The rheology of clay may be divided into three almost distinct fields; the study of liquid “slips,” i.e., the relatively dilute liquid suspensions used for casting, mainly in the manufacture of pottery, or of plastic bodies, mouldable but capable of retaining their moulded shape under their own weight, or of the deformation of the fired material at high temperatures. Consideration of time available made it necessary to restrict the present considerations to the second of these, the plastic state.

Suspensions of clays in organic liquids change considerably in rheological character from liquid to liquid or when additions of a second liquid are made; thus, for example when about 15 per cent. volume of ethyl alcohol was added to a dispersion of a sample of bentonite in water the specific viscosity and thixotropy (as measured by rate of the sol→gel transition) increased to a maximum in spite of the-fact that alcohol itself yielded low viscosity suspensions.

My contribution to the Exeter meeting consisted of two parts: (1) a summary of the X-ray work done at the Macaulay Institute for Soil Research in Aberdeen, on complexes of montmorillonite and halloysite with organic liquids; (2) a discussion of the free energy relations in such complexes. The second part was similar in scope to the contribution to the recent Clay Minerals Group meeting, so this account will be confined to the first subject. A brief account of it is justified, for although a full paper is in print, it has not yet appeared (Trans. Faraday Soc.).

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

Activated clays may be used as catalysts for the polymerisation of vinyl aromatic substances such as styrene, α-methyl styrene or mixtures of these two compounds when in solution in an aromatic solvent.

The catalytic cracking process, which first came into prominence during the recent war, was one of the means adopted by the petroleum industry to meet the demand for aviation spirit of high quality to fuel the Allies' air forces. It was applied mainly in the United States and quantitatively made the largest contribution of any process. The process is a modification of the older thermal cracking process, the presence of the catalyst bringing about at a lower temperature the breakdown of large hydrocarbon molecules into smaller ones (some in the petrol boiling range) whilst at the same time promoting side reactions which improve the quality of the spirit.

The yield of gasoline on distilling crude petroleum is of the order of 10 to 45 per cent., but in order to satisfy the enormous demand for motor fuel this yield may be increased by thermal treatment of heavy fractions such as gas oil at temperatures in the region of 500°C. Under these conditions, the oil is “cracked” or broken down to give gas, gasoline, and heavier fuels. Cracking increases the anti-knock value of gasoline as well as the yield.