The tensile strength is measured by molding the tempered clay into briquettes, of the form and dimensions shown in Fig. 26, and, when they are thoroughly air dried, pulling them apart in a suitable testing machine. The cross section of the briquettes when molded is 1 square inch, and, after being formed, they are allowed to dry first in the air and then in a hot-air bath at a temperature of 100° C. (212° F.). When thus thoroughly dried the briquette is placed in a machine, in

The outline and dimensions of a briquette made for testing the tensile strength of clay.

which its two ends are held in a pair of brass clips, and is subjected to an increasing tension until it breaks in two. Theoretically the briquette should break at its smallest cross section, with a smooth, straight fracture, and when this does not occur it is due either to a flaw in the briquette or because the clips tend to cut into the clay. In such event the briquette breaks across one end, and to prevent this it is necessary to put some soft material, such as asbestos, pasteboard or rubber between the inner surface of the clip jaws and the sides of the briquette. If the briquettes

are molded and dried with care, the variation in the breaking strength of the individual briquettes should not vary more than 15 or 20 per cent., but with some very plastic clays it is extremely difficult to keep the variation within these limits.

In making a complete test of the New Jersey samples the tensile strengths given are usually the average of 10 or 12 briquettes. In a number of cases where it was only desired to get an approximate idea of the strength of a clay, but two or three were broken. The greatest variation usually appeared in clays of high tensile strength, in which case the fracture nearly always occurred in the head, indicating that the briquettes broke before the limit of their strength was reached.

The tensile strength of clay briquettes is expressed in pounds per square inch, but, since the briquette shrinks in drying, the strength actually obtained in testing will be less than that for a square inch, and the result must be increased in proportion to the amount the briquette has shrunk.

The following figures give the range in tensile strength shown by clays from the different formations in New Jersey, and it will be seen from an inspection of these figures that the clays of any one formation may show a wide range in their tensile strength.

Range of tensile strength of clays from different formations in New Jersey expressed in pounds per square inch.

If the tensile strength tests of a number of clays from different localities are grouped according to the kind of clay, a somewhat similar variation is found.

The washed ball clays of New Jersey showed the lowest tensile strength of all the samples tested, while the clay from the railroad cut north of Alloway was the strongest of the series examined, and had an average tensile strength of 453 pounds per square inch, with a maximum of 506 pounds.

With such a variation existing in the tensile strength of clays, it becomes a matter of importance to know the cause of this variation. It is a well known fact that all clays shrink in drying, and that this shrinkage is accompanied by a drawing together of the particles. Indeed, some clays shrink to such a hard mass as to suggest a close interlocking of the grains, which, it seems to the writer, may be the explanation of the tensile strength shown; that is to say, those clays in which the interlocking of the particles is the tightest will show the highest tensile strength, and vice versa. If this is true it becomes necessary to determine, if possible, what arrangement or size of particles produces the tightest and strongest structure.

When any series of clays is tested it is evident that the highly sandy ones have a low tensile strength,' and very fine-grained ones are in all cases, as far as the writer's experience goes, also low in their tensile strength. Many samples having a high tensile strength have considerable grit, but still they shrink to a very dense body, indicating a considerable percentage of fine particles. E. Orton, Jr., attempted to determine the effect of the fineness of grain on the tensile strength of clays by taking a very fine-grained clay and mixing different sizes of sands with it, the sand being obtained by grinding and screening vitrified bricks. His conclusions were "(1) that the tensile strength of mixtures of a plastic ball clay with equal quantities of nonplastic sands

3

1See Asbury clay, sample 695 S, Table, Chap. XVIII.

198.

See Raritan clay, sample 723. Table, Chap. XVIII.

Transactions American Ceramic Society, Vol. II, p. 100, and Vol. III, p.

will vary inversely with the diameter of the grains of the sand from grains of 0.04 inch down to the finest sizes obtainable. (2.) That the nonplastic ingredients of clay influence its tensile strength inversely as the diameter of their grains, and finegrained clays will, other things being equal, possess the greatest tensile strength." In other words, the coarser the grains of sand, the less the tensile strength of the mixture containing them.

In order to obtain some information regarding the relation of texture to tensile strength the writer selected 5 clays at random from the New Jersey series tested for this report, the only precaution taken being to pick out samples ranging from high to low tensile strength. These 5 samples were the following:

1. Alloway clay from the railroad cut north of Alloway (Loc. 164, Lab. No. 680). This is a very plastic, slightly gritty, dense, red-burning clay, with an average tensile strength of 453 pounds per square inch.

2. Pleistocene clay from the brickyard near Somerville (Loc. 234, Lab. No. 659). This was also a gritty, plastic clay, but not as dense as the previous one. Its average tensile strength was 297 pounds per square inch.

3. A Cape May clay from along the river south of Millville (Loc. 181, Lab. No. 645). A gritty, plastic clay, with an average tensile strength of 289 pounds per square inch.

4. Sample of Raritan clay from Cliffwood (Loc. 220, Lab. No. 615). A black, sandy, micaceous clay, with an average tensile strength of 105 pounds per square inch.

5. A soft, powdery, washed ball clay from near Sayreville (Loc. 268, Lab. No. 723). It was plastic to the feel, with very little grit, and a tensile strength of under 20 pounds per square

inch.

Each of these clays was put through a mechanical analysis and separated into the 5 classes of grains,1 shown in the table below. The percentage of these sizes in each of the 5 samples is shown in the following table:

'For explanation of sizes, see "Texture" in this chapter.

These figures seem to throw some light on the relation of the texture to the tensile strength, but, while highly suggestive, are not to be taken as final. The writer hopes, however, to have the opportunity of supplementing them by additional tests at some later date. Before discussing the bearing of the mechanical analyses given above it seems desirable to plot them in the form of curves, as shown in Fig, 27. Here the horizontal lines represent percentages. Of the 6 columns, the first 5 represent the grain sizes and the sixth the tensile strength.

Taking No. 5 of the above table of analyses we find that it contains 87.96 per cent. of clay substance. This point is plotted in the first column. The point representing the percentage of fine silt is then plotted in the next column, and so on with the other sizes. These points are then connected with a curved line. In the same way the percentages of the different sizes of grains of the other samples were plotted and connected by curved lines. The lines are drawn in different ways so that those representing the different clays can be more readily distinguished at a glance. From a study of this table it is seen that the clay having the lowest tensile strength (No. 5) contains a very high percentage of the finest clay particles. Furthermore, the clay having the second lowest tensile strength (No. 4) contains the largest percentage of sand (42.9 per cent.). From this it appears that an excess of either coarse or fine grains lowers the tensile strength. On the other hand, in those clays having the high tensile strength the percentages of fine, medium and coarse particles are