Ah, so it's not just the expansion of water inside the pot I need to worry about but the oorous pot itself. Thank you!

just my opinion..my neighbor gave me a clay pot with a white glaze..I put it (empty) in my shed for the winter..it still got a hairline crack..eventually chunks of glaze started popping off by the crack..it was never out in the winter..my climate might be colder but I wouldn't leave clay pots out even if covered..

I second nicholsworth. I have heavy pots glazed inside and out that have survived for a decade, but others go in two years.

jana..I would say my glazed pot (outside only) that cracked was average thickness..our winter cold is so wicked I bring in every garden decoration and all pots..only 5 pots made of concrete stay out..I've seen ceramic pots that are labeled "frost resistant" but haven't bought any..it's not convenient but I would bring them in too..

Most pots will crack eventually when exposed to below freezing temperatures to some extent or the other. If water is present it will accelerate it. Unglazed pots will absorb water and lead to cracking in freezing weather. Glazed pots may also absorb moisture depending on quality of the glaze and how much surface it covers/protects. If fines cracks appear in the glaze then it will absorb moisture.

I have managed to save many clay pots to varying degree of success. The trick is to empty them early enough so that they can be let to dry in the Sun first. After that you can store them in a spot where they will not be exposed to moisture.

There are specially made clay based pots designed to withstand cold temperatures. They are expensive. They use special clay composition and fire at much higher temperatures. I have bonsai pots (with soil and plant) that they stay outside (somewhat protected), subjected to very cold temperatures and they have not cracked yet. But they cost like $50+ for a itsy-bitsy pot.

I do not know if there are sellers in US for large well made pots. I am sure there are but I do not think it is a big market for quality pots here in US. In Europe it is more common. Especially in England. The demand is high there and so the pots are a bit cheaper too.

I have a big (and valuable) collection of frost proof glazed ceramic containers, some that I've had as long as 30 years! All stay out all winter long and none have ever even chipped or splintered due to cold, let alone break or shatter. And it does get cold here, sometimes dipping down into the teens for a few nights. Maybe not a zone 6 cold but still well below freezing!!

Most of these are manufactured in Viet Nam and are shipped here by the container load. Every nursery or garden center in my area sells them and typically a very wide selection of shapes, sizes and colors. On the west coast, there is a very big market for these, likely because container gardening is a year round activity here. And there are numerous wholesale suppliers for these as well, focusing just on containers.

They are not inexpensive however, but since they last virtually forever (at least here) I consider them to be a great investment.

As an aside, I also have a collection of high fired terra cotta (Italian, not Mexican) that remain outside year round. Eventually they will develop some spalling or chipping and if cold enough, can freeze and shatter but many of these I have had for many years as well. Mexican terra cotta is low fired and will not tolerate cold or freezing at all well.

gardengal: is there a brand name for those vietnamese pots or similar ones?

No brand name :-) They come from countless small suppliers. The Chinese also produce them in quantity as well.