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

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