[Histonet] Concave paraffin blocks

[Timo Väisänen]

Dear all,

I mailed a while ago a question about concave paraffin blocks and received ample
of suggestions. Thank you all. Based on them the culprit for our problem was
suggested to be the excess of xylene in the blocks. 
I have now checked the tissue processors for faulty valves causing carry-over
etc. and they seemed to be fine. Concave blocks were also present when
processing was performed with totally fresh paraffin. Therefore, I think,
carry-over of solvents may not be the cause in our case. Somebody also
suggested that too low temperature of the cold plate during embedding or when
cooling the blocks before sectioning could cause the concave surface. I tested
this by adjusting the plate temperatures to -5-10C range. This did not help.
The only thing that seemed to have some kind of positive effect was using six
chamber biopsy cassettes during the embedding. The rationale in this experiment
was, that maybe during embedding and when paraffin solidifies the shape of the
cassette base changes a littel bit and causes tension to the solid paraffin.
This tension could then cause the concave shape of the block surface. The extra
plastic in the six chamber biopsy cassette base could make it more rigid and
resistant to tension/distortion. To my surprise using these cassette bases made
the blocks more even. Does this theory make any sense? One other thing that came
to my mind is paraffin itself. We routinely use Histowax 52-54C. Maybe I should
try another brand? 
Thank you for your comments in advance!

Timo
Univ. Hospital of Oulu
Dept. of Pathology
Finland