DMSO, dimethyl sulfoxide, a very volatile solvent (can't you taste it when
you open up a 60C oven?) is the culprit.  The DMSO is supposed to make
penetration easier (used for horses topically to reduce pain and inflammation
of joints) and probably works well with the paraffin as well.  BUT is 
does contribute to the rusting of metal, and ruination of solenoids of 
incubators, and other instruments other than just automated processors.  I
was more worried about my constant exposure to the DMSO and long term
effects, enough to work with something else along with watching some of the
metal parts of things turn rusty brown.  

There are other paraffins which are nice for infiltration.  Surgipath has
an infiltration media which contains little or no plastic polymers, is
used solely for infiltration purposes, works very well and for large bone
slabs, in our case the proximal and distal ends of tibias or femurs from
sheep, goats, and dogs.  We merely increased the time of infiltration on
the VIP to 4 hours per change (4 changes!) for these huge dense pieces of bone 
and embedded in Surgipath embedding media which contains
the polymers needed for good sectioning.  Worked very well.  Just don't
try to use the infiltration media for embedding, disaster, cutting poorly set
putty is a good comparison for that mistake.

The TIME of infiltration on dense tissues may be a big factor, and 
alternating vacuum and pressure, as most automated processors now have,
is very important.  I preferred to infiltrate whole rat knees, from mature
rats, for 2 hours per station, 4 changes.  In fact, all changes from 70% 
ethanol through clearing and paraffins were done at 2 hours per station,
and could cut at 3 micrometers, disposable blades, with excellent 
staining results.

Gayle Callis