MEASUREMENTS OF T H E PLASTICITY O F CLAYS B Y G . W . SCOTT BLAIR
Introduction The conception of that property of materials which we call Plastzctty* must have been a fairly important one, even in very early times. This is especially true in the case of clay, where the plastic properties of the material were of so great importance to the craftsman; but although the craftsman was able to judge the quality of his clay with considerable precision, Rrongniartl was right when he said of plasticity in 1844, “On a souvent paxi6 de cette propri6t6, on semble la connaitre, mais on n’en a qu’une vague id6e.” Ever since the time of Brongniart, scientific minds have been trying to define a n exact conception of plasticity, and to measure it. An excellent summary of these attempts is to be found in Mellor’s Treatise on Inorganic Chemistry, which it is not the purpose of this paper to attempt to reduplicate, but it seems that in spite of all the work that has been done, Brongniart’s remark is almost as true today as it was eighty-six years ago. There can be no doubt that the term “plasticity” has been used by different writers to mean very different things. I n some cases the conception may be that of a single physical property, but more often a convenient combination of properties, describing a workable condition of the material, is implied. Without being too dogmatic, we can agree with Mellor to exclude from our conception of plasticity those properties which do not belong to the wet material (in the case of an aqueous two-phase plastic such as clay.) “It is assumed [by certain authors] that the tenacity of the dried clay is proportional to the plasticity of the wet clay. This is generally, but not always true. KO known property of the d r y clay can be used as an infallible index of the plasticity of the wet clay,” (p. 485). We can scarcely agree that the proportionality is even generally true. Although more circumscribed definitions of plasticity have been suggested (e.g. Karrer*) it is thought that the most convenient formula is that of W i l ~ o nwhich ,~ is believed to combine an accurate description of the age-long meaning of the word with a comparatively simple scientific conception. Wilson says “Plasticity is that property which enables a material to be deformed continuously and permanently without rupture, during the application of a force which exceeds the yield value of the material.” I t is this general conception of plasticity that will be used throughout this paper. I t will be seen that such “plasticity” is not n e c e s s a d y related
* Plasticity, from the Latin, plasticus, Greek irXaurixbr = deformable, mouldable. Plasso (nXbuuw) = I mould. 1 “Treatise on the Ceramic Arts” (1844). See also J. W. Mellor: “Treatise on Inorganic and Theoretical Chemistry,” 6, 48 j (1925). 2 Ind. Eng. Chem., 21, 770 (1929); Anal. Ed., 1, I j8 (1929); Rheology, 1, 290 (1930). (This paper contains an interesting discussion on the relation of plesticity to flow data). 3 “Ceramics-Clay Technology,” 5 5 , etc. (1927).
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quantitatively either to the binding power of the dried material, as pointed out by Mellor, or to the amount of water which the material will absorb in order to attain its maximum plasticity. The latter property has so often been confused with plasticity, consciously or unconsciously, that many erroneous methods for measuring plasticity have not only been suggested, but have received a fairly wide popularity. This has been particularly true in the case of attempts to relate the plasticity of various colloidal systems to constants derived from curves relating volume of flow to shearing stress, when the material is caused to flow under shear. I t seems likely that some such relationship should exist, but previous attempts to define it have invariably resulted in the measurement of a property dependent on the amount of the dispersed phase present, or, where different materials are compared at the same concentration, on the amount of the dispersed phase remaining free; or in other words, on the amount of solvation of the material. I t seems generally to have been accepted at one time that plasticity is measured by flow data of this type. Bingham,‘ who showed that for many two-phase systems a t high rates of shear, the curve relating flow to shearing stress is a straight line which on extrapolation gives an intercept on the stress axis, headed his first discussion on the obtaining of these curves and evaluation of the slope and intercept “The Measurement of Plasticity.” Wilson3 states that the plasticity depends on a combination of both the properties measured by slope and intercept, and that plasticity can be measured comparatively by arranging that the materials are ident’icalwith respect to one of these constants, the other then defining the relative plasticity of the materials. (Ref. 3 . p. 108.) Bleiningerj is rather less definite, speaking of “plastic properties” rather than “plasticity” itself. I t is the object of the present paper to discuss the relationship between flow-stress curves and the plasticity of aqueous clay pastes, and then to consider how flow curve data can be used to investigate the causes and control of the plasticity of such clay pastes.
