On 16 Jan, 11:57, "Jeff Layman" wrote:

Basically, frostproof means just that - it will withstand any amount of
frost. *That usually means it has been fired at such a high temperature that
it is waterproof, and water cannot soak into it. *As water expands on
freezing, the ice formed will crack anything the water has soaked into.
However, it might be a different story if the pot fills with water which
then freezes. *If the terracotta isn't thick enough, the ice might just
force the pot apart from the inside.

I think that only truncated conical pots ('straight sided') can be
truly frostproof - anything O-G shape or with the top narrower than
the middle will always be under some sort of threat from the contents
expanding. Flower pots are the shape they are, partly so that they
don't do that - but mainly so there is a large aeration area for the
smallest amount of soil.