Clay in Fausset's Bible Dictionary

Tough plastic earth, containing silica and alumina. Used for making pottery in Israel (Jeremiah 18:2; Jeremiah 18:6). Vessels of dark blue clay are still made at Gaza. Used by Jesus in curing the blind man (John 9:6), a mixture of dust and spittle. Doors are sealed with clay in the East, to facilitate detection of thieves. Wine jars were so sealed. It may have been with clay our Lord's tomb, and the earthen vessel with the proofs of Jeremiah's purchase, were sealed (Matthew 27:66; Jeremiah 32:14). At Koyunjik fine clay cylinders with Assyrian impressions have been found, which were made by rolling the seals on the moist clay, which was then baked in the fire.