Published Dec 16, 2020
We had our first snow of the season the other day, so it seems appropriate to share this post that answers a common question of potters (in cold climates anyway!). When a clay body freezes and is thawed again, it doesn't look or feel like it did before being frozen. But contrary to what you may have heard, this doesn't mean the clay is damaged and unusable.
It may take a little work, but as Dave Finkelnburg explains in this excerpt from Ceramics Monthly, it is possible to get the clay to a usable state again. –Jennifer Poellot Harnetty, editor
When a clay body freezes and is thawed again, it doesn’t look or feel like it did before being frozen. That doesn’t mean the clay is damaged. It can take some work, though, to get the clay to a usable state again.
Bound Water: Chemically bound water in clays consists of hydroxyl (OH–) ions that are part of the clay mineral crystal lattice and are not liquid water. The OH- ions are only released from the clay crystal structure when it is heated well above the boiling temperature of water.
Free Water: The fraction of water, expressed as a percent of the total weight of a sample of a clay body, that can be driven off by heating to the boiling point of water and holding at that temperature until the sample ceases to lose weight.
There is a persistent belief among some ceramic artists working in cold climates that if a clay body freezes, it’s damaged and unfit for use. That belief is wrong! While there are reasons to get rid of clay, the fact that the clay has been frozen is not one of them.
A clay body contains two kinds of water—free water and bound water. The bound water is part of the clay mineral crystal lattice. It is oxygen and hydrogen atoms held by chemical bonds within the mineral’s crystal lattice structure.
Bound water only begins to be driven off from the most common clay mineral when it is heated above about 707°F (375°C). The process isn’t complete until that clay mineral is heated above 1202°F (650°C).
Free water makes the clay wet and workable. Free water evaporates from the clay body into the air when the clay dries. When water is added to this dry clay, it absorbs the moisture and becomes workable again. If the moist clay body gets cold enough, the free water is the part that freezes.
Drying clay to bone dry and adding water to it again is a fully reversible process. Freezing and thawing moist clay is also fully reversible. No chemical change occurs. What changes is the physical distribution of the moisture in the body. Here’s what happens.
As the temperature falls, moisture from the clay body condenses on cold surfaces. This evaporation and condensation of free water can go on even after the clay freezes. When frozen clay is thawed, we see that the clay’s surface has more free (liquid) water, but the rest of the clay itself is necessarily a bit drier. Before and during freezing, molecules of water have moved through the air in the clay body. This is the same common phenomenon that occurs when dry, fluffy snow falls to the ground and then is stiff and solid after a few hours. Water molecules evaporate and condense within the snow. Thermodynamics is involved but we don’t need to go there.