DEVELOPMENT OF PLASTICITY IN THE PRESENCE OF WATER.
Whatever may be the fundamental cause of this phenomenon we call plasticity, it is certain that it is manifested only when water is present. It has been shown that mere molecular attraction between the clay grains and the water molecules is not sufficient to account for plasticity. There must, therefore, be factors other than molecular attraction which becomes operative in developing this property, which, when water is not present, may be said to be latent. Since it is the presence of water that makes the development or expression of plasticity possible, it is important that we consider some of the fundamental and well-known hydrostatic forces.
There are at least four forces operating on the water in a wet, un burned brick : First, gravity, or the weight of the water itself ; second, surface tension, which is due to attraction (cohesive) between the molecules of water themselves; third, molecular attraction (adhesive) between the water molecules and the mineral particles in the clay; and fourth, surface pressure, which is the opposite of surface tension.
Gravity—Surface tension, or the contracting power of any exposed water surface, may act with gravity or against gravity, depending upon circumstances. Molecular attraction between the mineral and water molecules always acts in opposition to gravity. Since, as can be shown, the conditions of capillarity in a mass of clay compressed into the form of a brick is such as to make surface tension the very much greater force, and operating in opposition to that of gravity, gravity will not be considered as one of the component forces in our problem. If we were dealing with "slips" or even soft mud fixtures, the force of gravity would have to be considered.
lfolecnlar Attraction—Milton Whitney' says: "The potential of a single water particle is the work which would be required to pull it away from the surrounding water particles and remove it beyond their sphere of attraction. It is the total attraction between a single particle
and all other particles which surround it." It is called by some "mole cular attraction." Surface Tension—Because it has particles adjoining it only on one side, i. e., molecular attraction is affecting it only from one side, the potentiality of a water particle on the surface is, according to Whitney's definition, only one-half that of a particle in the center of a drop. That things tend to move from points of low to points of high potential is a well-known law of physics. The particles on the surface, will, there fore, strive to get to the interior of the drop. The results will be sur face tension.
Looking at this proposition from the mechanical point of view, the force of molecular attraction operating on the surface particles, is effec tive along lines that extend from the center of each particle, to the center of the surrounding particles. Since the particle on the surface of a drop of water is under the influence of other particles only from one side, the several lines of force would extend radically from its center to the center of adjacent particles, having as a resultant a line of force extending from the center of the surface particle to the center of the mass.
Surface Pressure—Suppose that instead of a drop we have the same mass of water surrounding a solid particle as a film, say, 0.0005 m m. thick. We should have in this system two combating forces, first, mole cular attraction of water molecules for each other, causing a pull on all water particles toward the center of the film, creating a tension on the outside surface as well as on the surface contiguous to the solid par ticles; second, attraction between the molecules of the solid particles and those of the liquid, tending to create a tension only on the outer surface of the glm. • Consider the water between four solid particles as shown in the following figure as having a potentiality less than that of the solid particles.