People have been using clay since the Stone Age, but what exactly is it? Why does clay make such good ceramics?
What is clay?
Clay forms wherever rocks that contain aluminum and silica are exposed to air, water, or steam. It’s all over the place, in other words – from mountains through volcanoes and geothermal fields to the ocean floor – and usually mixed in with a lot of other organic and inorganic stuff.
Not surprisingly, clay doesn’t have a single chemical formula. It is defined by its physical properties, and you can easily guess what these are. Clay absorbs or loses water easily, for instance, and it swells when wet.
Here are the two key properties of the clay group of minerals when it comes to ceramics:
- They’re soft and made of tiny crystals (less than 0.004 mm in size) that are arranged in sheets
- Some are plastic when mixed with a little water – you can deform them and they will hold the new shape
That plasticity, of course, is why we started using clay in the first place. No one is sure why it happens. Small grain size, chemical bonding, and water’s lubricating effect on the stacked crystal sheets all certainly have something to do with it.
There are three basic types of clay minerals, but we’re just going to look at the type of clay that’s usually used in ceramics.
Kaolinite forms when rock, especially granite, undergoes chemical weathering in a hot, acidic environment where there’s a lot of water, say, in a tropical rainforest or near a hot spring.
You’ll find it in a deep bed of bright white clay, though sometimes it’s stained yellow, brown, or even bluish-gray with impurities.
Its name comes from the Chinese Kao-ling, the name of the hill in China where it was mined for centuries.
From kaolinite comes kaolin, or China clay, the pure white powder that’s used to make porcelain. Other materials are sometimes added, depending on the desired results in the finished product.
The Royal Society of Chemistry describes kaolinite much more authoritatively than I ever could:
Crystallographic studies have established that the clay minerals are composed of sheets of tetrahedral silicon dioxide (SiO2) and octahedral aluminium oxide (Al2O3) linked through bridging oxygen atoms…Broadly speaking, there are two main categories of clay minerals: those with one sheet each of the silicon and aluminium oxides and those with two sheets of silicon oxide enclosing a sheet of aluminium oxide.
Kaolinite has one sheet each of silica and alumina. These sheets are bonded very weakly and so can be moved around easily.
Below is a graphic of parts of two kaolinite crystals. Each part has a tan sheet of silicon and a gray sheet of aluminum. The red is oxygen (see how everything is bonded to it?) and the white is hydrogen. Red and white layers are hydroxyls – pay particular attention to those.
When making ceramics, you want that plasticity long enough to shape the clay, but then you want to make your item hard. For starters, let it dry out in the air. This takes out all the atmospheric H2O, forcing those hydroxyls into more hydrogen bonds. The end result: “green ware.”
Your piece is still going to lose its shape if it gets wet again, so it’s time to heat it. Firing a piece of pottery not only drives off all the water, it also disrupts those hydroxyl-hydrogen bonds. The hydroxyls turn instead to oxygen, which makes a much stronger bonds.
By 500 degrees C, your ceramic piece is permanent but fragile. Turn up the temperature to 1000 degrees or more, and the hydroxyls now build cross-links that are three-dimensional and quite strong. Components in the clay are melting together, too, which will make it even stronger when the item cools down.
Clay makes good ceramics because it’s plastic enough to shape and then, with some heat, it turns into something that’s sturdy, long-lasting, and useful. Humankind first learned how to do this many tens of thousands of years ago, and we are still making progress in ceramic science today.
- The Clay Mineral Group. Amethyst Galleries
- The Chemistry Of Pottery. Stephen Breuer
- What Chemical Changes Happen When Pottery Is Fired in a Kiln? : Making Pottery. eHowArtsAndCrafts
- History of Ceramics. Andrea Silnes
- Weathering and Clay Minerals. Tulane University
- Environmental Characteristics of Clays and Clay Mineral Deposits. US Geological Survey