Re: Gum hardening -- top down?

Katharine,

I hope you wont think I'm persecuting you but from what you just wrote below
one or at least I would be inclined to think you have doubt for both
theories which is very reasonable considering we have no definite proof of
either.

I would just add which will possibly just add to the confusion that any
attempt to prove or disprove any of these theories one would have to
consider all other variables in play especially the pigment(s) color, the
amount of pigment, the thickness of the coating, exposure, etc. But it seems
obvious to me that when considering all these vars in the end if you
underexpose your gum "recipe" using a step tablet such that only the first 2
steps stick on the paper you should see all the other steps float away
during development (right). I assume if you can capture them as intack as
possible it should be possible to observe that some step would be thinner
then the next higher expose step up to the point that harden emulsion is so
thinly crosslinked (most probably from top to bottom) that the tiniest of
turbulence in your tray will distroy these links into so tiny flakes that it
would be impossible to discern them from disolve gum-pigment. Probably
everyone as underexpose gum at one time or another so it should be easy to
realise such an experiment with a relatively thick emulsion at least tick
enough that to expose it all the way to the paper and that suffient
hardening as happen in other step to hold them together in some layer or
large flake if you prefer. If this happen as I expect it would, you should
have your proof. One or two step stay on the paper, a few other steps would
not stick to the paper but would be well anchored with the first one or two
and then you should see that the least exposed step simply disolve totally
and gradually larger and larger pieces stay link with the previous more
exposed steps.

If any crosslinking happens at the paper surface elsewhere then in the first
couple of step the emultion should stick in those areas but I would
disregard them in the case of using a step tablet because of the unusually
long scale and that real negative will rarely if ever have such a long
scale. (Considering possible inversion here)

Regards
Yves

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Thursday, April 06, 2006 9:53 AM
Subject: Re: Gum hardening -- top down?

> Etienne,
> A couple of general comments first, then a couple of specific
> comments embedded below:
>
> I'm not sure why everyone is beating me over the head with top-down
> theory. I "get" top-down theory. And if it makes y'all feel better,
> I'll say that top-down has always made more sense to me than, say,
> the paper-attracts-dichromate theory. I'm just saying there's very
> little evidence either way, and also there is this observation of the
> tonal inversion thing that makes top-down theory problematic for gum,
> which problem none of the advocates for this theory have addressed in
> this discussion. I've been accused of being a blind advocate of top-
> down theory because I had some questions about the dichromate-
> attracts-paper theory, and now it seems I'm being called an idiot
> because I say that there are observations that make top-down theory
> problematic for gum.
>
> BTW, I did a side-by-side comparison yesterday, coating two pieces of
> the same thin paper with the same well-pigmented gum mix, and
> exposing one from the front and one through the back (this is what
> Sandy should do, rather than just exposing one through the back and
> congratulating himself on supposing he's gotten a better print than
> he would have by the usual method). Unfortunately I inadvertently
> overexposed the back-exposed one and had to discard it before it was
> fully developed, since I eventually needed the sink for something
> else. But even overexposed, the back-exposed print had less DMax than
> the front-exposed print. At any rate, it wasn't a fair comparison and
> I'll try to do it again sometime, but I didn't see anything in this
> initial attempt to make my socks roll up and down.
>
>
> On Apr 5, 2006, at 10:03 PM, etienne garbaux wrote:
>
> > Katharine wrote:
> >
> >
> >> AFAIK, it has hardly been "established" that gum hardening occurs at
> >> the paper surface, as Judy suggested, but it's also interesting that
> >> Sandy is so certain that hardening occurs from top down that he
> >> claims he will remain certain, no matter what proof is offered. :--)
> >> I myself am waiting for further evidence to illuminate the issue.
> >>
> >> In the past, the "proof" that has been offered here for hardening at
> >> the paper surface is no proof at all but Mike Ware's speculation that
> >> the dichromate may be strongly absorbed to the paper; he thinks it
> >> may move down through the layer and congregate at the paper
> >> surface, and if this is so, that is where hardening would take
> >> place, because that is where the bulk of the dichromate would be
> >> found. But it's just a hypothesis, and he has offered no evidence
> >> that I know of to support this hypothesis.
> >>
> >
> >
> > We are quite certain that in dichromated gelatin, the dichromate
> > concentration is approximately erqual throughout the thickness of the
> > coating. If the paper substrate really does leach the dichromate
> > out of
> > gum, this could at least partly explain greater hardening at depth
> > than at
> > the exposure surface. This suggests that pretreating paper with
> > dichromate
> > before coating with gum could enhance the effect.
>
> This is one of Ware's arguments for his theory: Demachy's claim that
> if the paper were treated separately, first with dichromate, then
> with pigment/gum, that this two-part emulsion was 4-5X faster than a
> mixed emulsion. But I have tried and tried to replicate that result
> and it doesn't work for me; in fact for me the two-part emulsion is
> slower than the mixed emulsion. I asked here for other experiences
> with this a year or two ago and the few people who replied reported
> varying results; the one thing that seemed consistent, as I recall,
> was that no one reported that the two-step emulsion was faster. So
> I'm not sure what to do with that, but at the very best I can't
> consider it a reliably replicable finding, and I suspect that the
> reason it never caught on after it was published in 1898 is that it
> didn't work for everyone.
>
> >
> > I am more inclined to think that the effect is mostly macro and
> > mechanical.
> > To wit: relatively less-hardened gum can avoid being washed away
> > when it is
> > "reinforced" with the matrix of paper fibers lower in the layer
> > than toward
> > the top of the layer where such fibers are fewer or absent
> > entirely. Think
> > of washing paint out of a brush. At the ends of the bristles,
> > where they
> > are free to move, it is relatively easy to wash out the paint. But
> > down at
> > the root, where they form a dense matrix, it is much more difficult
> > -- not
> > because the paint is more hardened there (in fact, because air cannot
> > penetrate, it is probably significantly *less* hardened there), but
> > because
> > it is harder to wash out even unhardened paint from the dense matrix.
> >
>
> Mmm, no, I don't buy this one. Gum is extremely soluble in water,
> unlike paint, and if it's not hardened, it washes out of paper (and
> brushes) very easily. No, in the tonal inversion phenomenon, the
> hardened gum that's left in the paper even after a minimal exposure
> that's way insufficient to harden the gum layer as a whole
> (suggesting that there must be hardening at the bottom of the layer
> from the very beginning) is very definitely hardened gum. You can
> tell because where there's less exposure (where there is less or no
> hardened gum) the paper stains (remember, we're talking about tonal
> inversion, which IME is a type of pigment stain). Yes, I'd say that
> the hardened gum at the bottom of the layer adheres because it
> attaches to the paper fibers, but that's not quite the same as
> saying that unhardened gum gets gummed up in the paper fibers, which
> is not what I think is happening here.
>
> To say that there is apparent hardening at the bottom of the layer
> from the very beginning is not the same as saying that hardening
> proceeds from the bottom up. I don't see any reason to suppose that
> would be the case, and I don't think anyone is arguing that. I'm
> just saying that my observation of the tonal inversion thing has
> convinced me that there is some hardening at the bottom of the layer
> from the beginning of the exposure, which is the first thing that has
> made me question whether top-down theory is right for gum.
> Katharine
Received on Thu Apr 6 09:49:02 2006