Where to find clay?
Clay is a product of erosion of feldspar and other minerals. Feldspar, a large group of silicate minerals, is by far the most abundant group of minerals on Earth making up of about 41% earth crust by weight.
Clay is found in subsoil below topsoil and above rock bed. Topsoil, usually dark brown and black, are rich in organic matters from decaying organism, which is not suitable for plastering. Subsoil can usually be found in a depth of 30cm below ground.