FotoDave@aol.com
Tue, 02 Nov 1999 22:13:29 -0500 (EST)
> Most such photographic information tends to be anecdotal in nature. How
> does one go about measuring the effect of hardening in a gelatin thin
> film? Has anyone here actually done it?
It might be difficult to measure it quantitatively since most photographers
don't have the scientific equipments to do that. They'd rather spend their
money on equipments that helps them to create their art works. :)
However, they are many evidences that show the importance of the hardening.
The fact that Judy saw that sizing with unhardened gelatin give worse
staining (and I have the same experience), that Luis wrote to use more
hardened sizing for single transfer and less hardened sizing for double
transfer, that Pollmeier suggested adding some alum to partially harden the
gelatin coat for oil prints all show that the hardening effect is real.
> However, I think that for
> gum printing the primary reason for hardening the (optional) gelatin size
> is to make it less water soluble. You don't want the size to wash out of
> the paper while developing the print since that would take at least some
> of the image with it.
Well, not exactly. I think the function of hardening the size have been
misunderstood by many. At least in gum printing, the size don't really wash
out because the gelatin is thin and grasps the tooth of the paper and because
gelatin is not soluble in cold water where the gum prints are developed. If
the gelatin washes out, then not just some of the image will wash out, but
the whole image would frill out since the gelatin coat is at the bottom.
The real reason for hardening is, if you use an unhardened gelatin, then when
you coat your gum emulsion, since it is a water-based emulsion, it basically
soak and swell your gelatin just like when the gelatin is prepared. This will
soften the gelatin and so the gelatin will bond with the gum because gum and
gelatin has similar characteristic. (The effect is just like when people make
colorful jello with designs!) Because of this bonding, the gum doesn't
completely wash out during development and hence the stain.
But not all people have seen the problem because it depends on how hard your
gelatin is and how liquid your gum is. You noticed (or can find from the
archive) that some people said they used harder gelatin and don't need to
harden their size. That's because the harder gelatin takes longer to swell
up, but before it swells, the gum coat already sets and dry, and there is no
chance for that bonding.
This, by the way, is also the reason why Judy sees the "solarized effect"
with gum (see the journal, issue #3). I have wanted to get to this for a
while but kept forgetting but maybe now it is an appropriate time. She saw
that in a step-tablet exposure, after the "normal" exposure region, there was
a few steps which clears nicely, but after that there started to be staining.
Mike Ware's explains with the concept of viscocity caused by exposure, but
this explanation doesn't explain the phenomena well (at least not completely)
because if it was the low viscosity that stains the paper, it should have
stained it while it is being coated regardless of exposure.
What is actually happening, I believe, is the bonding and staining effect
that I described above. However, in the area where there is a little
exposure, the top layer is hardened a little (imagine a crust over the
progressively softer gum below it). Now during development, this hard crust
will be pulled away, but remember that it is not a completely hard crust
again soft gum, it is connected to the gum below it and gradually soften, so
as it pulls, it also pulls the gum below it. The cohesiveness within the gum
(same material) is stronger than the adhesiveness between gum and gelatin, so
it breaks the bond between the two (if you imagine the colorful jello, one
side is made of gelatin, the other side is made of gum, if you can pull the
gum side, the whole section can be pulled off). In the area where there is
completely no exposure, there isn't any "crust" so there isn't any pulling.
Development is simply by dissolving the gum, but since the gum has some bond
with the soft gelatin below, it stays and there's the stain.
As I said, it doesn't always happen because it depends on how hard your
gelatin is (or how hard you have hardened it), how liquid your gum is, and
how hard your gum is (that is, how compatible it is with the soft gelatin
that it will bond). Judy doesn't see it all the time, and she reported a few
pigment only, but from her description, it looks like I have seen that effect
happened more than she has, and the explanation for that is simple too. She
hardens her size more than I do. She used to use formaldehyde at one point
and later switched to glyoxal. I used alum (not even chrome alum, just simple
grocery alum), so my size is not hardened that much, probably just enough,
but that makes it more susceptible to the effect that I just described.
And this same principle is also the same principle that is used in Pete
Frederick's trick. Actually I am not sure if Pete is the one who discovered
or invented it, but I got it from his post. (Pete, are you reading? Hello,
long time no talk) The trick is in clearing some pigment stains, he coats the
gum print with another clear coat, expose, and develop. The same thing
happens there. The clear gum coat bonds with the gum beneath it, which in
some highlight area bonds with the sizing. With exposure, the "crust" is
formed, and with development and because of cohesiveness within gum, the bond
is pulled broken and the stain is pulled away.
And I know now I will get some flame because of the long post. Oh well, I am
stronger nowadays.... :)
Dave S
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