[Histonet] Why do some paraffin sections of brain wrinkle on the water bath?

[CM Bush]

Dear Histonet,
 
Seeking some some basic info:
 
Sometimes, when cutting 8 um sections of brain, mouse or human (in standard size cassettes), the section will either wrinkle or pucker once on the water bath (set at ~43C), and then other sections (from different blocks) are great and flatten out, very well, looking pretty much like the tissue did at the face of the block. 
 
The mouse brains seem to wrinkle more randomly, where human brain sections pucker in a more regular pattern-does this depend at all on how the piece of human brain was cut-in as far as which section of brain, or ratio of white matter to gray, contours of the brain, or dehydration issues, or..?  
 
For the mouse brains, some extra time on the bath seems to iron out the wrinkles pretty much.  Same for the human brain but not completely.  If I could draw a picture, once the section of human brain has dried on to the slide there are regular patterns of slight folds where the puckers where, arranged around the, I think, tracks of white matter.
 
Like I said, sometimes these wrinkle and puckers take care of them selves while on the water bath or drying on the slide- I'd like to try to improve the sections, what I can do to fix this, or at least find out what is going on with this, just to know.
 
Thank you so much for the help.
 
CMB