No, if you are starting with a cold furnace, the flasks will not
explode. Trials have shown me that the ramping process is largely
non-functional as well. I placed thermocouples at different points in
a test investment as well as at several points inside the burnout
furnace and started at high setting… I found that the temperature
inside the flask lagged behind the furnace to range around the
boiling point of water until all water was gone, then rose steadily
to the final temperature (1200F.). I hold the temperature at that
point for 2-3 hours but am not sure at this point that the holding is
I issue the caveat that this is with my investment and furnaces. I
have as a general rule used thermocouples inside the large flasks I
use for bronze and aluminum castings. These may weigh hundreds of
pounds (the investments, not the thermocouples). I heat these at
full heat and hold at 1100-1200F. for three days.
The trials with jewelry investments were burned out in an electric
muffle furnace and were the size for single pieces of jewelry such
as a ring, etc. For this kind of thing, incidentally, I use the cans
or tins that are used with tuna or similar products. They work very
well and can be used several times before being discarded.
I find that there are so many opinions as to how things "are done"
that I find it easier to run a few quick experiments or tests to see
how things work for me, myself, in my shop. I advise others to think
on this but to stay safe.