On Fri, 24 Aug 2001, Renault, Peter wrote:
> precipitate which is forming on the brush boarder in the rat kidney tissue
> ... perfused through the aorta ... 4% buffered paraformaldehyde ...
> slices are dissected and immersed for 60 minutes in the same fixative on
> ice, and placed at 4C in ethanol overnight. ... paraffin ... 

You didn't describe the appearance (colour, texture) of the
precipitate, but a simple answer might be sodium phosphate
from the fixative. It isn't soluble in the higher alcohols.
After a buffered aqueous fixative you need to go through
enough water or lower alcohols to dissolve out the buffer
salts.

----------------------------------------
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