Sewer brick shall be whole, new bricks of the best quality, of uniform standard size, with straight and parallel edges and square corners: they shall be of compact texture, burned hard and entirely through, free from injurious cracks and flaws, tough and strong, and shall have a clear ring when struck together. The sides, ends and faces of all bricks shall be plane surfaces at right angles and parallel to each other. Bricks of any one make shall not vary more than 1/16th of an inch in thickness, nor more than 1⅛th of an inch in width or length, from the average of the samples submitted for approval. The truest bricks shall be used in the face of the masonry and the exposed surfaces shall be true and smooth planes. All bricks delivered for use shall be culled by the Contractor when required. No brick thrown out in the culling shall be used in any work done under any contract of the Sewerage Commission, except that the best of the culls may be used in manholes, above the level of the top of the sewer, if permitted by the Engineer. The average amount of water absorbed by the bricks, after being thoroughly dried and then immersed for 24 hours, shall not exceed 6 per cent. All bricks shall be uniform in quality and percentage of absorption. Whenever vitrified bricks are required in the invert of the sewer, they shall be smooth, hard, tough, and of such durability as will fit them for this use. They shall be of standard size, well and uniformly burned, thoroughly vitrified throughout, and free from warps, cracks, and other defects. The surfaces and edges shall be true and straight and the corners sharp and square. They shall be in every respect satisfactory to the Engineer, and in all respects equal to the sample in the office of the Engineer. The remaining paragraphs of the specifications deal with the manner in which samples shall be submitted and the necessity for conformity between the samples submitted and the bricks used. A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the variations in size are many. The bricks in use on any one job should be as near the same size as possible as the extra mortar filling necessary to make up for small brick detracts from the strength of the sewer. Small brick are undesirable as the cost of laying small and large bricks is the same, but the thickness of the finished sewer is less. Sewer brick should not absorb more than 10 to 20 per cent moisture by volume, in 24 hours; except the special paving brick used to prevent erosion at the invert which should absorb less than 5 per cent moisture. =97. Vitrified Sewer Block.=—Blocks and bricks are manufactured in a manner similar to the manufacture of vitrified sewer pipe described in Art. 91. J. M. Egan describes two types of sewer blocks[62] as follows: