Re: Gum hardening -- top down?

Katharine,

Although I don't do gum printing any more I have a lot of experience
with it. It was the first alternative process I learned and I have
carried out many comparisons of the type I suggested to test the
validity of top down hardening. I would never had suggested the test
if I had not previously done it myself.

I have made it clear that there are questions about how a gum image
forms that have to so with issues of surface adhesion that can not be
explained by top down hardening, and I don't know how to explain
them. However, the basic nature of single coat gum printing, which
limits either Dmax or tonal range, is a consequence of top down
hardening. It is simply not possible to get high Dmax and a long
tonal range with single coat gum. And I am not talking about the puny
Dmax of 1.1, which might be obtained with a single exposure, but
Dmax of 1.7 or 1.8. In the history of gum no one has every been able
to get a Dmax that high, with a long tonal range, with single coat
gum exposing from the top. And the reason is simple and straight
forward. If you add that much pigment to your emulsion, and expose
from the top, the insoluble gum of the shadows, at the very top of
the emulsion, will simply float away from the soluble gum below.
This is just a fundamental fact of gum printing, amply explained by
theory itself, and I am quite honestly astonished that it has even
become a matter for discussion.

Sandy

>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 09:56:17 2006