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66 ISDUSTRIAL TESTS FOR PLASTlcIjy

the test is not fully understood, but it has • useful infonnation, not only for ceramic clays^K? ałso in the studying of soil-fonning processL’. of the effects of climatic factors in soil plastik in the field. For example, in the process ^ podzolisation, the morę plastic constituents of j? soil are washed down from the layers nearest th surface to lower layers; and this process ca_n be followed by studying the changes in flow-pIasticitv with distance from the surface. The method is ałso used by Scott Blair and Yates to study the effects of rainfall, temperaturę, etc., on the plasticity of soil in the field.

Mellor describes an empirical method for measuring plasticity by sąuashing a sphere of clay under stress until cracks appear. Modifications of this method are numerous in ceramic practice: for instance, Pfefferkom describes an apparatus for erushing clay cylinders. Bowmaker follows another of Pfefferkom’s methods in dropping,a weight from a standard height on to a cylinder of clay. _ This is done at a series of moisture contents, and the height of the cylinder after deformation is plotted against the moisture content. The plastic moisture rangę is thus defined, and a plasticity factor evaluated

R(R-^r),

where R is the amount of water reąuired to ,give complete sąuashing, and r the amount ęeąuired to cause just appreciable deformation.

A great many similar methods are used as measurements of plasticity, as well as othęr niethods which, instead of measuring plasticity directly deal with various other colloidal feperties,’

»XT^th_P^yiSticf_y:^which “^{e more}

EFFECTS of electrolytes

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„    ' and Atkinson compare three methods for

^BT^in^he plasticity of hot coa1. The first Amost generally accepted method is the Davis e in which a cylinder containing coal is rotated Sowly, and the torąue on an inner cylinder is jneasured. The Agde-Damm method measures the compression under weight, and the Layng-Hathome the pressure reąuired to force gas through hot coal.

Electrolytes have a veiy marked effect on the plastic properties of clays. In the casting process, a clay body is shaped on an absorbent casting of plaster of paris, enough water being removed by the conseąuent absorption to make the clay firm enough to retain its shape. Sodium silicate, carbonate, etc., are added to cause a condition in which a minimum change in moisture, and hence in shape, is accompanied by a maximum change in consistency.

Simonis measures the flow of Żettlitz earth sus-pensions through an orifice, treating the materiał with different ąuantities of caustic soda. His flow curves are linear, and the yield-value decreases with increasing concentrations of caustic soda; but the method is not a sound one for obtaining flow curyes.

Many later workers have studied electrolyte effects. Demolon and Barbier study the changes in viscosity, in an Ostwald viscometer, of z per cent. clay suspensions, and find that the viscosities pass through maxima with increasing concentrations of hydrochloric acid and calcium chloride, and minima with sodium salts.

In the case of clay, adds and polyvalent cationsv which tend to flocculate the suspension, usually increase the shearing strength and decrease the mobility; whereas substances such as caustic soda, caustic potash and ammonia, which defloc-

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