Dear Susan,
I would be extremely careful with the propane. I hope you´re a non-smoker!!
But serious: propane is "safely" liquid at 120-180°C. The only thing I can
imagine, without the proper machinery, is cooling down the propane by
leading it into a dewar with liquid nitrogene. 
If you are planing to do these things more frequently, then I would do it
more professionally and safely, by buying the Leica CPC. This is a nice
machine which allows you to shoot tissue specimens into liquid propane or
liquid ethane under controlled temperatures. Afterwards this machine can be
used for deep-temperature embedding of tissue into plastic (only small
specimens). In Germany the CPC costs something around 30.000 $.
Good luck!!

Jochen Schuck