|Colonial Fort Dorchester, circa 1757|
In 15th century Iberia and North Africa there existed a strong tradition of rammed earth construction inherited from the Romans: taking clay, sand and larger granite aggregates and tamping a nearly dry mix into wooden forms. In Spain this type of consturction was called 'tabia', a borrowed word from the Arabic 'عتاب' (attab). When the Spanish began to establish colonies along the Southern coast of North America they modified the 'tabia' construction as suitable clay was rare along the southeast coast and Florida, the general condition being a thin layer of topsoil covering over sand. Although the 'pluff mud' found in the 'back barriers' and tidal lagoons is partly composed of clay, it also contains large percentages of fine sands, even finer silts as well as organic matter that make it entirely unsuitable for construction. However, pluff mud does provide a perfect nesting bed for a natural resource that would prove extremely useful as a building material: oysters.
Whereas in Spain readily available limestone was quarried and dressed as masonry units or burnt for lime, in the colonial outposts it was largely unavailable (the Coquina of St. Augustine providing an exception). However, there were plenty of oysters available whose shells were a ready source of lime. The oyster shells were harvested, oyster removed and the shells left outside for weeks to allow the rain and insects to clean out any organic matter. Most often 'middens', Native American waste oyster shell heaps, were already available and exploited for raw material.
The shells were used in three ways. Shells were broken up to provide a gradation of small to medium aggregates although many were left intact for the larger aggregates. A good percentage was set aside for a lime burn. Shells were stacked atop a 'rick' of alternating logs not unlike a funeral pyre and the logs would be set alight, burning for a couple of days. Carbon dioxide calcines from the shells with the intense heat (over 1500° F) leaving a caustic highly alkali compound, Calcium Oxide commonly known as quicklime.
Making Tabby is a lot like making pancake batter. One starts with hoeing up the dry ingredients:
1 part quicklime
1 part broken shells
1 part whole shells
1 part sand
Next comes the water. One needs to be very careful at this step because with the addition of water the slaking process, conversion of quicklime to Calcium Hydroxide of slaked lime, is a very rapid, exothermic reaction. Larger pebbles of quicklime can quickly release steam that can cause the mix to pop and splatter the highly alkali mix onto exposed skin or eyes.
The mix should be just wet enough to be able to tamp. Excess water will make the lime matrix weakened by voids as the water evaporates and take longer to achieve a sufficient compressive strength. Tabby can be hodded to the site with barrows. Balls of Tabby are made by hand and slung into wooden forms. The mix is then vigorously tamped from above.
With all of the rough oyster shell aggregate exposed at the surface, tabby walls are vulnerable to erosion from streaming water. Typically, like the wall itself, they were covered with a lime plaster using just the smaller crushed oyster shell and sand for aggregates. This render served as a sacrificial coat to protect the wall. The entire system is both aesthetically pleasing and very durable, proof of which being a number of colonial and early federal sites that have endured hundreds of years even without maintenance.
Contributed by Patrick Webb