In the casting class that I teach we invest late in the class period
(usually near 9PM) and don't cast them until the next class the
following week. The burnout is done during the day prior to the
students coming in at 6PM. Prior to starting the burnout the
invested flasks are put in a bucket of water for 15 to 20 minutes.
So far (12 years) there has been no problems with this procedure.