You may be making an insoluble salt (zinc phosphate) that could
perturb the structure of the tissue and make for difficulties
when cutting sections.  Paraformaldehyde is a polymer of
formaldehyde, and it depolymerizes. It doesn't "dissolve," and
the actual fixative agent is the formaldehyde released in the
depolymerization reaction. The depolymerization needs an acidic
or basic catalyst (it won't happen in water alone),  A sodium
phosphate usually serves as the required base. You should not
move a piece of tissue from a phosphate-buffered formaldehyde
fixative into a zinc-formalin, because the zinc ions will make
insoluble zinc phosphate as soon as they bump into phosphate ions 
derived from the perfused liquid.

Why do you say "4% paraformaldehyde" is expensive and unstable?
It is rarely necessary to use paraformaldehyde to make a 
formaldehyde-based fixative, but the cost of paraformaldehyde is
not that much different from that of formalin. It isn't in any
way "unstable" either as a solid or as a solution of formaldehyde 
(which is what is when you have a transparent solution).

On Wed, 16 May 2001 dgaupp@tulane.edu wrote:
> Has anyone out there in histoland ever heard of perfusing mice with Zinc 
> Formalin instead of 4% paraformaldehyde?  Where I work at, they perfuse mice in 
> 4% paraformaldehyde, then fix tissue overnight in 4%para in refrigerator and 
> then give it to me to process; it gets processed with zinc formalin.  I would 
> think 4%para. would not be stable in tissue processor because its kept at room 
> temperature overnight.  4% Paraformaldehyde is so expensive and the stability 
> is less than zinc formalin(which is kept at room temp).  Going from 
> refrigerator temp to room temp constantly can not be good. Any comments, 
> suggestion, or advice would be greatly appreciated. 

----------------------------------------
John A. Kiernan
Department of Anatomy & Cell Biology
The University of Western Ontario
London,  Canada   N6A 5C1
   kiernan@uwo.ca
   http://publish.uwo.ca/~jkiernan