15

more nearly equal. This is perhaps what might be expected, for if the tensile strength is due to the interlocking of the grains, a mixture of different sizes would fit together more closely than if particles of one size predominated, as in Nos. 4 and 5 of the table. It is rather difficult, however, to compare these results

289

Fig.27. Curves showing relation of texture to tensile

strength.

with Orton's, as in his artificial mixtures the nonplastic particles were of uniform size, while in the natural mixtures a variety of sizes existed.

If the theory of interlockment is true, then it should be possible to make a mixture of two clays, whose tensile strength is

Sand

Tensilestrength lbs. per sq.in.

higher than that of either of the clays alone, or vice versa. While no experiments were made with the object of proving this point, some results were obtained in the course of the physical work on the New Jersey clays that have an important bearing, as follows:

Of several clays tested from the Asbury clay, near Asbury Park, one was a slightly gritty, black clay, with an average tensile strength of 182 pounds per square inch. The other was a plastic loam, whose average tensile strength was 137 pounds per square inch. A mixture of the two in equal proportions, however, had an average tensile strength of 258 pounds per square inch. Another clay from a different formation (Lab. No. 695) had an average tensile strength of 108 pounds per square inch, while a mixture of equal parts of this clay and sand showed a tensile strength of but 65 pounds per square inch. In the latter case the decrease in strength was due to the excess of sand.

The tensile strength of a clay is used by some as a means of measuring its plasticity, but there are probably fewer advocates of this idea than there were a few years ago. It is true that many very plastic clays have a high tensile strength, and that many very lean ones have a low tensile strength, but we cannot say more than this. Many clays, as, for example, some of the Clay Marls, have a lean, gritty feeling when wet, and yet show a high tensile strength when air dried. Again, many of the Raritan clays feel very sticky and plastic, and yet their tensile strength may be under 100 pounds per square inch. Some clays of high bonding power will, when tested alone, develop little tensile strength, but this is because the high plasticity of the clay causes it to shrink, warp and crack so much in drying that it is impossible to get a briquette free from flaws. When mixed with sand or grog, its shrinkage and cracking are greatly reduced, and the proper texture is developed for high tensile strength. Some of the No. 1 fire clays of the Woodbridge district, which are very fine grained, crack so much when dried alone that it is impossible to get a flawless briquette for testing.

SHRINKAGE.

All clays shrink in drying and burning, the former loss being termed the air shrinkage, and the latter the fire shrinkage.

Air shrinkage.—When a mass of clay is mixed up with water, each of the grains of clay can be considered as being surrounded by a film of the liquid, which may prevent them from coming into close or actual contact. As soon as the wet, molded mass is set aside to dry, however, evaporation of the water contained in the pores of the clay begins, and, as it passes off, the particles of clay draw closer together, causing a shrinkage of the mass. This will continue until all the particles come in contact, but since they do not fit together perfectly, there will still be some spaces left between the grains, and these will hold moisture, which cannot be driven off except by gentle heating. The air shrinkage, may, therefore, cease before all the water has passed off. This fact can be told by the loss of weight which takes place in the clay, if put in a hot-air bath (at 100° C.) after the air shrinkage ceases. The amount of air shrinkage is usually low in lean clays, and high in very plastic clays, for the reason that the latter absorb considerable water in mixing, which they then give off in drying. At the same time, however, it must be noted that all clays which require a high percentage of water in mixing, do not show a high air shrinkage, as is shown by the diagrams on Plate XIII A.

In these, the horizontal lines indicate percentages; the numbers at the top of each diagram represent the locality, arranged according to the percentage of air shrinkage which they showed, the lowest being placed at the left end, and the highest at the right.

Taking the diagram for the Cohansey clays, the first one is No. 219 (Lab. No. 699). A point was marked indicating its air shrinkage, viz., 2.5,2 and above this a second point was marked.

1 Whenever the term air shrinkage is used in this report it refers, unless otherwise stated, to the linear shrinkage, expressed in terms of the length of a freshly molded brick, and measured on its greatest dimension.

'The percentage of both air and fire shrinkage is expressed in terms of the length of the freshly molded bricklet.