In moist clay bodies the clay particles are layered like a deck of cards and as you know, clay bodies contain both physical and chemical water. The physical water is what allows clay platelets to glide back and forth on each other. These water molecules surround each clay platelet. It is also called water of plasticity as it lets the clay stretch and move and keeps your pots in one piece.
Physical water makes up between 30-40% of the total weight in a bag of moist clay. But what happens when that water freezes? How does freezing affect the bag of clay you left in your car overnight? When water freezes it expands to become a solid crystalline structure (ice). The expansion of the physical water pushes the clay platelets apart. Since water is the binder for the clay platelets, after it freezes and thaws, the little clay bits are left stranded. The clay will now look fractured because the thawed ice leaves a network of tiny cracks, sort of like layers of shale. Because of capillary action, some parts of the clay will be dry and crumbly while the bottom of the bag will be super squishy. The clay will look like a total mess and many potters would throw it away. But surprisingly the frozen and thawed clay can be just as good, if not better, than it was before.
However, there is a catch. The clay platelets have to be forced back together through mechanical means. The thawed clay must be re-wedged or run through a pug mill before it can be used. And be prepared, it takes quite a bit of wedging. But it is worth the effort. The frozen and thawed clay may be more plastic than when it was fresh. It will certainly be softer. I have used freezing and thawing to my advantage in order to save stiff clay. A few cold nights outside with some good wedging and presto- I've got great clay!
Keep in mind that even if the clay was only slightly frozen, it has to be reworked. To prove this point, I placed a wedged two-pound ball of clay in the freezer for about an hour. After it warmed up enough to be worked, I rolled it out into a slab. It looked fine at first but once it dried there was a network of minute cracks on the surface. Along the folded planes the cracks were more pronounced.
So what about leather hard and greenware pots in your frigid studio? They won't fare much better than a bag of clay. If freshly thrown and leather hard pots freeze they will develop the same type of flakey cracks as the bag of clay. They might look really cool, but it is best to throw them in the slurry bucket. If you proceed with bisquing , the cracks will get worse and the whole piece will be crumbly. Dry greenware can sometimes survive a freeze if your studio is especially dry. But if you work in a humid environment, the increased moisture in the atmosphere will cause the greenware to crack, too. Bummer.
All things considered, to save yourself quite a bit of work it is best to protect your clay and wares from the elements. But if you do find yourself with a few frozen boxes of clay, don't despair! After all, our materials have gone through millions of years of erosion, freezing and thawing. What's one little night at 25 F going to hurt?