Check it for cracks of any kind since flint is schist it flakes it
should be a combination of flint,quartz and sometimes even tuffa
since these can stand considerable heat using normal wet wadding
and cooling techniques should work.
Just to clarify a couple of points-
Flint technically is the form of cryptocrystalline quartz found in
the Cretaceous chalk formations of southern England. The name has
been loosely applied to somewhat similar quartzes (cherts, jaspers,
and chalcedony, for example) found elsewhere around the world, and
often used in making the tools and points so useful as markers of
various prehistoric cultures.
Schist, on the other hand, is a high pressure metamorphic rock
consisting of micas, quartz, garnet, and various accessory minerals,
depending on the chemistry of the original rock and the proximity of
other mineralizing bodies. It is unlikely that a low temperature
quartz like flint will be found within a schist, unless near-surface
hydrothermal processes have emplaced that form of quartz in cavities
in schist. (I have personally found banded agate in cavities in vein
quartz eroded out of schist or phyllite.)
Tufa is a general name for surface deposits of calcium carbonate
left as a result of spring activity, especially near limestone
bedrock. It sometimes encloses plant materials. Travertine is
actually a bedded form of tufa used as an ornamental rock similar to
limestone or marble. The name is also sometimes erroneously applied
to the eruptive volcanic rock rock known as tuff, essentially a
solidified pyroclastic flow.
With respect to the application of heat, modern flint knappers
sometimes use heat to make their raw material more brittle and
easier to flake. Any cryptocrystalline quartz is somewhat porous, and
might contain enough water to cause fracturing if heated enough. So
care is necessary if using a torch in its vicinity. Heating is
deliberately used to make carnelian redder, so it is certainly
possible to heat these materials without destroying them.