Comments on brick tests.-In studying the above tests number of interesting points are to be noted.

1. Taking the two largest groups of bricks as a whole, that is, the stiff-mud brick and the soft-mud brick, we find that the crushing strength in the former ranges between 1,694 and 13,873 pounds per square inch, with an average of 4,856 pounds, while

1 2 3 4 5 6 7 8 9 10 11 12

Fig.38.Diagram showing lack of close agreement between crushing strength and transverse strength.

the latter range from 661 pounds to 5,909 pounds per square inch, with an average of 3,703 pounds. Even if the two exceptionally high tests, Nos. 1 and 12, are deducted, the average of the stiffmud crushing tests still remains higher, being 4,055 pounds.

2. Comparing the transverse tests in the same manner, we find 17 CL G

that the modulus of rupture in the stiff-mud bricks ranges from 513 to 1,750 pounds, with an average of 950 pounds, while in the soft-mud the variation lies between 141 pounds and 1,042 pounds,

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Fig. 39.

Diagram showing lack of close agreement between crushing strength and transverse strength of stiff-mud brick.

with an average of 571 pounds, or nearly 50 per cent. less than that of the stiff-mud.

3. If the crushing and transverse tests are shown graphically, as in Figures 38 and 39, it will be seen that the transverse breaks

do not seem to stand in direct relation to the crushing resistance, bricks of high crushing strength in some cases showing a low strength on the transverse test, and vice versa. This is notably true of the stiff-mud bricks tested.

4. In testing the individual bricks it will be noticed that there is sometimes a great difference between the maximum and minimum figures, as in tests Nos. 5 and 19. The lower breaks are in many cases due to carelessness in the manufacture, and wherever a low break occurred, it was found in the majority of instances to be due to pebbles in the brick three-fourths of an inch or an inch in diameter. These come from the loam that is mixed with the clay, and could be removed by a proper preliminary screening. 5. The hardness of the brick and porosity as shown by the absorption test are not necessarily an index to its crushing strength, except within very wide limits. Thus No. I is of low porosity and great hardness, but its crushing strength is very high, whereas No. 19, which is also of low porosity and great hardness, has a crushing strength of only about one-half that of No. 1, due perhaps to the fact that it is made of a much more plastic clay, which tends to warp and split somewhat in burning. Again, No. 12 is extremely porous, and hard burned, but has a crushing strength almost equal to No. 1. On the other hand, examples of high porosity and low crushing strength are shown in Nos. 2, 3, 7, 8, 13, 15, so that, while it is perhaps safe to say that high porosity is more frequently accompanied by low crushing strength, and vice versa, nevertheless, these tests prove that there are many striking exceptions to this generalization.

I.

If we compare the transverse strengths with the porosity, we find the same lack of any close accord, although generally speaking the least porous brick shows the higher transverse strength, and vice versa. Thus, No. 21 has the least strength and the greatest porosity, and No. 19 has the least porosity and next to the greatest transverse strength, which accords well with our generalization. A striking exception is to be noted in the case of No. 12, which stands first in point of strength and is, also, one of the most porous, standing eighth in a list of 26, instead of last, as the generalization demands. So, too, No. 4 stands low in

strength, and also low in porosity, contrary to what we might expect.

6. Re-pressing increases the strength of the brick so far as the New Jersey experiments go; it also increases the density and, therefore, decreases the absorption. As an example of this, we may take samples 26 and 27, representing the red brick made around Trenton. In this case the re-pressing has increased the crushing strength about 40 per cent. and the transverse strength nearly 30 per cent., while the absorption has decreased 2.24 per cent. A similar difference is observable in specimens 30 and 31, where the difference is still greater, but in this case it has been increased somewhat by harder burning.

7. The material added to decrease the shrinkage may seriously affect the strength of the brick. Thus Nos. 20 and 21 are common bricks made from the same clay bed, at two different, but not widely, separated points. No. 20 contains clean, sharp sand as an anti-shrinkage ingredient, while No. 21 has a sandy loam added to it in tempering. The latter has evidently made the brick more porous, softer, and less ringing. It also lowers its crushing strength more than 66 per cent. and its transverse strength over a half.

8. In one instance the same yard was found to be using both the stiff-mud and the soft-mud process; the clays used were obtained from the same bed, and the mixtures used differed but little. No. 4 represents the stiff-mud, and No. 22 the soft-mud brick. The greater crushing strength of the latter is probably due to its more homogeneous character, for the former contains many small cracks.

9. The effect of hard burning is seen in Nos. II and 12. Both lots came from the same down-draft kiln, but No. 12 was taken from the top, where it had been subjected to greater heat. Its strength is more than double that of No. II.

For the sake of completeness of data, and also to indicate the character of the clays used, the conditions surrounding the manufacture of each brick are given here in detail, the numbers corresponding to those given in the first column of the table of tests.

STIFF-MUD BRICKS.

1. A front brick, made from a plastic, gritty clay, having a high tensile strength, and vitrifying at above cone 12. The bricks are burned in down-draft kilns, at about cone 6 or 7. The crushing tests ran quite uniform, and the lower figures of the transverse tests were due to fine cracks in the bricks.

2. A common brick made from a mixture of black clay and loam, in the proportion of about two-thirds of the former and onethird of the latter. Little or no water is added to the clay in the stiff-mud machine. The bricks are dried on pallets and burned. in up-draft kilns. The brick are all fine grained, but some showed a laminated structure. The minimum modulus of rupture was caused by a one-inch pebble in one sample.

3. Common brick made from Cape May clay. The raw material is a highly plastic, gritty and sometimes pebbly clay, with an average tensile strength of 289 pounds per square inch. The linear air shrinkage was 8.4 per cent. and the fire shrinkage 1.5 per cent. The bricks are dried on pallets and burned in scove kilns. On the fracture they showed a coarse grain, with small clay nodules and gravel, and also slight laminations.

4. A common brick made from a mixture of Raritan clay and surface loam. The mixture is gritty, moderately plastic and not of high tensile strength. No temperature or cone measurements were made on the kiln, but the laboratory bricklet at cone 05 shows about the same absorption as the large brick. The linear air shrinkage is 0.7 per cent. and the fire shrinkage o per cent. The cubical air shrinkage 0.7, and fire shrinkage 5.2. The bricks show numerous fused specks of limonite, and the centres are sometimes black and shelly. The minimum transverse break was caused by a one-inch pebble.

5. Common brick made from a dense-burning clay of high tensile strength and red-burning if fired slowly. The clay was molded as taken from the bank in a small stiff-mud machine, stacked up to dry under sheds, and burned in up-draft Dutch kilns. The linear air shrinkage in drying is 5.4 per cent. and the fire shrinkage, as measured on the greatest length, is o per cent., but