PLASTICITY.
There is probably no property of unburned clay which has been more widely discussed than plasticity. To plasticity the clay owes its re sponsiveness to every touch of the potter's hand and its adaptability to the preservation of every line of the artist's tool; it is this quality that permits of its being drawn out into sheets and of the most astonishing thinness.
Of the many theories advanced as to the cause of plasticity the fol lowing are the most tenable: Molecular Attraction Theory—To properly appreciate this conception of the cause for plasticity, suppose clay to be blunged into the form of a slip, as is the practice of the potter before casting a vase. In this slip or fluid condition each grain is surrounded or enveloped by a film of water. If the volume of water is large compared with the total vol ume of clay particles, the mass will behave in every respect like a fluid; indeed, as will the turbid water of the Mississippi. Suppose that, by evaporation, or adsorption by a plaster mold, the volume of the water be decreased. The clay particles will be brought closer and closer to one another, causing the mass to pass from a fluid state through var ious stages of consistency until it assumes a stiff plastic condition; a process to be observed in mud roads. after every rain. When in this stiff condition the particles still have an envelope of water or, in other words, they are still suspended in water just as truly as they were when the mass was more of a fluid. But, owing to their proximity, it is assumed by those advancing this theory of plasticity, that they are held in position by the molecular attraction which each particle of clay sub stance exerts on the other.
Molecular attraction is a known force, and there has been no adequate proof advanced upon which positive claims can be made against such a force operating between clay particles when brought into close proxi mity. The popular conception of a bar of iron is that it is a rigid homogeneous mass, but, as is shown in magnetization experiments, it is made up of individual particles which can be turned about or set up endwise, thus acting independently of one another except in the matter of the molecular attraction that each exerts upon its neighbor, binding or holding the whole together. Aside from composition the degree of molecular attraction determines the hardness of the iron. Iron, then, is a solid fluid, that is, it will flow. The force of gravity is not sufficient to overcome this molecular attraction and cause flowage, but when a force that exceeds that of the molecular attraction is applied, flowage follows in the direction of the greater force. It is in this respect that iron is a fluid.
If similar flowage is attempted when the grains in a clay mass are practically dry, or, in other words, not surrounded by water, except per haps that held by absorption, pressure sufficient to overcome the force binding or holding the particles together will disrupt the ware. That is,
instead of flowage of the particles in this comparatively dry state, rup ture is a possibility. Further, maximum plasticity . or ability to flew is not attained until the maximum number of particles is enveloped with the least amount of the suspending medium. This same is to be noted with almost all fine insoluble powders. Wheeler' has shown, for instance, that the non-plastic slates, Iceland spar, propyllite, gypsum and halloysite can be made to develope a much smaller but still a fair degree of apparent plasticity with water as a floating medium. When dried, the force required to disrupt these masses, while small, is yet comparatively great. The difference, however, between the behavior of clay and these finely pulverized minerals is that the latter can be molded by pressure alone into a shape that will have a comparatively higher tensile strength than if they were caused to acquire that shape by flowage due only to assumed plasticity. But we know that maximum density and consequent strength can be best developed in plastic clay by the combined influence of pressure and plasticity. Now i,5 it mole cular attraction in the case of clay, as in that of iron which can be bent, stretched, rolled, etc., in the cold without rupture, or is it merely that clay grains may be pressed so close together that flowage is permitted so long as water is present in excess, but is resisted by fractional force when dry ? Text books on physics give as an "expression"' for the force of mole cular attraction between two molecules, M and M', MM'f (r). "All that is known about this funtion of r is that it is very large for insensible distances, that it diminishes very rapidly as r increases and that it vanishes while r is still very small. The maximum value of r at which molecular action ceases is estimated by Quincke to the 0.00005 mm. If the particles then were 0.00005 mm. or 0.00002 inches apart, they would be at the extreme distance through which molecular attraction can possibly operate. Grout' says, however, "Now a simple calculation, based on the mechanical analysis of the clays, will show that the amount of water needed to place a film 0.00005 mm. thick around each grain is often nearly equal to the amount added in tempering, so that in ordinary plastic clay, it is necessary to consider practically all the water as being under this influence." Grout' bases his reasoning on the following calculations : He found that his "mechanical analyses frequently show a large percentage of grains below 0.001 mm. in diameter, also from 0.001 to 0.005 mm. The average diameter of grains below 0.001 mm. is 0.0005 mm. If these are considered spherical and of specific gravity 2.5, it would re quire 25.5 per cent by weight of water to place around each grain a film 0.00005 mm. thick." On making these same calculations the following was obtained : This calculation, so far as the validity of Grout's argument is con cerned, checks his results.