Examine the bricks and learn the date–or so says this myth. If you see glazed headers, you know the buildings was built before 1750, because a law was passed in 1750 against burning hardwood. Hardwood is necessary to make fire hot enough to glaze bricks in a kiln. (A header, by the way, is a brick turned so the short end faces out instead of the long side.)
It is not possible to date a building this way. “I have seen plenty of buildings built after 1750 with glazed bricks,” says Matthew Webster, Colonial Williamsburg’s Director of Historic Architectural Resources. (see above example) It is true that hardwoods burn hotter. “Softwoods burn fast,” he says. “Hardwoods create a better and sustainable high heat.” But it is the potassium contained in hardwoods that helps the sand in the brick vitrify into glass.
“It’s not the heat necessarily that is required for glazed bricks,” explains Jason Whitehead, supervisor of Colonial Williamsburg’s Historic Masonry Trades and Brickyard. “It’s the potash that is released from hardwoods such as oak and hickory that then builds up on the bricks and reacts with the clay. This combo creates the glazing seen on the bricks that are making up the fire tunnels in the bottom of the kiln.”
However, the real problem here is that there was no colonial law against burning hardwoods. “I have never heard of or seen evidence of any law forbidding the burning of hardwoods in brickmaking,” says Whitehead. Hardwoods gradually became scarce in colonial America because of their desirability in both England and the colonies. From the earliest years, English colonists burned hardwood to produce potash, used in making glass, for export to England where few hardwood trees remained.