Re: Colors of Dichromate Stain (was:Re: Sodium Bisulfite

From: Katharine Thayer ^lt;>
Date: 10/18/04-05:26:32 AM Z
Message-id: <>

Thanks Alberto, it's helpful to be able to see what we're talking about.
Just on quick glance, it looks like our colors are fairly similar. There
is more difference between the sulfite and bisulfite in yours than in
mine, but that's because I gave the sulfite more time to achieve the
same effect as the bisulfite.

Sometimes in these endless discussions it's good to go back and remember
what the original question was. Someone asked, does sulfite work as well
as bisulfite for clearing gum? An answer given was that sulfite cannot
work for clearing gum because it is alkaline in solution and simply
won't work unless acid is added to change the pH.

You can read the answers from our tests two different ways: You can say
yes, sulfite does work as well as bisulfite if you give it enough time,
or you can say no, it doesn't work as well as bisulfite because it takes
longer, but you can make it work as well as bisulfite by adding acid.
The difference between these two ways of looking at it does strike me as
a good example of half full or half empty.

I'm not sure I understand the point with the plain dichromate on paper.
There you are looking at dichromate reacting with an internal or
external size, or with organic material in the paper; it's not
surprising that the visible effect might be somewhat different from
dichromate reacting with gum.

I think that if there's a lesson for everyday gum printers to take away
from this discussion it's this: both Alberto's tests and mine show that
when you think you've "cleared" a print of dichromate stain, the gum may
still not be really clear; there may well be a lingering grey or
grey-blue or grey-green tinge to the cleared gum.

The comment about my disinction between fading and clearing being like
the distinction between half full and half empty points up a substantive
difference between us: Alberto apparently thinks it's a continuum, and I
think there is a qualitative difference between what I call "clearing"
and what I call "fading," since the colors are so different. I don't
have any idea which of us is right, whether it's a continuum or a
qualitative difference, but if I could prove to myself that if you left
the stain in water long enough, it would go to a faint grey-blue or
grey-green rather than just a pale tan, then I would have to agree with
Alberto, at least theoretically, although practically I still think it
would amount to a qualitative difference in the practice of gum
printing. Furthermore it seems to me that the logic that says it's all
a continuuum would require us to say that all the bisfulfite does is
just speed up the fading process rather than reacting with the stain
chemically to "clear" it, and that logic doesn't make a whole lot of
sense to me.

I'm not convinced either that the color differences we're describing are
just semantic. As a painter and a color photographer for decades, I like
to think I can tell the difference between blue, green, blue-green, and
brown, but you're right, people can describe colors different ways.
Yesterday after I posted the strips, I thought it would be helpful to
make some dichromate stain and scrape it off wet, both untreated and
treated, so that you could see the dramatic difference between the deep
green and the definite blue. But I only got a few minutes of sun
between squalls, and when the resulting stain was washed, it was only a
very light brown. This stain was very deep brown when scraped wet, and
very deep green when treated with sodium bisulfite, not the deep green
for the untreated stain and the bright blue for the treated stain that I
was getting with more severely stained gum. So what I'm coming to think
is that both the crosslinking and the clearing reaction are a continuum
where there are different things going on and different products being
produced (which is typical with chromium) and that the color of the
final product depends on the the proportions of the different products.
I would guess that if more oxides, or what the heck, chromium chromates?
are produced, then it's more green, and if more chromium sulfate is
produced, then it's more blue. And it depends how much stain you have to
start with, how the balance comes out. That's my tentative hypothesis at
the moment to account for the different colors. I can understand
perfectly that if you haven't seen the different colors yourself, you
might be skeptical about this, but I have to wait for the sun before I
can show them to you.

As for chromium chromate, I have some stuff I could look up about that
in the literature. It was suggested 125 years ago that this is the
product that combines with colloid to render it insoluble, but that was
long before we began to understand photo-redox reactions, and I don't
believe most scientists today would consider this a reasonable
explanation of the process. But whether this material plays a part in
the clearing of dichromate stain, I couldn't say.

I disagree that this is of interest only to the two of us. The purpose
of having an open forum is to have open discussions that we can all
participate in and benefit from, and I would like any discussion about
chemistry as it relates to the gum process to be out in the open so we
can all participate and benefit from an open discussion of the issues.
Those who aren't interested can hit delete, just as I hit delete for
digital and dageurrotypes.

Alberto Novo wrote:
> 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 (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 12:22:06 2004

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