yes most of my unglazed ones crack and flake even in mild winters.
If you really treasure them they will have to be taken under shelter.

Quite right but for some reason the hand-made terracotta pots seem to be weather-proof. I wonder if they use different clay to the mass-produced ones. The old Victorian ones just seem to go on for ever.

Probably depends on the firing temperature when they were made. Frost proof ones are usually sold as such these days. Porosity is more significant than thickness as its water freezing in the pores that does the damage. If you remember where you bought them I'd ask them.

For me, modern clays fall into two groups. There are the ones on a special display table in garden centres, hard and smooth skins and with a distinctive white lable with bar code that is the devil to remove! These are produced in Germany and fired in a very hot kiln and are frost proof - at least for a good number of years. Then there are the ones often sold in superstores, more of a soft biscuity texture and colour and often with the word Italy stamped in the base. These are produced in the Mediteranean region, fired in a cooler kiln and are not frost proof.
Agree with Jimmy, I am fortunate enough to have a number of old pots and they are very tough.

I had a beautiful painted clay pot & that cracked, I was told if you leave them outside you must put those little feet under them to stop them cracking, I now do this & have been lucky, no more have cracked :o))