Some Empirical Plasticity Tests In order to compare flow constants with the plasticity of a clay, it is necessary to have some simple tests of plasticity depending fairly strictly on the definition, or accepted by the experts as giving on the whole a good rough measure of plasticity, as a standard of comparison. The author has developed a test for this purpose which will be described first, as it seems the most satisfactory of those used. Atterberg6 has described a test in which a plastic mass of clay in water is rolled out into a fine “wire,” the moisture content a t which the wire or thread just tends to crumble being recorded. This is called the “Lower Plasticity Limit” of the clay, and gives a measure of its hydration capacity. To measure “Fluidity and Plasticity,” 320 (1922). J. Ind. Eng. Chem., 121, 436 11920). 6 Internat. Reports on Pedology ( 1 9 1 1 )etc. See also Kinnison: U. 5. Bur. Standards, Tech. Papers, No. 46 (1925); Wilson: Ref. 3, p. 114. 4
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plasticity roughly, this test has been modified in the following way. .I small pellet of plastic clay is rolled into a thread or wire very carefully with the finger, on a piece of smooth paper. This is done at that water content which gives the thinnest possible thread before crumbling takes place. The determination of the right moisture content is done by trial and error in the hands, and with a little practise becomes fairly easy. The moisture content need not be measured, but the diameter of the thinnest “wire” of clay that can be rolled is measured with a gauge. This test is believed to give a good rough indication of the extent t o which the most plastic mass which can be made from the clay can be “deformed without rupture”, and appears to correlate well with other information about the plasticity of clays, For convenience this will be referred to as “The Wire Test”. Two other empirical tests have been used which give some measure of the plastic properties of a clay. The first, called “The Slaking Test”, has been useda good deal by practical ceramists. (It has also been called “The Bancroft Test.”)’ In this test, a mass of clay is kneaded under the thumb into a ball at what is termed the concentration of maximum plasticity, (though actually this is rather drier than the concentration of maximum plasticity as used for the wire test), allowed to dry out at room temperature over night, and then placed in distilled water in such a way that the material which falls away as the water replaces the cementing material in the ball, is able t o fall clear of the mass of the clay. The time taken for a ball made from a given weight of clay to disintegrate completely is termed the “Slaking-Time”, and it has been claimed that this gives a good measure of the plasticity of the clay. In view of what has been said earlier in this paper, it is not surprising that although in many cases this test measures something so near to plasticity as to make it highly useful, experiment shows that in certain cases the slakingtime does not depend directly on plasticity at all, but rather on other properties of the material.* The second, the “Oil Filtration Test”, depends on the phenomenon investigated by Nutting,* who claims that the extent to which a clay is able to remove coloured impurities from a heavy oil by filtration, depends on the presence of hydroxyl groups, and open bonds in the clay. Dr. Xutting suggested to the writer of this paper that this property might be related to the plasticity of clay, and although the method is quite indirect, and although the author has not been able to obtain such regular and reproducible results as has Sutting, yet there does appear to be a very close correlation between the oil filtration capacity of a clay and its plasticity. Since work on t h k test is still in an early stage, no detailed technique will be described. Having described a few rough tests by which some idea of the relative plusticity of a batch of clays may be obtained, we can now proceed to give an 7
Bancroft and Jenks: J . Phvs. Chem., 29, I
Z I(192;); ~ Jenks:
33, 1733 (1929)’ Boyd:
J. Am. 8oc. Test. Mat., 22, 337‘(1922); hliddlrton: E. S: Dept. Ayri. Tech. Bull., No. ii8
(1929). * Bancruft anti Jenks are quite clear on this point. 8 J . Washington .-lead. Sci., 18, 409 (1928). See also Haseman: J. Phys. Chem., 33, 1,514 (1929).
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account of the way in which flow-constants are obtained. The modified Binghanl plastometer used in this work has already been described.g d paste of the clay is caused to flow between two pipette bulbs through a glass capillary tube of known dimensions at a series of different pressures, the volume of material flowing through the capillary at each pressure being determined by means of a flowmeter somewhat similar to that described by Green.’” The shape of the flow curve thus obtained has been much discussed, both from the theoretical and experimental points of view (for references see (9) ), but in general, the following is a brief outline of its character. Up to x certain small, but quite definite pressure (a), there is no flow a t all (Fig. I . stage I , ) ;then follows a stage (11) in which the material flows as a solid plug through the tube, the flow-curve being rectilinear. This plug may
FIG.I
be regarded as sliding through a water-envelope of constant thickness. The intercept (a) may be regarded as the shearing-strength or yield value of this water-envelope, or as a measure of the adhesion of the plug to the wall of the tube, It appears that these water films show rigid properties up t o a thickness of at, least IO& cni. (minimum value).* K h r n another critical pressure ib) is reached, the curve slopes sharply upwards, and streamlining takes place near the wall of the tube (stage III.), the material still moving as a plug in the centre. .Is pressure is still further increased this plug diminishes at the expense of the streamlining sheath surrounding it, until finally almost the whole of the material is streamlining, the flow curve being again rectilinear (stage IV.). The extrapolated intercept (c) of this last straight line to the pressure axis gives Binghani’s “yield value”,4 or as we prefer to call it, thc shearing strength of the material. G . IT. Scott Blair: Rheology, 1, 12; (1930); G. W. Scott Blair and E. W,Crowther: .J. Phys. Chem., 33, 321 (1929); G. W. Scott Blair: J . Phvs. Chem., 34, I j o g (1930); R. K. Schofield and G. IT. Scott Blair: 34, 248 (19.30); B. A . ‘Keen and G. I?. Scott Blair: J .
Agric. Sei., l?, 648 (1929). i o .J. Am. hoc. Test. Mat.. 20,450 (1920). A detailed description of the apparatus and technique are given in the paper by Keen and Crowther, listed under Ref. 9. * This is c 50.
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