On 25 Mar 2018 14:00, Chris Hogg wrote:
On Sun, 25 Mar 2018 13:24:50 +0100, Jeff Layman
wrote:

I believe that all these
frost-proof tubs are fired at a higher temperature than the others. I
wonder if that introduces internal forces which sometimes result in
stress fractures?

Pots that are fired to a higher temperature are naturally stronger
than those fired lower. This is because the pores between the grains
of sand and clay particles that go to make up the pot get smaller with
rising temperature. The ultimate is for the temperature to be taken so
high that the whole pot just melts to a puddle of black glass, which
has virtually no pores at all. I've not done that with a flowerpot,
but have done it with a red house-brick by way of a demonstration.

But to some extent you're right to wonder about internal forces.
Nearly all pottery shrinks on firing, with the possible exception of
earthenware tiles, as the pores decrease in volume. What can happen is
that a slightly uneven temperature across the item in the kiln can
result in uneven shrinkage. In its most obvious manifestation, the
item becomes distorted and is rejected, but sometimes it's not
visually apparent. My wife used occasionally to restore porcelain
dolls whose heads had been broken. Even though she might have all the
pieces, sometimes they just would not fit back together because there
had been tension in the porcelain after firing that was relieved when
the head broke, and the pieces sprang very slightly out of shape.

I remember many years ago in the gents urinals at work, one of the
vertical porcelain partitions separating the individual stalls split
right down the edge and a half-inch gap opened up right down, showing
that there had been a lot of internal stress, relieved when it
cracked.

But as far as flowerpots are concerned, it's water freezing in the
pores that causes them to crack. The smaller the pores, the higher the
inherent strength of the pot, and the less water there is in those
pores to expand, so the more resistant they are to freezing and
cracking.

We have 5 large clay pots unaffected by this years weather, however
they are glazed all over and I have no doubt that is the reason. They
are waterproof in effect.

--
Regards
Bob Hobden