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.