74 CLAYS AND CLAY INDUSTRY. 



and carbonaceous character of the organic matter found in the 

 clay is due to its having decayed, out of contact with the air, 

 such conditions bringing about a slow carbonization of the 

 material. In some clay beds a considerable layer of organic 

 matter may accumulate, producing a bed of coaly material, which 

 represents the early stage in the formation of true coal. 1 



Many surface clays contain organic matter in the form of 

 plant roots that have grown down into the clay from the sur- 

 face, but the effect of these is probably not very material. The 

 amount of organic matter which a clay contains is usually small, 

 and 3 per cent, or 4 per cent, may color a sandy clay deep black. 

 The depth of color will, however, be influenced by the way in 

 which the organic matter is distributed in the clay, finely divided 

 and evenly distributed particles producing a deeper and more 

 uniform tint. As long as any organic matter remains in the 

 clay, it will tend to exert a reducing action during the burning, 

 because the carbon, in changing to carbon dioxide, requires 

 oxygen, and takes it greedily from the atmosphere within the 

 kiln. Under such conditions, there is little opportunity for other 

 compounds requiring it to get any, until the organic material has 

 been consumed. The organic matter in clays will, however, pass 

 off during the early stages of burning, if sufficient air can enter 

 the kiln, and the clay is not heated too rapidly. In fact, if the 

 clay contains considerable organic matter, the combustion of 

 the latter may even add to the temperature of the kiln. In burn- 

 ing clays, which by their color clearly show that much organic 

 matter is present the burning should proceed slowly until all the 

 organic matter is driven off, for, if this is not done, the surface 

 of the clay may burn dense, and form a skin which the oxygen 

 cannot penetrate. Consequently, the carbonaceous matter will 

 not be consumed, but will remain in the interior of the ware. 

 Moreover, the clay may swell up and become porous, due to the 

 presence possibly of hydrocarbons (compounds of hydrogen and 

 carbon), which are decomposed by the heat, and the gases, in 

 endeavoring to expand and escape, bloat the clay. Many of the 

 Clay Marls show this phenomenon, if heated too rapidly. 



See description of Raritan clays, Chap. XVIII. 



