A clay soil must contain at least . . . . . . . . Clay separate, to be designated a clay

Clay soil must contain at least 35% clay separate, to be designated clay.
Clay soil is comprised of very fine mineral particles and not much organic material, which makes it quite sticky and it does not drain well at all.
Clay develops plasticity when wet, due to a molecular film of water surrounding the clay particles, but becomes hard, brittle and non-plastic upon drying or firing.