DISCUSSION OF TESTS AND COMPARISON OF QUALITIES.
A comparison of the various tests may be made by studying the results of the extensive series of tests of brick of a wide range of char acter and quality made at the University of Illinois for the Department of Ceramics and State Geological Survey. 'These tests are more fully reported elsewhere. The brick were obtained from twenty-seven man ufacturers in the states of Illinois, Ohio, Indiana, Missouri, and Kan sas. From one to five grades of each make of brick were obtained. Duplicate rattler tests were made for each grade, and five or more brick were tested in cross-breaking and in crushing for each grade. The bricks used in the tests were generally selected and graded at the yards by a representative of the Ceramics Department, who was skilled in selecting and grading brick. When more than one grade was ob tained, the first selection made was the best grade for paving pur poses, according to the judgment of the representative, care being taken not to select too hard burned a brick. A grade harder or some what overburned and one softer or even underburned were selected. When there seemed to be an opportunity for error in judgment, inter mediate grades slightly harder or softer than the first were also picked out. The N. B. M. A. Standard rattler test was used, and the other tests were made by the methods already described. Rattled brick were used in the absorption tests.
The general results of these tests are plotted in Fig. 3. The three makes of brick on which transverse and crushing tests were not made are not included in this diagram. The average for the tests on a par ticular grade are shown. The brick were placed on the diagram gen erally in the order of the rattler loss, the grade which gave the lowest rattler loss being used to fix the order of any make of brick. The crushing strength is plotted in connection with the modulus of rupture, (cross-breaking test), to enable a ready comparison between these two tests to be made, the scale for the crushing strength being one-third of the actual value. The figures given with the modulus of rupture show the average variation of. the modulus of rupture for the
individual brick in any grade from the mean of the test on that grade as given in per cent of the mean value of the modulus of rupture. In studying this diagram attention should be given to the amount of variation in the absorption test for each make of brick, to the range in the amount of absorption producing little change in the desirable qual ities in some brick and to the rapid change in quality for small changes in absorption for others, and to the relation between the rattler test and the other tests.
Attention is called to the following particulars shown on the diagrams.
Brick No. 2.—A range of absorption from to 3% gives an excellent quality of brick, as shown by the rattler tests, the cross bending test, and the crushing test. Even with 6% absorption this brick gives a good rattler test and a high crushing strength. It is apparent that there may be con siderable variety of burning with this brick and yet secure a good article, providing, of course, that the heat treatment is otherwise suitable.
Brick No. 5.—In this make a change in the absorption amount is accom panied by a considerable change in the quality of the brick as shown by the rattler test and the other tests. Much care must then be used in selecting the right degree of burning.
Brick No. 7.—This is a fire clay brick and its strength can not well be com pared with the other brick. It seems probable that the smoothness of this material gives it a higher, rating in the rattler test than the brick should have.
Brick No. 10.—In this brick the grading for hardness as made secured a brick with but a small range in the absorption test, three grades varying less than 1% in the absorption test. All of these were of very good quality.
Brick No. 12.—Absorption up to 5% has little effect upon the quality of the brick, the cross-breaking strength being good for the grade having 5 per cent absorption. The overburned brick is of poorer quality. The range in absorp tion from one to five per cent allows considerable latitude in the selection of the brick.