Re: Gum image has reversed

A couple more comments about "tonal inversion":

To say it's somehow related to exposure is to say the obvious, as the
whole inversion thing requires different levels of exosure to be
present. With no exposure, the entire paper would be stained, rather
than there being a reverse image, which sort of goes without saying.
But it also doesn't make sense to say that it's caused by
underexposure, because underexposure can't cause a reversal image
unless the gum is overloaded with pigment to start with. And if there
is excess pigment present, exposing more won't make the problem go
away, as is shown by the partial inversion on the "for fun" page I
showed the other day:

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

where the image is correctly exposed, but is still partially
reversed as the result of overpigmentation.

For a complete reversal of tones, where what should be the darkest
tones are the lightest, the bulk of the emulsion has to have left
from the areas of greatest exposure, one way or another. This
leaving of the emulsion can happen as a result of underexposure, but
it can also happen as a result of flaking off of the pigment, as a
result of severe overpigmentation resulting in a too thick layer, or
from printing on a hard surface resulting in the floating away of the
hardened gum, as in the experiments on glass some of us were doing a
couple of weeks ago.

Tonal inversion will occur on well-sized papers; it isn't a question
of sizing or not sizing. It's all about pigmentation, how much
pigment a particular gum can hold in suspension. If you're not
getting staining or inversions, your pigment loads are no doubt well
within the capacity of the gum. I don't think it's useful to make
characterizations about the amount of pigment we use in ordinal
terms, because we all mean different things by those terms. For
example, some time back, Chris said she uses a huge amount of
pigment, and gave the amount of pthalo she uses as an example.
Curious, I figured out how much pthalo I use, and found that we used
the same amount of pthalo paint as a percentage of total emulsion,
although when I talk about how much pthalo paint I use, I say I use
a very small amount, since it's so much less than I use of many other
pigments. My point is that it wouldn't be very useful to assume
that because Chris says she uses a lot of pigment and I say I don't
use very much pigment, that means Chris uses more pigment than I do.
My feeling about this whole pigment concentration issue is that gum
knows how much pigment it can hold, and will let you know if you've
given it more than it can carry, and that all of us who successfully
print gum are probably using about the same amount of a given
pigment, regardless of the label we give to that amount.
Katharine

On Jan 8, 2006, at 8:56 PM, Katharine Thayer wrote:

> Hi Bruce,
> 1. The negative image, in my opinion, is the result of too much
> pigment. There's been some discussion of this fairly recently (in
> the last two months) on this list. Not everyone agrees with me
> about this, but I'll bet if you'll cut back on your pigment, you'll
> get a positive image. The reason it's black where there should be
> white, or light tones, is pigment stain, and the reason it's white
> where it should be black is that the emulsion was so thick (pigment-
> laden) that it flaked off.
>
> 2. I don't know what to say about your gum. I don't know the
> current Photographer's Formulary gum, although I loved an earlier
> version. I think the two problems are related to pigment
> concentration; with a heavy (but not overly pigmented) pigment
> load, you will get a high contrast image. It looks like with one
> gum (the prepared gum) you're just under the limit for the amount
> of pigment that the gum will hold in suspension, and with the other
> one, you're just over the limit and that's why you got the inverted
> image.
> Katharine Thayer
>
>
> On Jan 8, 2006, at 8:40 PM, Bruce Pollock wrote:
>
>
>> I'm new to the list and will apologize in advance for the barrage
>> of questions which I'm going to unleash over the next little
>> while. My frustration level is running quite high. However, for
>> the moment, I will try to limit myself to two main problems.
>>
>> First, a little background. I dabbled in gum dichromate about 25+
>> years ago and had some satisfactory results, but never really
>> stayed with it. I always wanted to get back into it and so, here
>> I am. I still have the 1 lb. jar of Potassium Dichromate I bought
>> back then and still have some Gum powder as well. Whether the age
>> of my chemistry has any bearing on the results I've experienced,
>> I'm not sure. I really can't see how Pot Dichr can "go bad" but,
>> you never know.
>>
>> My first question relates to the first usable print I've obtained
>> after many, many failures. I finally have something that suggests
>> I have a chance of success, but the print has reversed to
>> negative. Can anyone explain this? The detail is quite good, but
>> I have a negative, not a positive. Here are some details about my
>> method:
>>
>>
>>
>>> Lanaquarelle Medium Watercolour 140 lb. Paper, *not* sized
>>> Analogue 4x5 negative made in-camera on Tri-X film
>>> #2 blue photoflood light source about 25 inches from neg
>>> 20 minute exposure
>>> Emulsion made from 2.5 ml gum, 2.5 ml potassium dichromate
>>> solution and about 0.5 gm Daniel Smith Lamp Black pigment
>>> Still development for about 10 minutes brought up reasonable
>>> density -- my fear in letting in go on much longer was that I'd
>>> loose the entire image (just like the first dozen or so failures).
>>>
>>>
>>
>> I realize that I should probably size the paper and that Lamp
>> Black is not the best starting pigment, but I was just trying to
>> get myself into the ball park on emulsion and exposure times.
>> However, I'm stumped by the negative product.
>>
>> My second question relates to the type of gum arabic I should be
>> using. This success (if you can call a negative print a success)
>> came after I switched back to my old gum arabic powder (25+ years
>> old). The gum was a fine, white powder which I put into solution
>> using the formula in "The Keepers of Light" by William Crawford.
>> My earlier failures had been done using some Gum Arabic pre-mixed
>> solution recently purchased from Photographers Formulary. Using
>> the pre-mix I got virtually no results -- anything I got was
>> basically a "soot and chalk" type print where the shadows went
>> black and the highlights blew out. Nothing in between. No detail
>> at all.
>>
>> Can anyone explain the lack of results with the pre-mix Gum
>> Arabic? Should I stick with the powdered form and mix it myself?
>> The fact that I got *some* results from the powder suggests to me
>> that the pre-mix is somehow at the root of my earlier problems
>> where I got soot and chalk.
>>
>> Many thanks in advance -- all advice gratefully received.
>>
>> Bruce Pollock
>>
>>
>>
>>
>
>
Received on Mon Jan 9 12:43:28 2006