I really don't know what the "correct" answer is, but I have had
the best results from making 24 hours my shortest time to wait. In
our class we invest one week then burnout and cast the next.
Ken, at 24 hours, it’s actually drier than most casters would like,
though still OK. Your class is a special situation, since the
instructor has to arrange the timing of when to invest and when to
burnout and cast, to fit the class schedule. In many such school
programs, flasks are allowed to sit sometimes for days, as is done
with your class. In the last one I taught, while in grad school, the
routine was also invest one class session, burnout and cast the next
week, but flasks were dunked in a sink of water and allowed to soak
up enough water they’d stop “bubbling” before going into the oven.
I think I had a flask that was still too wet and when I burned it
out it exploded and made a mess all over the inside of the kiln.
It actually had dried for overnight. But the work place was cold
and I suspect it didn't dry as much as it needed.
As other posts in the thread have also pointed out, moisture in the
flask helps drive the wax out, rather than letting it soak into the
investment, so burnout is more easily complete. As well, in the
initial stages, water is a better heat conductor than is dry
investment, so initially, it helps even out the heat transfer into
the flask during the initial period when wax is being melted. Flasks
can “blow” for a number of reasons. Often, the initial heating stage
is too rapid when people burn out in kilns not equipped with a
programmable controller. The quartz (silica) in the investment goes
through a structural change while passing through the temperature
range in the 300 to 350 degree area and there is a slight dimensional
change to the crystals that goes with this. Going through that temp
range too quickly puts stress on the investment. Also, when
investing, it’s important to pay decent attention to the water to
powder ratio. Too much water gives a weaker investment, more prone to
cracking. And carving waxes are also harder on the investment than
the softer and lower melting injection waxes.
For what it’s worth, virtually all the manufacturers of casting
investments used for silver or gold recommend a wait time, after
investing, of just a couple hours or so before beginning burnout, and
don’t suggest needing the stuff to be fully dry. It reaches full
strength in just an hour or two if properly mixed.