Clay was undoubtedly the first binder discovered and used to make plasters. Likewise, earthen, clay-based construction is our oldest continuous building tradition. Clay is readily available only a few feet or as little as inches under the surface of the ground in most areas humans inhabit. Because of this, clay-based products such as roof tiles, pottery and bricks are benefits of clay technology that we almost take for granted. The topsoil in your backyard is likely a mix of organic matter, sand and clay. Having clayey soils is vitally important for most kinds of agriculture. As such, there is hardly a habitable region of the earth that doesn’t have vast, readily accessible clay deposits suitable for construction.
Understanding a bit of the geology helps to explain why clay deposits are so widely dispersed. Clays form from millions of years of mineral erosion. Mountains break down into boulders, boulders into rocks, rocks into pebbles, sand, silt and eventually, when the silt reaches a certain size of fineness, an amazing transformation occurs. Instead of just being a loose mix, the fine particles manifest an attraction for water and each other at a molecular level. Clay can be thought of less as a material and more of a behavior: the phenomenon of very finely eroded minerals to agglomerate into a sticky mass.