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

From: Alberto Novo ^lt;[email protected]>
Date: 10/19/04-12:32:04 AM Z
Message-id: <20041019063204.3227.qmail@webmail.aruba.it>

Dear Katharine,
I think that many things have been explained between us.

> 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.
Acidity changes the sulphite ion into bisulphite, so that one cannot refer
to sulphite anymore if the pH is changed. If so, the clearing definidively
comes from bisulphite.
As for the speed of the process, I think that a clearing process should rely
on reasonably short times of action. What is "reasonably short" is again a
matter of taste.

> I'm not sure I understand the point with the plain dichromate on paper.
Well, if the dichromate in the gum layer reacts with the paper it will
introduce an additional stain. This is for printing purpose, not for a study
on the gum layer only.

> 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 suspect that the colour of chromium chromate is brown. This might account
for the first colour of the stain, but without any information on its
solubility all this topic is only a formal exercise.

> I disagree that this is of interest only to the two of us.
Sorry, but I said that "We could send each other our proofs for a visual
comparison, but this would be useless for the other people in this list",
because we can compare the results only between us. Instead, if we were able
to define in a unique way the colour of our proofs, also other people will
be able to reproduce and see them in an undistorted way.

I think that this matter is interesting in particular if referred to prints
made with subtle, transparent colours, which may be shifted towards unwanted
hues by the gum stain.

Alberto
Received on Tue Oct 19 00:32:41 2004

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