Terra Incognito: The Method Behind the Form

There are various forming methods than can be used when working with clay.  As a self-proclaimed clay nerd, I can tell you that one of my favorite parts of the journey is actually forming the piece. There is a “form” and there is a “surface” and if you learn to treat the two as separate aspects of the same whole, you will have a successful composition. In ceramics, there are many ways to form something from clay. For example, a piece  of clay can be thrown on a potter’s wheel, press molded, slump molded, pinched into shape, coiled, built with slabs or extruded.

For this article I want to focus on just three forming methods: Throwing, Press Molding, and Coiling.

Throwing
To “throw” is to form clay using a potter’s wheel.  As the clay spins on the wheel it is then “lifted” into shape. This is a very popular method, usually used to fashion functional or utilitarian pieces, such as plates, cups, bowls, teapots, or containers.

The word “throw” comes from the old English meaning to twist, turn, or propel—a perfectly apt definition. The potter’s wheel is a relatively new invention in the 30,000 year history of ceramics, and not used by all clay cultures. Much of the Greek pottery at Eye of the Day is wheel thrown.

Press Molding

Press Molding is when a piece of clay is actually pressed into a mold or form. The mold is made from an original form which is made from clay, then reproduced in Plaster of Paris, which becomes the mold so the shape can be replicated over and over again. This is a particularly helpful method when forming large pieces that would otherwise be hard to reproduce. The Italian terracotta carried at Eye of the Day is traditionally press molded.

Coiling

Coiling is one of the oldest methods for forming clay. The material is rolled back and forth until it looks like a “rope” of clay. You begin with a shape in mind, then the coils are placed on top of each other until the desired shape is formed. This is a particularly slow method, the clay needs to harden a bit before more coils can be stacked, so a potter will begin one piece and then move to another to begin, then move to another, and so on. By the time he has begun say the fourth or fifth shape, the first one will be stiff enough to continue the next phase of the shape.

Eye of the Day has an array of vessels produced using each of these traditional methods, authentically manufactured and imported directly from the very best potteries of the world.

As the days grow shorter, and we inch toward the certainty of a cold winter, it seems fitting to talk about fire. I love fire. It takes a deep understanding of this primary element to master the process of “firing” clay. In my last installment we talked about frost proof ceramics and even that conversation must begin with a discussion about fire.

Clay begins as granite, decomposes and compressed for a millennia, then becoming a sticky, malleable mud with memory. When it passes through human hands, turning it into functional art… the real fun begins: the firing. Once the clay morphs all the way back to stone, it changes from stone to mud and back to stone again, transformed by fire. I feel lucky to be in the middle of this cycle, the one that gets to instruct the clay. Kind of a big responsibility really, manipulating the earth while it regenerates itself.

It takes a lot of heat to fire ceramics and even more heat to fire clay to vitrification (refer to Baked Earth installment How It Works: Frost proof Terracotta). Firing clay depends on what material we burn. This is why so many ceramic wares are under-fired: limited combustibles. In the developing world many traditional peoples have to travel further and further to harvest the wood or brush. As the forest recedes, they begin to use less fuel to burn the clay, because transportation is more difficult. Travel can take three or four times what it used to for procuring wood, or other combustibles, to fire the pottery, cook and heat their homes.

In the US, you will see many types of firings: wood, salt, recycled automobile oil, sawdust, dung, natural gas, propane, passive solar, and electricity. We are fortunate to have so many options within reach when it comes to firing pottery. Regardless of how the work is fired, some basic transformations occur on the clay’s long journey back to stone.

At 212°F the water in the clay converts to steam and is slowly released. At approximately 400 °F, all organic matter in the clay has burned out, and the cristobalite (a crystalline form of silica found in all clays) suddenly shrinks. Between 1000°F and 1100°F the quartz crystals change from an alpha crystal structure to a beta structure. Referred to as Quartz Inversion, the temperature must increase gradually.

From 1500°F to 1650°F, “sintering” begins. This is the stage where clay particles begin to cement themselves together to form a hard material called ceramic. From this point on, depending on the temperature range of the clay, vitrification occurs. For earthenware clays, this happens at approximately 2050°F. For stoneware clay, about 2300°F, and for porcelaneous clay, 2500°F.

The transformation of clay from stone and full circle back to stone is a long, hot journey that no other material makes. And here’s a little point of reference: Kilauea, the volcano on the big island of Hawaii, spewing lava into the Pacific Ocean was recently tested for temperature. The orange lava is only 1800°F—low fire in the world of clay!

See you next month!

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!