Re: Chemist question for all of you who love to figure things out

Potassium cyanide is a very strong silver complexing agent and als is
an agent very capable of fixing even silver iodide. In the context you
cited, it might also have the function of a bleach.

Silver iodide is hard to fix with thiosulfate fixer. There are several
well known compounds that scavenge iodide ions, and those might work
if added to an ammonium thiosulfate fixer. (On a different aspect of
those compounds, they are used in some emulsions, like those used for
medical films, as a blue-black agent because they can make the image
very cold tone when added to emulsion. Unlike benzotriazole or other
antifoggants they don't lose speed or slow development process.)

For geleral image bleach, my favorite of slow working gentle bleach is
an ammonium thiosulfate solution containing sodium metabisulfite,
citric acid and disodium EDTA. This is a lot slower than dilute
ferricyanide bleach but it's the best when brightening silver gelatin
prints.

--
Ryuji Suzuki
"Well, believing is all right, just don't let the wrong people know
what it's all about." (Bob Dylan, Need a Woman, 1982)
From: Christopher Lovenguth <chris@chrisportfolio.com>
Subject: Chemist question for all of you who love to figure things out
Date: Thu, 02 Jun 2005 00:47:34 -0400
> I've come across something recently about "brightening" highlights of a
> daguerreotype plate.
> 
> Here is the text: 
> 
> "The daguerreotypist sometimes immersed the plates in a solution of
> potassium carbonate and potassium cyanide with "a little alum, borax and
> some other things" (Rinhart and Rinhart 1981
> <http://aic.stanford.edu/jaic/articles/jaic35-01-002_appx.html> , 188). This
> procedure may have brightened the highlights by slightly etching them." 
> 
> Can anyone tell me what chemical reaction is going on here in a way someone
> who doesn't really know chemistry would understand. Also does anyone have an
> idea of what not so dangerous and toxic chemicals could be used to maybe
> recreate the same reaction?
> 
> Christopher Lovenguth
> www.christopherlovenguth.com <http://www.christopherlovenguth.com/> 
> 917.721.4768
>