OXIDATION.
Definition of terms—"Oxidation" and "Reduction" are chemical terms referring respectively to taking on and giving off of oxygen. When a piece of iron is rusting it is becoming oxidized, i. e., the metal (Fe) is being converted to an oxide of iron which is red in color. Iron rust can be reconverted to the unoxidized metallic state again by application of heat under reducing conditions, i. e., condi tions that favor separation of the metallic iron and oxygen. When the quantity of oxygen in combination is reduced, then it is said that re duction has taken place. When the quantity of oxygen in combination has been increased then it is said that oxidation has taken place.
Evidence of reduced condition in raw clay—"Blue" clay and dark gray shale owe their characteristic blue color, in the main, to two classes of substances, (1) the ferrous compounds, principally ferrous car bonate and (2) carbon. Partially metamorphosed carbon adds to a clay mass its characteristic black, just as does lamp black when added to what would otherwise be a white mass. Lamp black, an amorphous form of carbon, is the product of decomposition of carbon compounds under the influence of heat, resulting from conditions that prevent its complete oxidation. The carbon in shales, at one time a part of the fibrous tissue of living plants, was buried in deposits of sea mud, and is found today in this same mud hardened into shale. Therefore, the dark
iron compounds and the metamorphosed remains of carbon compounds combine to give the characteristic blue color to shales and many fire clays.
Evidence of oxidation in raw clay—Where the shale is covered with only a very thin "stripping," the color of the upper three or four feet of the bank will be red. In the lower portion of these red strata the color shades off gradually into the blue of the more solid strata below. In this red portion near the top of the bank the ferrous compounds have been oxidized to ferric compounds by the action of the oxygen from the atmosphere. Below the belt of weathering, the clay retains its blue color owing to the fact that either air cannot penetrate to those depths or that its oxygen is largely spent before it can reach the lower limit of the belt of weathering. It is observed that oxidation starts at the surface and proceeds downward. The depth to which evidence of oxidation can be seen depends upon the nature and amount of the oxidizable mineral present, the solidity of the rock mass, the prevailing atmospheric conditions and the length of time of exposure.