Re: dichromate stain and chemistry question

[Katharine Thayer <kthayer@pacifier.com>]

On Jan 7, 2007, at 7:29 AM, Christina Z. Anderson wrote:

> To the chemists of the list, does this chemical explanation sound  
> plausible--it's Greek to me:
>
> "Light acts the same as heat, and in the presence of organic matter  
> sets free the ammonia from the double salt, which then becomes acid  
> by the formation of chromic acid, which is reduced by the organic  
> matter to chromate or chromic oxide.  If the washing is continued  
> this chromate is reduced to chromium oxyhydrate which is a pale  
> greenish color."
This doesn't look like Greek to me, but it's incomprehensible just  
the same.  There's no reference given, but from the sound of it I  
would guess it dates back to the early days long before chemistry  
advanced to the understanding of the photochemical process that we  
know today to explain the process.  Just the beginning statement,  
"Light acts the same as heat" is incomprehensible in the light of  
this knowledge.  So I'm not sure how useful this information could be  
to us, but since no chemists here have responded, I've forwarded it  
to my physical chemist colleague for comment, and will report back  
his comments when he replies, whatever they are.



> Way back when I talked of putting a gum print out in the sun which  
> will get rid of stain, too--it was scoffed at, as is usual, but try  
> it--sun fades the stain.
The record will show that I was skeptical in the beginning because  
you were talking specifically about YELLOW dichromate stain, which is  
unreduced dichromate or chromate, which of course would turn brown  
when exposed to the sun, as the light would reduce it in the presence  
of the organic materials in the paper or the size.  When I said that,  
you corrected yourself and said actually you didn't mean YELLOW  
stain, you meant brown stain.  So I said okay, I've got to try that,  
and I did and found that it worked, and reported back to the group  
that sun does indeed work to fade the stain,  although I was only  
able to fade the stain, not to eliminate it.   There was no scoffing  
involved, and I'm really rather tired of an honest disagreement on  
issues being characterized as "scoffing,"   as well as things being  
misrepresented and taken out of context, in that you only referred to  
my initial skepticism in characterizing my position, rather than  
taking the discussion as a whole, in which I did do the test and  
agree that sun fades the stain.
Katharine