How It Works: Frostproof Terracotta

It’s hard to believe that it is already Fall. In California, leaves don’t change much in a drought, no matter what time of year. But as another October becomes a thing of the past, much of the country can already feel the promise of winter. So let’s talk about frostproof terracotta.

Many of you live and work in places that freeze, and much has been said, both by homeowners and garden designers about the freeze/thaw capability of ceramic plant containers. Remember that ‘clay’ is the earthen material, ‘ceramic’ is the clay when it has been fired. Terracotta is low fire red clay found all over the world. There are low fire clay bodies, and high fire clay bodies. Low fire is also referred to as earthenware, high fire is stoneware. Once clay is fired it becomes ceramic no matter what temperature it is fired to. But that doesn’t mean it is mature. I want to introduce you to two new key words: “vitrify” and “absorption.”

Every clay, whether earthenware or stoneware, has an optimum maturation point when fired. When this point is reached, the clay is “vitrified.” When clay becomes vitreous, it means that all the different ingredients in the clay have melted and bonded together to form a mature, very strong, “ceramic.” If a clay is over-fired it becomes flakey and worthless. If a clay is under-fired (like many of the pottery containers that are called “terracotta”) it is weak and porous and highly susceptible to cracking, even without being frozen. So when we talk about frostproof ceramic or terracotta, interestingly enough, we have to talk about fire first. In ceramics it always, always, always, comes back to the fire.

The key equation: the more vitrified, the less absorption. Did you catch that? Absorption yields expansion/contraction and hence cracking, during a freeze/thaw cycle. That’s basically it. You’ve got clay, you’ve got fire, you’ve got maturation/vitrification, and you’ve got frostproof ceramic. Or, more plainly put, if a ceramic vessel is not fired to its target temperature, it will die an easy death no matter what the weather is doing.  But if it will last longer than you do.  Just think about that for a moment.

Here is what you can do when you are ready to purchase frostproof ceramic containers:

1) Research the manufacturer’s credibility. If you are buying real, frostproof ware, know where it comes from and how it was fired, and get something on paper, like a warranty.

2) Protect your investment! Do not add moisture to a vessel that you know is going to freeze. And if you are going to take the time to cover the plants, take the time to cover the container too. Start to think of the ceramics as a living, breathing, expanding and contracting entity, just like the plant that it holds.

3) IMMEDIATELY call Eye of the Day. They have been selling frostproof terracotta for more than fifteen are not only knowledgeable, but relatively good looking and congenial, and carry the best frostproof terracotta on the market (guaranteed to withstand temperatures down to minus 15° F).

4) Move to California!

What is it about Italian terra cotta? Living in Southern California, we have all seen plenty of Mexican terra cotta, or should I say Mexican CLAY products (since they really aren’t terra cotta). It looks great in the California sunshine, but the clay seems to melt or disintegrate immediately with the constant spray of water from the Rainbird sprinklers working overtime throughout the summer, and winter can be even more destructive.

But not even all Italian terra cotta is created equal. Much of the pottery arriving in the United States from Italian factories is machine-made from mediocre clay found throughout the many regions of the country, but the terra cotta pottery at Eye of the Day is made from Galestro clay, found only in the Sienna Grossetto area of Italy, near and around Florence.

Most terra cotta (whether Italian,  Mexican, Greek, or French) requires a sealant to be applied at least on the interior of each pot to be planted in order to protect the integrity of the material. Terrecotte San Rocco products do not need to be sealed, the quality of the clay will hold up to extremes in temperature as well as constant contact with water.  Terra cotta from San Rocco is long-lasting and frost-proof, guaranteed to -15 degrees; the mineral content found in the clay, when combined with high firing temperatures, insures products of the finest quality and highest durability.  And it is beautiful.  Just say no to poor quality terra cotta, say yes to terra cotta for the centuries.