Re: Gum image has reversed

Bruce,

In addition to what Christine and Katharine told you, I would suggest
three things:

1. Get a Stouffer 21 steps tablet or equivalent. It is a priceless
instrument for testing and sharing information.

2. Avoid lamp black at least for your first experiments, if you can.
It is a difficult pigment. If you must use black, use one of the
others, such as Ivory black.

3. For almost the same quantity of gum and dichromate (i.e. 2cc gum +
2cc dichr.) I have used 0.05g of lamp black. Even allowing for
different brands etc. this is almost ten times less than what you
have used and even so, under the more dense zones of the tablet I got
tone inversion. In my opinion you are using far too much pigment.

Tom Sobota
Madrid, Spain

At 05:40 09/01/2006, you 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 04:29:43 2006