TESTING THE BRICK. It is important to have a definite method of testing the qualities of any artificial material, since then all parties may know exactly the grade called for, and since the results obtained by different observers with different materials may be compared. This is particularly true of brick, since the clays differ greatly in quality and also since a slight variation in each step of the manufacture materially affects the result. Definite methods of testing paving brick are less than ten years old, and probably the best method has not yet been discovered; but in the past five years methods have been developed which give fairly satisfactory and definite results. Several tests formerly employed have now been practically abandoned; but for the sake of completeness these will be briefly considered. The object of testing paving brick is two-fold: (1) to determine whether the material is suitable for use in a pavement; and (2) to enable comparisons to be made between different classes of brick.
The brick should have reasonably flat sides and square corners. They should be nearly uniform in size, since the unduly large size indicates under-burned or soft brick, and the unduly small size usually indicates over-burned or brittle brick. However, as most paving brick are die-molded and as the die wears larger by use and must ultimately be replaced by a new one, a change of size from this cause must not be confused with that due to deficient or ex cessive burning. The edges of the brick should be smooth and free from serrations or "ragging," due to friction in the die; and the " kiln marks," or impressions from the over-lying brick in burning, should not one eighth of an inch. The quality of the brick
can be judged by striking it a sharp blow with a hand hammer, or by striking two together, or by dropping the flat side of one upon another.
The interior of the brick should be homogeneous, free from uncrushed or lumpy material, especially if such material is not united by vitrification with the balance of the material of the brick. The brick should be vitrified clear through to the center; and should contain neither unfused spots, which indicate sand or fire clay, nor glassy or spongy spots, which indicate imperfect crushing and mixing of some fusible mineral in the clay. The structure should be free from "shakes" or marked laminations. Fire cracks, caused by too rapid firing, if small and superficial, are not of much importance ; but they should be limited in number and extent. There should be no lumps of lime, due to the presence of limestone pebbles in the unburned clay, since these will slake when the brick becomes wet and probably disrupt it.