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CLAY. A word that translates a number of different Hebrew words and one Greek word and is often used in the Bible in a literal or a metaphorical sense—in the latter sense meaning “dust” or “flesh” (as made from earth). Clay was widely used in OT times for the making of brick, mortar, and pottery, and, in some countries, for the making of tablets on which inscriptions were impressed (see Clay Tablets). Mud bricks were not always made of true clay, but of mud mixed with straw. True clay was variable in composition, giving variety to quality and color, and thus was suited for different uses. As a building material, clay has been used from very ancient times. Babylon was made wholly of brick, either baked or dried in the sun. Nineveh, the capital of Assyria, was made mostly of brick. The villages of Egypt were constructed of sun-dried clay.
CLAY, the name given to a group of minerals and to rocks composed essentially of clay minerals with grain size<0.004 meter. All clay minerals are hydrous aluminum silicates with a sheet-like crystal structure. There are three main groups: (i) kaolinite group with sheets (7Å thick) which are electrically neutral and made up of two different layers—the main clays in ceramics, china and pottery; (2) montmorillonite group with three layer sheets which are not electrically neutral and which have variable quantities of water molecules and exchangeable ions taken up in the interstices between the layers—the main clays used in pharmacy and cosmetics; and (3) illite group which are clay minerals similar to the micas—the main constituent of many brick clays. References to different uses of clay, such as the potter’s clay (
If mixed with water, clays become plastic, after standing for a short time. There are three stages in the hydration of clay mineral powders: (1) water is bonded completely to the surfaces of clay minerals leading to coherence; (2) additional water between the particles acts as a lubricant enabling them to move somewhat relative to one another, imparting plasticity to the substance, which can be shaped (
Clay minerals heated to 150o C. lose porewater and absorbed water. Water bound up in the crystal lattice is driven off between 400o and 900oC with the alteration or partial destruction of the crystal structure. Above 900oC there is extensive destruction of the crystal lattice with the development of amorphous substances, then new crystalline phases. Pottery resulting from the heating of clay provides a major record of past civilizations and has played an important role in survival of early MSS. The brittle nature of these clay products would explain such references as
Clay minerals are formed by the alteration of rocks, chiefly of igneous origin, most resulting from the processes of surface weathering. The weathered material either remains where it is formed, giving rise to residual clays, or is transported, mainly by water, and deposited as beds of clay, or mud, which when compacted are referred to as mudstone and shale. As clays represent extremely fine, broken down crustal material, and as they make a large proportion of soil which provides the mechanical and chemical environment for almost all plant growth so essential for human existence, ideas of the human form being made “out of the clay” (
H. Wirsch, Applied Mineralogy (1968), 102-106, 119-126.