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.

For several decades clays have been used for decolorising mineral oils. A simple method is percolation of the oil through a layer of clay. Some twenty times the amount of oil is usually treated with such a layer. The first oil filtered through the fresh clay is much more changed than the rest of the oil. It has a very light colour and its low refractive index indicates a high content of paraffinic and naphthenic hydrocarbons.

Most of the aromatic hydrocarbons are left behind in the clay. This suggests adsorption analysis of oils using clay in the same large quantities that have been successful with other adsorbents (Mair and White, 1935; Willingham, 1939; Allibone, 1941). Quantities of the order of 100 gm. alumina per gm. hydrocarbon were used by Winterstein and associates (1934) when separating a number of aromatic hydrocarbons.

The term colloid is now generally applied to any system which contains a large area of phase boundary per unit volume. There can be no doubt that clay minerals rank as colloids and it is very desirable that clay mineralogists and colloid chemists should be brought together as in the present meeting.

The skill displayed by clay mineralogists in overcoming the technical difficulties with which they were confronted has been remarkable, and the benefits derived from their painstaking investigations are already being felt outside the immediate field of their studies and particularly in other branches of colloid science. To have precise information about the dimensions and crystal structure of the particles in a colloidal system can make all the difference between success and failure in distinguishing between alternative explanations of colloidal phenomena.

It is nearly a year now since we commenced the systematic investigation of our local soil clays in the North-East of Scotland by the powder method of X-ray analysis. In Aberdeenshire, one of the most wide-spread Soil Associations is the Insch Association, and this is the one I want particularly to talk about to-day. An association consists of soils derived from similar parent material, and the parent material of the Insch Association is till derived from basic igneous rock—mainly hypersthene gabbro, but sometimes with olivine also present.

The absorption of dyestuffs by montmorillonite can be attributed to two different processes, namely (1) base exchange and (2) van der Waals adsorption. There are some mixed cases, but usually one of the two effects is predominant and can be easily distinguished by several of the following features (see table 1.).

This paper deals with the base exchange type of adsorption only and in particular with the quantitative relations between the adsorbed dye and the released exchangeable cations. The simplest assumption, which is very often taken for granted, is that the two quantities are equivalent and correspond to the base exchange capacity of the montmorillonite.

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.

The mode of formation and the particle size distribution of potters' clays, particularly china clays and ball clays, were discussed in this paper, emphasis being laid on the proportion of colloidal material present. From this information, it was found possible to estimate the proportion of colloid in pottery bodies. After a brief indication of the structure of the clay minerals involved, the colloidal properties of clay were considered in some detail, particular attention being paid to the development of the Gouy-Freundlich diffuse double layer and the effect of exchangeable ions on this.

The theory of deflocculation of clay slips is based on the diffuse double layer and the development of an adequate potential. The anions are considered to be located on the surface of the particles which have associated lyospheres the size of which is dependent on the cation present.

When moist clay bodies are heated on one side it is often observed that moisture moves towards the cooler side. This phenomenon has been studied using clay-water mixtures of initially uniform moisture content packed into sealed, vertical tubes, along which a constant temperature gradient is maintained, the tubes having a reservoir of water at constant temperature at their lower, cool ends. Figure 1 shows the moisture distributions obtained after various times in columns of a commercial kaolin (Supreme Kaolin) which contained initially 16-18 per cent. moisture. A “stepped” type of distribution results from the movement of the moisture, the step moving down the column with increasing time.

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.).

From this investigation it was hoped to obtain information about the position of the exchangeable cations. That they take up a position between the layers of the mineral seems obvious from the varying c spacing of dried montmorillonite saturated with different ions, but in the complexes with water and organic liquids it was not known whether the ions remained close to the layers or were removed from them. It was also hoped that the Fourier syntheses would give some information about the actual layer structure of the mineral (montmorillonite) and thereby lend support to one of the two suggested structures, those of Hofmann, EndeU and Wilm on the one hand and of Edelman and Favejee on the other.

The montmorillonite used was a sample of Wyoming bentonite separated on the Sharples supercentrifuge. In order to get as large a contrast as possible between the two syntheses it was decided to use Li+-saturated montmorillonite and Ti+-saturated montmorillonite.

The investigations on clays progressing at the Macaulay Institute are fundamentally connected with pedological studies. The general aim of these investigations is to provide evidence which we believe will give us a better insight into the fundamental differences between the soil types as mapped in the field and provide us with an explanation of certain of their characteristics and properties. Within the last year a systematic investigation has been commenced into the clay minerals associated with each different soil type in Aberdeenshire. The methods being used are X-ray examination and differential thermal analysis. A brief account of the X-ray work has been given by Mr G. F. Walker.

The hydroxides of the divalent metals have an octahedrally coordinated structure of hexagonal symmetry typified by the “cadmium hydroxide” structure, familiar to clay mineralogists as the “brucite” structure. The weakly electropositive metals form insoluble or slightly soluble basic salts in which the ratio increases as the initial concentration of the unhydrolysed salt is decreased. A pseudo-Cd(OH)2 structure is formed when this ratio is of the order of 20 to 25 in which the octahedral sheets are separated by a constant distance proportional to the radius of the anion of the unhydrolysed salt. The resultant characteristic interplanar spacing is stabilised by a very small number of anions, the remaining interlamellar space being filled with amorphous hydroxide. The latter is known as the “α -zinc hydroxide” structure since zinc shows its similarity to its fellow transitional group metals by forming basic salts with this structure (Feitknecht, 1938).

The function of the bond in moulding sand is that it should make the sand sufficiently plastic for moulding and give it sufficient strength in the green (moist) and dry states after compaction to retain its shape and later to support the weight of the metal. It must do this while still leaving the sand permeable to gases, and it must further retain its strength in contact with the hot metal for a sufficient time for it to solidify, after which it should be sufficiently weak to be readily removable from the finished casting.

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.

The use of thermal analysis for the identification of clay minerals differs from its use for the determination of phase transitions in that the latter are, in theory at least, reversible and occur at definite temperatures while the dehydration reactions utilized in the identification of clays are monotropic, irreversible decompositions proceeding at a velocity which increases continuously as the temperature is raised. There is, therefore, no definite transformation temperature and the temperature of the arrest registered on a heating curve depends on the heating rate employed.

In view of the increasing importance of thermal analysis it was decided that an attempt should be made to study the implications of this. An investigation of the kinetics of the dehydration reaction seemed the most obvious approach.