Re: Gum image has reversed

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 Sun Jan 8 22:57:27 2006