From: Dave Rose (photo@wir.net)
Date: 03/26/01-09:55:39 PM Z
Dear Amanda,
Whenever I mix a new gum coating, I make a 4x5" control test print from a
'normal' negative of a waterfall (which prints beautifully on Grade 2 silver
gelatin paper). I also include a Kodak Control Scale T-14, to better judge
exposures.
I'm looking at four single-coating test prints, all made with dry-powder
pigments:
1. Pigment: Sennelier 'Black for Fresco' 1g/100ml gum + Ammonium Dichromate
30% solution (Mix 1:1)
2. Pigment: Sennelier 'Black for Fresco' 1g/100ml gum + Potassium
Dichromate 10% solution (Mix 1:1)
3. Pigment: Winsor Newton 'Burnt Sienna' 1g/25ml gum + Ammonium Dichromate
30% solution (Mix 1:1)
4. Pigment: Winsor Newton 'Burnt Sienna' 1g/25ml gum + Potassium Dichromate
10% solution (Mix 1:1)
Observations: The color is identical in both cases - the sensitizer used
does not make a difference. The ammonium dichromate prints are weaker and
muddy, with less separation between individual steps on the Kodak scale.
The ammonium dichromate is much faster. An ammonium dichromate exposure at
1.75 minutes is roughly equivalent to a 5 minute potassium dichromate
exposure.
I use potassium dichromate for the majority of my gum printing. It gives
cleaner results, stronger separation of tones, and is better suited to my
enlarged negatives. However, ammonium dichromate is useful, and I do use it
in certain situations, e.g. printing a subtle tone into highlights....
without unduly affecting shadow areas. With the pigments I use, I've never
seen color shifts due to use of either sensitizer. However, that's not to
say the pigment you're using is not reacting to ammonium dichromate in a
negative way. Is it possible that your prints are overexposed when using
the stronger sensitizer? What exposure times are you using for AD vs. PD?
Best regards,
Dave Rose
Cactus Cowboy
Big Wonderful Wyoming
> I have been having problems with my coloring. Normally, I use potassium
> bichromate and notice that the orange color of the chemical has only a
> slight affect on the overall finished color. However, I recently switched
> to using ammonium dichromate and have noticed a very strange reaction
> with this orange chemical. My colors have been coming out quite different
> that one would expect an orange color mixture to produce. I am using
> Winsor & Newton water colors. Is this color difference normal when using
> the different chemicals? Why is there such a difference when the
> chemicals are the same orange hue?
>
> Amanda
>
>
>
>
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