Katharine Thayer wrote:
> Hi Giovanni, Alberto and all,
> I ran another comparison, adding more conditions, and this time I used a
> different scanning program that doesn't add the red cast that my
> SilverFast has been doing, and have got what looks to me a fairly good
> approximation of the pale colors, although the blue still doesn't show
> very well.
Instead,I am not able to properly tune the scan of my proofs. If I try to to
change the colour of the chromium hydrate in order to be as similar to the
original, the other hues and the paper base look yellowish. Nevertheless, my
image is in http://www.albertonovo.it/images/CrIII-VI.jpg (attention to the
capitals!)
It shows the results of some proofs made on Fabriano 5, 300g/m2.
I coated the paper with a mixture of gum arabic and sat. potassium
bichromate (1:1), then I dried it in the dark and exposed for 20' with 4 UV
tubes, 15 W each. I added also a brush of sat. pot. bichromate on the top of
the sheet to see what happens to the paper base if the bichromate penetrates
it. I shelted the center of the sheet from the light in order to have a
reference for an unexposed region.
The proof has been spoiled for 30' in tap water at room temperature,
changing it 3 times. Once spontaneously dried, I cut it in the middle and
soaked one half overnight.
Finally, I cut again pieces of the two portions, clearing them in
(from top to bottom of the image)
a) sodium chloride 10%, 10'
b) a mixture of 0.1% sodium sulphite + 0.1% sulphuric acid, 10'
c) potassium metabisulphite 5%, 10'
d) sodium sulphite 5%, 1 hour
The image shows at left the spoiled-only part, at right the spoiled and
overnight soaked one. You can see a very faint colour also in the unexposed
part. Interestingly, the bichromate-only part has a deeper colour than the
gum arabic one, also after the prolonged washing.
For comparison, the image contains also a piece of Fabriano F5 rubbed with
some chromium oxide and a filter paper with some chromium hydroxide. This
last is grey shifted towards blue-green. I prepared the chromium hydroxyde
reducing some pot. bichromate diluted in distilled water with an excess of
sodium sulphite and sulphuric acid, then precipitating the hydroxide with a
solution of sodium hydroxide.
> Contrary to my expectation, I did find, as Alberto suggested, that
> leaving the stain in water 12 hours longer did fade it, although the
> water didn't take on even the palest of yellow tint, and I wouldn't say
> that the water actually "clears" the stain, it just fades it. I also
> tried salt water, for reasons related to a private discussion, and found
> that it worked much as the clear water, fading the stain but not
> altering it. Both the sodium sulfite and the bisulfite removed the brown
> stain and turned the gum a bluish-grey color, although the sulfite took
> longer to get the same effect.
I don't know if in the U.S. you use the same, but in Italy we say that two
persons, one optimist and the other pessimist, referring to the same glass
half filled of wine, say respectively that it is "half full" and "half
empty".
Perhaps the same difference applies to "clearing" and "fading".
> These are the colors of dichromate stain that occur in my gum universe.
> It's interesting to consider that there are other gum universes in which
> the colors are different.
... or our perception of "green", "blue", "yellow" and "brown" are
different. We could send each other our proofs for a visual comparison, but
this would be useless for the other people in this list.
We should find a more "independent" way to define the colours of our proofs.
Any suggestion?
To add a further disturbing element...
I did not believe to the existence of "chromium chromate", or "chromic
chromate", whose formation I found speculated in some articles of "Progresso
Fotografico" in the period 1920-30. Instead, I see that it has a CAS number
24613-89-6, so it is a real thing. I am not able to find other
characteristics of this subsance (i.e. colour and solubility) if not it is a
carcinogenic one.
Is anyone in this list able to look at Gmelin for more information?
Alberto
Received on Mon Oct 18 05:02:56 2004
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