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Lime Burn

The Colonial Williamsburg Brickyard successfully built and burned a limerick recently. Lime was an important product in the 18th-century building trades, being the key ingredient in both mortar and plaster. The week of February 9-13 was spent constructing the rick, which consisted of a wide circular platform of firewood about four feet tall, topped with a large mound of oyster shells. On Saturday, 14 February, the central chimney was lit and acted like a fuse to ignite the firewood, and over the course of the day, burned the oyster shells. The wood burned down, and on the morning of Sunday, 15 February, the Brickyard staff was left with a large, still-warm pile of a burned shell. Chemically, burned oyster shells are quicklime. Sunday found us “slaking” the lime by shoveling the burned shells into tubs and gently pouring warm water over them. An exothermic reaction takes place, breaking the burned shells down into what the 18th-century lime burner would have termed “slaked lime”: a white, caustic substance that looks and feels kind of like ricotta cheese.

The slaked lime was separated from the ash and the non-reactive leftovers of the burned shell on a large mesh screen and stored in a pit in the ground. These lime pits filled rapidly over the course of the following days. Any impurities in the lime will sink to the bottom of the pit, leaving us with good, usable lime for plastering the Charleton Coffeehouse this summer. In the pits, the lime is covered with water, which will keep the lime from absorbing carbon dioxide and hardening. Differing amounts of sand are added to lime to make mortar for laying bricks, or in this case, for the plaster to plaster interior walls.
Limericks are burned every couple of years by Brickyard staff, on an as-needed basis. The design of this rick was different from ones burned in the past; it is based on Jamaican style oyster ricks. Colonial Williamsburg architectural conservator Matt Webster suggested the design and helped the Brickyard staff construct it. By the way, the term “rick” is an old English word meaning a stack, usually referring to hay or cordwood.

For anyone who might be interested, here is a brief summary of the chemical processes involved:

Common Name Chemical Formula
Calcium Carbonate
(often seashells or limestone)
is heated to approximately 750-900° C
releasing Carbon Dioxide – CO2
and leaving Lime
(Calcium Oxide)
= CaO
adding Water+ H2O
creates Slaked Lime
(Calcium Hydroxide)
= Ca(OH)2 + significant heat energy
exposing it to Carbon Dioxide+ CO2
releases Water– H20
leaving Calcium Carbonate once again
(in this case, dried mortar/plaster)
= CaCO3
A close-up view of the shells before the lime burn.
A barrel full of shells after they’ve been fired.

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