This word is used of sediment found in pits or in streets (Isaiah 57:20; Jeremiah 38:60), of dust mixed with spittle (John 9:6), and of potter's clay (Isaiah 41:25; Nahum 3:14; Jeremiah 18:1-6; Romans 9:21). Clay was used for sealing (Job 38:14; Jeremiah 32:14). Our Lord's tomb may have been thus sealed (Matthew 27:66). The practice of sealing doors with clay is still common in the East. Clay was also in primitive times used for mortar (Genesis 11:3). The "clay ground" in which the large vessels of the temple were cast (1 Kings 7:46; 2Chr 4:17) was a compact loam fitted for the purpose. The expression literally rendered is, "in the thickness of the ground,", meaning, "in stiff ground" or in clay.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Clay'". "Easton's Bible Dictionary".