T H E PLASTICITY OF CIA.lY1 BY WILDER D. B A S C R O F T . i S D L. E. J E K K S
-1plastic substance is one that, can be inoultletl under a finite premmre. This connotes that any break due t o the deformation is self-healing. ,1 niohile liquid i s not plastic because it will no-t retain its shape. With increasing viscosity, a liquid or a suspension passes through the point of zero fluidity and thew is a change, cs Binpham has shonn. from viscous to plastic flon-. Molasses candy is plastic until it crys-tallizes. Glass becomes plastic when heated t o a suitable temperature. These are cases of plastic liquids. ('rj-stalline solids may become plastic under suitahle conditions. I\Ie tals are squirtetl into rods or pipes because they are plastic under high pressures. The flow of :Iglacier is clue, at least in par.t, t o the plast'icity developed in ice tintier high pressures. Retncen the estreme cases of a solid such as a metal and a liquid such as glass or molasses candy, we have the intermediate case of putty, which is a mixture of whiting and oil, a solid with a liquid filiii rou~iclit. If a liquid is adsorbed strongly hy a solid, thin films of the liquid will hold the solid particles together, while still permitting thein t o move relatively to one ano-thcr. Thc tentlency of the liquid surfaces to coalesce causes any break to heal at one('. -1liquid film will therefore act as a bond for solids and may make thr. inass plastic. One kind of oil is used as a hinder in roads and another t o make putty out of calciuin carbonate. Everybody knows that dry sand cannct 1w moulded whereas wet sand can. The clay workers use the word plasticity in a special sense to denotc that a clay is plastic and that it will burn t o a coherent niass. From this point of view a metal is not plastic. because it melts when heated: and wet sand is riot plastic. because it falls t o pieces when dried. Clay particles must therefow have 011 the surfaces some form of gelatinous ma-terial which burns to a solid hond. Since clay is essentially a hydrous aluminum silicate, so-called, the gelatinous material must he essentially hydrous alumina, hydrous silica. or soine intermediate coniposition. The difficulty is that neither hydrous alurnina, hydrous silica, nor any mixture of t,heni will take up water again to form a satisfactory gelatinous mass after having been dried out pretty thoroughlj. in a hot sun. If we designate our gelatinous material as a so-called hydrous aluminum silicate with hydrous alumina and hydrous silica as the two possible end ternis, there must be present some other substance or subs-tances which keep the so-called aluminum silicate from losing its plasticitmywhen dried in the sun. The two possible things to consider are electrolytes anti organic matter to which we will give t8hegeneral name of humus. The problem that, N r . Jenks set himself was to find out whether any electrolyte would make and keep a non-plastic clay plast,ic when added in Paper presented :it the Plasticity S y m p v i u m , I,af,1yette College, Oct. 1 7 (1924).
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TT'ILDER TI. B A S C R O F T .%SI) L. E. J E S K S
moderate amouiit,s. Before st8arting on this it was necessary t80have soiiic t,est for plasticity. X modification of Sokolow's method i m s used. .A weighed amount of clay was moulded t o a ball with the optimum quantity of n-at,er, dried a t 110' and placed in a beaker of water. The longer the time before the clay ball disint'egrates. t'he inore plast.ic the claJ-. Of course this process does not m.easure plasticity but it apparently rneaswes something which runs parallel with plasticity because it, was possible, in t'his \yay, to arrange a nuinber of clays in the exact order in which t'he clay workers arrange them. As a non-plastie clay, we used kaolin furnished by the S o r t o n Company. I t was found that, addition of ahout a quarter of one percent of a lithium salt, usually lit'hium chloride, togcthcr with a little sodium chloride, gave kaolin a very high plasticity as measured by this method, higher than that of t,he most plastic ball clay tested. S o burning experiinenk were made, so that we do not know what effect lithiiiin salts have on t'he burning properties. I t was hoped at first t,hat the problem of the plasticity of clay had been solved; but. this proved not to bc the case. Experiments by Dr. Papish showed no connection between t8he lithiurii contents of the different' claps and the plasticities. Special experiments showed that lithium chloride kept alumina gelatinous rat'hcr than silica. which is a further proof t,hat,this is not S a ture's way of n-::hing a plastic clay, liecause the lateritic or alumina-rirh clays are not plastic as a rulr. It seems certain therefore that we cannot account for the plasticity of ordinary clays without t'aking huiniis into account and it is probable t'hat the plasticity of clay will be due t,o the combined effect of humus and some electrolyt,e. Khile our experiments have not yet hrought u s to the desired goal, it is quite certain t,liat( vie arc on the right track and it is solliething to have increased the lowtemperature ple,stjritjr of kaolin so iiiuch bv such relatively small additions. Coine!l Cniwmit~j,
