You can use all of the above and a few other things. Here’s a method
that’s gotten me thru a couple of emergencies. You need: 1) an
electric hot plate without a thermostat (it must be “on” at all
times); the nichrome element type is recommended, not the calrod
type. 2) a terra-cotta flower pot big enough to cover the flask,
allowing about an inch of extra space both vertically and
horizontally; 3) a sheet of heavy-duty aluminum foil big enough to
wrap the inside of the flower pot; 4) a well-ventilated area with a
power supply for the hotplate, and 5) at least 6 hours of your time
for an average 2 x 3-inch flask with, say, a single gent’s ring wax
or an average ladies’ ring or two.
Place the flask on the edge of the hotplate with the sprue-former end
downward. Situate it so the molten wax does not directly contact an
element when it liquefies and flows out. (It’s best to place a little
aluminum foil below the flask opening to catch the bulk of the wax –
wax that sticks to the hotplate can be removed with denatured alcohol
or nail polish remover).
Turn the hotplate on a low setting and leave it uncovered until most
of the wax has been eliminated. Every couple of minutes lift the
flask with tongs and observe to see if the wax is flowing freely,
not boiling out. Adjust temperature if necessary. This process should
take 30 minutes or so on average, but don’t rush the process!
When little or no wax is flowing out, gently increase the temperature
setting and move the flask incrementally toward the center of the
hotplate with your tongs. If the wax starts sputtering and
splattering you’re going too fast and can ruin the mold. Reduce heat
and go back to Go. Allow up to 90 minutes or more to complete moving
the flask to the center of the hotplate, increasing the temperature
to the highest level as you go.
When the flask is at the center of the hotplate, cover it with the
inverted flower pot to create a mini-oven. Burn-out from this point
should require about 4 hours (more for larger flasks, of course).
The advantage of using an electric hotplate instead of an open flame
or barbecue briquets (it’s been done!) is that the investmen is not
carbonized by the flame, so it’s easy to judge when all the wax has
been burned out. All traces of gray will be gone and the investment
will be a uniform chalk-white. It’s usually best to cast immediately
after removing the flask from the heat, but this is a very imprecise
process and experience is the best teacher. Practice with commercial
waxes first, and don’t start with your priceless originals!
Warning: do not use sections of galvanized pipe for flasks.
When heated they can give off poisonous fumes. The best source of
cheap flasks I’ve found is my friendly neighborhood muffler shop.
They have a bin of cut-offs they let me pick through. I have them do
all my muffler work, of course, and send all my friends there.