The industrial uses of clay minerals as catalysts date from the early'thirties. The application of catalysis to the thermal cracking of oilstarted in about 1931; pre-heated oil was passed downwards through fixedbeds of granular catalyst, often attapulgite, under pressure (80 lb. per sq.in.) and at about 900°F., as in the Houdry and Hydroforming systems. Thecatalyst had to be periodically burned off to regenerate it, and wasreintroduced at the top of the reaction column. An improvement on thisstatic process came when the granulated catalyst was kept moving countercurrent to the pre-heated oil. This led about 1939 to the fluid-flow methodof catalysis in which powdered catalysts are used in a fluid, free-flowingcondition, circulated by the air-rift method well-known in the movement ofliquids.