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.
Wedging Versus Gym Membership
Studio clay tends to be stored in plastic bags these days. When it is frozen and then thaws out again, there is some water in the bag. This may make the clay appear as if something is wrong with it. In fact, though, the only thing wrong is the gooey wet surface of the clay we see in the bag is the outside of a block of clay that’s now drier on the inside than it was before freezing. The freeze/thaw cycle has changed the distribution of water in the body. Thorough wedging to remix the moisture so it is uniform throughout the body will have it looking and working like new again.
An efficient way to wedge clay for any reason, including clay that’s been frozen, is the stack-and-slam wedging process. This technique mixes the clay thoroughly. The physical effort required is minimized for the mixing achieved. (See the method on page 59.)
This is where the artist has to seriously consider whether it’s worth the effort to wedge clay that’s been through a freeze-thaw cycle. Access to tools like a clay mixer or pugmill can affect the decision. The willingness to make a regular wedging session an alternative to a gym membership and workouts there may also affect the decision. The value of the clay and the value of the work being produced are also factors.
If wedging thawed clay appears to be too daunting a task, one may want to consider donating the clay to a student artist, a school, or a community studio, provided the clay is usable in their program and they have the willingness and possibly tools to take on the task.
the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.
The Stack and Slam Wedging Process
by Michael Wendt
Choose a comfortable amount of clay for wedging. I seldom wedge less than 3 pounds because it is too slow to wedge each piece one at a time. I prefer to wedge enough clay for several pots at one time because it is just as easy to wedge a large amount as it is to wedge a small amount with this stack-and-slam wire-wedging technique.
First, block the clay into a roughly rectangular shape. Next, use a wire tool to cut the block in half. Place the two halves with the cut sides parallel to the front edge of the table. Take the piece closest to the table edge and carefully place on top of the other half. Be sure not to make any dimples in either of the surfaces. Roll the joined pieces toward you, flipping them over so the bottom is now on top.
Pick up the clay and slam it down with enough force that it ends up the original thickness you started with. Repeat these steps (from blocking the clay to slamming it down) at least 30 times. This will give you over a billion layers of clay particles!
It is very critical that you pay attention to the lamination pattern since the final goal is to layer the clay rather than cross the layers with each other.
Once you have completed the required number of cycles, place your left hand on the top of the piece and roll it to the left onto its side. Now your right hand can be placed on the area that was the bottom on the table surface. The goal is to keep track of these two surfaces while converting the block into a cylinder by repeatedly tapping it onto the table surface and finally rolling it round.