John, sorry, I am not a chemist.  It is an observation that was immediately 
noticed when when we switched from NBF to Zinc formalin.  Many of the 
biopsies we process are what we call outside specimens... some come from 
across the country.  And they are already fixed in the zinc formalin for 24 
to 48 hours minimum before we receive them in the lab and also, the specimen 
bottle most likely sits on a shelf for a period of time before use.    The 
metal caps on the small specimen bottles were exposed to long periods of 
time, since the pathologist insisted on reusing them.  We have since replaced 
those bottles with plastic caps.  The pathologists here normally do not have 
a problem reading around the formalin pigments.  They are willing to put up 
with the annoyance inorder to get the beautiful immunos we produce.  
Periodically, we will remove the formalin pigments when the problem is 
interfering with the reading of the slide.     pearl