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 07:54:22 2006