Finally, before I tear myself away from this digression, IME printing
with a digital or pixilated negative is QUITE different from a continuous
tone negative. The dot neg makes tone range by size and spacing of the
dots, EACH ONE OF WHICH CAN BE FIRMLY ANCHORED -- a very different
proposition from contone which has partially exposed highlights ready to
float away...

Anway, IMO, whatever extra benefits may come from this experimenting, and
at worst it sounds like fun, beware of premature "explanation."

Judy

> > This implies that the gumbi emulsion layer, when exposed, is no longer
> homogeneous, and probably has a higher concentration of the photoactive
> Cr(VI) species just near the paper surface - a distribution which will tend
> to work contrary to the 'top-down hardening' phenomenon. The Beer-Lambert
> Law no longer strictly applies in this system, because of this concentration
> gradient. It's probable that relatively more light is absorbed at the paper
> surface in consequence, and therefore relatively more hardening goes on down
> there than would be predicted by a homogeneous 'top down' model.
>
> This is why I think a comparison with the method described in Maskell &
> Demachy's postscript is quite interesting.
>
> All I'm offering is a physico-chemical reason/mechanism for questioning the
> assumption that gumbi prints harden 'top-down' just like carbon prints do.
> I've no interest in prolonging the dialogue, it's all speculation anyway,
> until someone performs some real science, like electron microprobe analysis
> on transverse sections."
>
> If anyone can interpret this commentary as advocating "bottom-up hardening",
> then I'd be fascinated to hear from them.
Received on Sat Apr 15 20:08:02 2006