Stain and exposure (was: Re: Gum image has reversed

On Jan 9, 2006, at 6:30 PM, Dave Rose wrote:

> Way too much pigment IMO. I'm using 1gram carbon black per 100ml
> of gum.
> At that concentration it's very strong. I'm surprised you're
> getting any
> kind of image.

This brings up something that's been puzzling me since we started
talking about this inversion thing, and that's where things fall in
the stain-inversion continuum. For example, think about Ed Buffalo's
first gum print, which was also lamp black, and was also quite
overpigmented, and as I recall was also overexposed, and which was
solid black from corner to corner, no image at all. Was Ed's
print more overpigmented than Bruce's, or less overpigmented.

The discussions about exposure, and the assertion that "it has been
found lately that underexposure causes inversion" makes me think I
must have missed something. I thought I saw all that discussion, and
the visuals that people posted, and I didn't get that from anything I
saw there. In my experience, stain is not related to exposure,
except for this inversion thing which is a special case.

So then I stopped typing and went out to the studio and made a series
of prints with lamp black (three different concentrations, all highly
pigmented, at three different exposures: (1) 2 minutes, which would
be a good exposure for a normal pigment mix with this kind of
negative, but probably was too short for these heavily pigmented
mixes (2) twice that exposure, or 4 minutes and (3) half that
exposure, or 1 minute).

The results don't give very clear answers, for several reasons.
First, I didn't actually get any inversions, although the starting
pigment mix here was the same mix I used to produce quite nice
inversions on glass a few weeks back. So this series of test prints
can't address the question of where inversion comes in the continuum,
which was the question I was most interested in, or the question of
how exposure relates to inversion. Second, all three mixes made a
very heavy, hard to brush out coating which tended to break up and
flake apart in the water at all three of these exposures. So, for
the most pigmented coating,which was the only one that showed
significant stain, I added two more exposures, one at 3 times the
"normal" exposure for a normal coating mix, and one at 4x this
exposure, to see if it's true that more exposure reduces stain (it
didn't). Then I developed that last one longer, to see if a longer
development would somehow miraculously yield an unstained print, but
an hour of development revealed such severe speckling in the
highlight areas that it was quite evident that further development
would be fruitless.

For the most pigmented mix, while there were fluctuations in the
lightness and darkness of the stain, probably as a function of how
much of the gum emulsion had flaked off (which didn't seem to be
directly related to exposure, the overall impression is: stain. All
of these images have severe overall stain, which doesn't improve with
more exposure.

The second pigment mix, which had half as much lamp black in it as
the first, stained a light grey at all exposures (I personally would
bet that inversions might occur somewhere between this concentration
and the more pigmented one). This mix flaked severely after the
shorter two exposures; I arrested them as the images were breaking
up, and dried them with the image flaking and running. So I can't be
sure how much of the grey on these two prints is actual stain and how
much of it is just running pigment that got dried in place. The 4
minute exposure for this mix printed quite well, although with some
grey stain in the highlights and some blocking in the shadows.

The third pigment mix, which had 1/3 as much lamp black in it as the
first, flaked off so immediately and severely at 2 minutes and 4
minutes that I didn't think there was much point in doing the 1
minute exposure for this mix. Why the 4 minute exposure for this
pigment mix flaked off when the 4 minute exposure for the less
pigmented mix above it didn't, is a mystery I have no explanation for.

I normally print lamp black with less pigment than any of these and
get nice, black prints with good tonality and clear highlights, so
it's not necessary to overload the gum with pigment in order to get a
good black, and in fact I wouldn't recommend it, precisely because of
this problem that too much pigment makes too thick a coating and will
tend to flake off, even if it doesn't produce an inversion or overall
stain.

I did learn one thing: to get a print like Ed Buffalo's first gum
print, use a way overpigmented mix and expose the heck out of it.

Oh, I brushed one of the coatings onto a newspaper; you can see that
it doesn't pass the newspaper test.

http://www.pacifier.com/~kthayer/html/lampblackstain.html

This one won't stay up long because it doesn't seem like there's a
whole lot to be learned from it, but I figured I might as well show
them, for whatever they're worth, since I spent much of the day
doing them.
Katharine
Received on Wed Jan 11 00:20:05 2006