One note on LR White embedding...it CAN be done in the poyethylene molds,
if your samples are such that you are able to re-use the molds.  We
pre-treat polyethylene molding trays by "embedding" straight resin (no
samples); we use the aluminum chucks that fit the molds to seal the back.
The blocks can be popped out of the molds like ice cubes, then the molds
are wiped to remove excess unpolymerized resin and stored to be used
again. The Al chucks can easily be broken off the backs of the blocks, or
could be left on if your microtome will accept them. If you soak the
chucks in methanol for a few hours, the polymerized resin can be scraped
off and you are ready to go again.

We embed cells on coverslips (both glass and Thermanox) and tissue samples
this way. 

Tamara Howard
CSHL