A.John Carroll, a builder and mason living in Durham, N.C., responds: As late as the mid-19th century, bricks were stacked and fired in temporary, site-built kilns, then sorted later. The ones closest to the fire — "clinker" bricks — were very hard, but often distorted and sometimes fused together. They were considered ugly and were mostly used where they wouldn't be seen — though nowadays they are valued for their dark, uneven color and irregular shape. "Face" bricks were located a little farther away from the fire, so they were hard and dense but retained their rectangular shape. These bricks were reserved for the exterior face of outside walls. The bricks farthest from the fire — "fill" bricks — were used on the inside of exterior walls and for partitions inside buildings.

In general, fill bricks are larger than face bricks and pinkish-orange or salmon in color. They have a softer surface than face bricks; when you strike them with a hammer, you'll hear a thud rather than the ping you would hear when hitting harder, denser face bricks. The biggest problem with fill bricks is that they are porous, so they soak up...

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