Interesting discussion here.  Four sources of cracks have been noticed here. 

If cold plate is too cold, cracks have shown up but with some paraffins and
not others.  Most embedding centers have adjustments for temperatures. I
think you may have to adjust your embedding center cold temperature for
your paraffin by a few degrees. Blocks are never put in a -20C freezer,
this produced a spider web of cracks throughout a whole block.  We have
even let a block cool at room temperature and did a gradual cooling for
sectioning - ice with water, but beware of going onto really cold freezer
ice or cold plate before cutting, cracks can form.  I never had problems
with Surgipaths two system infiltration/embedding medias. 

Using deeper metal molds has produced cracks, but the overall depth is
greater while cooling so the paraffin crystals may not be forming correctly. 

60C paraffin temps are not excessive. 

The worst cracks (not spider web variety) are after embedding with plastic
cassettes I consider absolute junk, obviously the vendor will not be
selling these to us again! They are flimsy, overly flexible, and when
clamped into a block holder, cracks form in the block and ruined ribbon
formation.   Other sturdier brands of cassettes never have this happen. Be
aware cassettes can be very different and try samples from all companies.
Filling backs of cassettes is critical, it must be done with enough
paraffin or you clamp onto empty plastic back, hmmmmm cracks! 

If one trims a block down to reduce size of block face (really not
necessary for all blocks!), pressure on paraffin block and too much trimmed
away at a time cracks block.  I have a very strong sturdy body type young
man who had cracked blocks. When he trimmed paraffin block down, he used
extreme pressure with teflon coated razor blade, took away huge chunks of
paraffin rather than smaller slices. He now trims less away with each slice
without crack formation. Obviously flexing, stressing a paraffin block in
block holder, or trimming can produce cracks.   
Gayle Callis
MT,HT,HTL(ASCP)
Research Histopathology Supervisor
Veterinary Molecular Biology - Marsh Lab
Montana State University - Bozeman
S. 19th and Lincoln St
Bozeman MT 59717-3610

406 994-6367 (lab with voice mail)
406 994-4303 (FAX)

email: gcallis@montana.edu