With winter approaching, it might be time to think about bringing in any outdoor ceramic art you might have (if you live in an area with cold winters, of course). Unless you know that the clay body it is made of is formulated a way in which it can survive freeze/thaw conditions.
If you are not sure if your clay body is up to the test, today’s post will help you figure that out. In this post, an excerpt from the Ceramics Monthly archives, David Scott Smith explains what makes or breaks (literally) a clay body in cold winter months. – Jennifer Poellot Harnetty, editor.
What Makes a Clay Body Weatherproof?
Why doesn’t vitrified ceramic survive freezing? First, we need to realize just how destructive the smallest amount of water can be. If trapped water freezes in ceramic, no matter how miniscule the amount, it will expand and can break the ceramic. The reality is, it’s impossible to ensure that zero amount of water will invade the small nooks and crannies that exist in all ceramic objects.
Why do some vitrified clay bodies withstand freezing water in some places while other vitrified bodies do not? Tile murals or sculpture that are sheltered and drain properly are less likely to succumb to freeze/thaw damage. In northern climates, sub-zero weather often means very little moisture, and fewer episodes of freezing and thawing.
However, the solution to the problem of creating a fired clay body that can withstand freeze/thaw conditions is evident in our surrounding architecture, in almost every fired brick and every terra-cotta roof tile—a testament to the fact that a clay body can be designed to withstand freezing temperatures. And yet, go up and lick a red earthenware brick or a terra-cotta roof tile. Your tongue sticks, right? Because the ceramic is still porous and absorbing moisture.
The solution, it seems, is not to make a completely vitreous clay body, but to create a clay body that can both absorb and exhaust water.
In Cushing’s Handbook, Val Cushing describes a successful freeze/thaw clay body as having a “pore structure.” Basically, a successful clay body must be riddled with capillaries and tiny veins that can push out the expanding water as it freezes. The fired ceramic can absorb water, but is also designed to expel water before it can cause damage.
A successful freeze/thaw clay has a pore structure that allows the water to naturally vent as it freezes then expands—it is never trapped. The clay moves out in a similar way that water freezing in a garden hose moves out of the end of the hose. With a vitrified clay body, the clay is almost completely fused, it will not absorb water. However, (like solid rock), small cracks formed during firing, or formed over time due to natural expansion and contraction caused by temperature changes, will allow tiny amounts of water to enter. This water expands when it freezes, and if it cannot vent properly it will break the ceramic.