2. CaSO4 + BaCl2 = CaCl2 + BaSO4.

We thus see that in both cases we get compounds which are insoluble or nearly so. If soluble sodium compounds are present the addition of barium carbonate or barium chloride will form either sodium carbonate or sodium chloride (common salt), but since both of these are easily soluble in water they can be washed off without much trouble.

Method of use.-As carbonate of barium is insoluble in water, in order to make it thoroughly and uniformly effective it should be used in a finely powdered condition and distributed through the clay as thoroughly as possible, because it will only act where it comes into immediate contact with the soluble sulphates. While only a small quantity of barium is necessary, still it is desirable to use somewhat more than is actually required.

According to Gerlach,1 a clay containing 0.1 per cent. sulphate of lime, which is the same as 0.4 grains per pound, would need 0.6 of a grain of barium carbonate per pound of clay. For safety, however, 6 or 7 grains should be added to every pound of clay. This would be about 100 pounds for every thousand bricks, based on the supposition that a green brick weighs 7 pounds. As a pound of barium carbonate costs about two and one-half cents, the amount of it required for 1,000 bricks would be $2.50. It is cheaper to use barium chloride, for the reason that the salt is soluble in water, and hence can be distributed more evenly with the use of a smaller quantity; the chemical reaction also takes place much more rapidly when it is used. There is this objection to it, however, that as near the theoretic amount as possible must be used, for if any remains in the clay unchanged, that is, without having reacted with the soluble salts, it may of itself form an incrustation.

In the case of a clay containing 0.1 per cent. calcium sulphate, it would require 26 pounds of barium chloride per thousand bricks, and this, at two and one-half cents a pound, would mean an outlay of $0.65. With the barium chloride treatment, chloride of lime is formed, but this is decomposed in burning.

'The Brickbuilder, 1898, p. 59 et seq.

Since in drying molded-clay objects the evaporation is greatest from the edges and corners of the ware, the incrustations may be heaviest at these points, but the more rapidly the water is evaporated the less will be the quantity of soluble salts deposited on the surface. Incrustations which appear during drying are found more commonly on bricks made from very plastic clays, which, owing to their density, do not allow the water to evaporate quickly.

Under physical properties there are included plasticity, tensile strength, shrinkage, fusibility, texture, color, slaking, absorption, and specific gravity.

PLASTICITY.

This property permits the clay to be molded into any desired form when wet, which shape it retains when dry, and while manytheories have been advanced to explain its cause, none of them are perhaps wholly satisfactory. As is well known to all, clays show a wide range in their plasticity, some being but slightly plastic or lean, and others highly so or fat. Very sandy clays containing but little clay substance are usually lean, and the same is true of some fine-grained ones, such as many washed kaolins.

Several theories have been advanced to explain the cause of plasticity. For a long time it was supposed to be directly connected with kaolinite (the hydrated silicate of alumina) and clays high in kaolinite were said to be very plastic, but it was found before long that the plasticity does not stand in close relation to the amount of kaolinite. High-grade koalins containing 13 per cent. to 14 per cent. of combined water, and therefore a large percentage of kaolinite, are usually lean, while clays with but 4 per cent. or 5 per cent. of combined water may be very plastic. That there is some connection between plasticity and the presence of minerals containing combined water would appear from the fact that if a clay is heated sufficiently high to lose its combined water, it also seems to lose its plasticity. Another theory considers it due to fineness of grain. If, however, we take quartz and grind it very fine in a ball mill it is not plastic, although it is true that if an excess of water is mixed with it, it becomes soft and pasty. If this wet mass is pressed it will pack to a hard mass, entirely lacking the mobility of form possessed by a plastic clay, so that fineness of grain is not the sole cause of plasticity. Still another theory considers that it is due to a plate structure, the idea being that the clay particles are plate-like in form, and that the plasticity is caused by these plates sliding over each other. One serious objection to this theory is that many clays when examined under the microscope do not appear to be made up entirely of plates.

In studying clays from different formations, and different parts of the same layer even, some interesting facts bearing on this subject were obtained.

A residual clay (kaolin from Webster, near Dillsboro, N. Car.) was compared with a sedimentary clay of the same composition (Florida ball clay). The residual clay was found to possess a decidedly less degree of plasticity than the sedimentary clay, the particles of which had been transported long distances and had been more or less rubbed and ground together previous to their deposition. Inasmuch as the kaolinite particles, as shown by the microscope, are more or less bunched, it is to be expected that this rubbing and grinding action during transportation would break up the bunches to some extent, and it is

believed that the increased plasticity of the sedimentary clay is due to this cause. This conclusion is in accord with facts noted1 by the former State Geologist, Prof. Geo. H. Cook, who found that by rubbing a mass of kaolin in a mortar the bunches of kaolinite plates were easily broken apart and that the mass afterwards showed increased plasticity. From what has been just said it must not be inferred that all residual clays are of low plasticity, for some are just the opposite.

If an exceedingly plastic clay is examined under the microscope, it is found that in addition to the regular particles of either a scaly or irregular shape, there are often a large number of extremely minute, apparently spherical, structureless particles, which may be of colloidal character. It seems to the writer that a mixture of the mineral particles and these spherical particles might develop a high degree of plasticity.

This same view has been independently expressed by A. B. Cushman.3

TENSILE STRENGTH.

The tensile strength of a clay is the resistance which it offers to rupture or being pulled apart when air dried. It is an important property by virtue of which the unburned clay ware is able to withstand shocks and strains of handling and the shrinkage in drying. Through it, also, the clay is able to carry a large quantity of nonplastic material such as flint or feldspar.

There may be some relation between the plasticity and the tensile strength of a clay, but it is neither a constant nor a simple one. While high tensile strength and high plasticity often go together, there are marked exceptions. A clay low in tensile strength may yet have high plasticity, as is seen in the case of some New Jersey clays, and, on the other hand, however, a clay of only moderate plasticity may have high tensile strength.

1 Report on Clays, 1878, pp. 281, 287.

'Maryland Geol. Surv., Vol. IV, p. 251.

Jour. Amer. Chem. Soc., XXV, No. 5, 1903.