All pipe used shall be straight, smooth and sound, thoroughly burned, vitrified and salt glazed, free from lumps or other imperfections and for the size hereinafter mentioned with no applicable variation from the specified dimensions or the true cylindrical shape. All straight pipe shall be straight in the direction of its natural axis, and its interior and exterior surface shall be concentric. All pipes except as hereinafter mentioned shall be with butt joints at square and at right angles with the axis, and each piece of pipe shall be furnished with a "sleeve" or "ring" not less than four inches wide, and whose internal diameter is at least one and one-half inches greater than the external diameter of the pipe to be encircled and joined. The Engineer shall decide when it will be necessary to use ring pipe or socket pipe. All curves, slants or junctions shall conform to the foregoing requirements as regards quality, form and workmanship, and in general their thickness shall be equal to that of pipes of the same caliber into which they are to be worked or joined. All said pipes shall be subject to inspection and approval or rejection by the Engineer, or his duly authorized representatives and shall be equal in quality to the best pipe heretofore used in the construction of sewers.