We used to use the mercurchrome also but now since ours disapperared, we
use a few drops of eosin until it tints the formalin pink.  I don't know
why it doesn't come out in the dehydrants on the processer but it doesn't
and it does not interfere with later staining either as it will come out
when you de-paraffinize  the  slides.

Karen Dulany
Eppley Institute for Cancer Research
Omaha, NE