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Plasticity, which is only developed in clay when wet, is best estimated by feel. To get a numerical expression of plasticity several methods have been suggested. Stover (1905), following Bischof, suggests forcing the wet clay through a die and measuring the length of pencil extruded before it breaks by its own weight. Another method is founded on the fact that, generally speaking, the more plastic a clay the more water required to bring it to the degree of softness at which it can be worked. The pro- portion of water required to soften one hundred parts of 38 CERAMIC CHEMISTRY. dry clay expresses the plasticity. To get this, Langenbeck suggests the use of Vicat's needle, and states the proper consistency is reached when the needle under SOOgrams. penetrates 4cms. in 5 mins. Ashley's theoretical plasticity "R X P formula is ^ , where A is the Jackson-Purdy surface factor, B the cast air shrinkage, and C the relative colloids found by malachite green adsorption. Tensile Strength is measured by moulding the clay into briquettes, and, when dry, pulling them apart in a machine. The result is given in pounds per square inch. High tensile strength and high plasticity often go together, but not invariably so. Transverse Strength. Ries and Allen (1910) recommend this in preference to the older tensile test as being easier to carry out, and as giving more uniform results. The clay