Clay was a mineral substance which could be moulded.
According to Romana II, limestone and clay was combined with water to form cement. (TV: Destiny of the Daleks)
In 1918, Banham, under the control of the Dark Forces, animated clay to form walking "corpses" which he unleashed in Yorkshire. (PROSE: Casualties of War)
Mehendri Solon had a clay bust of Morbius. (TV: The Brain of Morbius)
The Seventh Doctor said that when he felt depressed, he would make clay models of Zygons to cheer himself up. (PROSE: Human Nature)
Golems were usually made out of clay. (AUDIO: The Beast of Orlok)