Tabby- Historic Oyster Shell Concrete

I took a vacation to Savannah, Georgia about four years ago– after a couple months of a New England winter, I can’t help but start thinking about memories of warmer places. As with any good vacation, it’s the odd and unexpected things that stick in your head for years afterwards. One of my most salient memories of that vacation was an idiosyncratic concrete-like building material called tabby, which is as much a part of the historic landscape on the southeastern coast of the US as Spanish moss.

Tabbly Blocks

Tabby blocks on Sapelo Island, Georgia

Tabby is a mixture of unslaked quicklime (calcium oxide, produced by burning locally abundant oyster shells at about 2,000 degrees Fahrenheit), sand, water, and whole unburnt oyster shells which served as a coarse aggregate. The recipe is elementary, with the ingredients mixed in roughly equal proportions by measure (not weight), and then poured into structural forms or cast into large blocks and allowed to dry in the sun for several days before use. The result was a durable concrete-like material, which could be handled like concrete blocks or ashlar stones wherever something more durable than wood was desired along the damp and hurricane-prone southeastern coast. The tabby was then finished with coats of stucco (also a locally-available mixture of lime, sand, and water) to make a smooth surface and to keep water from draining through the porous tabby and eroding the material.

Tabby is also sufficiently durable that the US Army Corps of Engineers was content to use it instead of concrete for an 1880s-era underground bombproof bunker at Fort Pulaski, located between the coast and the riverside port city of Savannah, Georgia (see photo).

Bunker

Interior of a late 19th Century bunker at Fort Pulaski, Georgia

Coquina, a similar but naturally occurring cementitious material made of geologically consolidated seashell fragments, was likewise used to construct the walls of Castillo de San Marcos in St. Augustine, Florida.

Tabby Economics

Tabby was known in Europe in the early Middle Ages; the now-ruined Wareham Castle in Dorset, England was built of tabby in the early 1100s. It was introduced to North America in the colonial era by English and Spanish colonists in South Carolina, Florida and Georgia and was widely used from the seventeenth century through the post-Civil War era.

Tabby was an ideal building material for the time and place for a number of reasons. Durable building materials such as brick and stone were not locally available on the coast of the southeastern states, which is for the most part a vast sandy plain. Brick and stone had to be imported at a premium cost. The technology for making and using concrete had been lost for a thousand years after the fall of Rome only gradually rediscovered in the early 19th Century. As an interesting aside, this scarcity of durable materials in the coastal South is why 19th-century seacoast forts along the coastline of the southern states were typically built of brick (some 25 million bricks for Fort Pulaski) while those north of Virginia were typically built of granite blocks, since brick was easier to transport and brickworks could be put up wherever there was a suitable clay pit somewhere inland.

By contrast, the ingredients for tabby were readily available—vast buried oyster beds can be found along the shores and islands, with live beds offshore— and although the process was labor-intensive, it was simple enough to mix and pour that it could be prepared by unskilled labor. Thanks to its simplicity and available ingredients, tabby remained in common use until the 1920s, well after Portland cement and concrete became available, although later uses of tabby were apparently more an aesthetic or decorative choice, rather than for structural reasons.

Sapelo Island Examples

The most conspicuous use of tabby I saw during my brief stay was on a visit to Sapelo Island off the Georgia seacoast, where I saw several examples of tabby used for former slave quarters and a mansion, as well as roads where oyster shells were used as gravel. Old tabby blocks marking the ruins of buildings are scattered among the live oak trees along many of the island’s roadsides. The plantation house is now known as the Reynolds Mansion and (along with most of the rest of the island) is owned by Georgia Department of Natural Resources as part of the Sapelo Island National Estuarine Research Reserve. The mansion was originally constructed of tabby circa 1802 by Thomas Spalding, an architect and tabby enthusiast, anti-abolitionist politician, and plantation owner who died in 1851. The building was subsequently rebuilt by Detroit-based Howard Coffin, owner of Hudson Motors, in 1912. Richard “Dick” Reynolds, heir to the R.J. Reynolds tobacco fortune, noted philanthropist, and one of the eventual founders of Delta Airlines, acquired the mansion in the 1930s and renovated the plantation into a sprawling private retreat, leaving it in the form it retains today.

Reynolds Mansion

Reynolds Mansion, Sapelo Island, Georgia

Most of the surviving tabby buildings are now considered historic structures or to have cultural or architectural significance. The upkeep and repair of these buildings is something of an art, as some modern materials may prove incompatible with the tabby and cause the historic material to deteriorate.

One thought on “Tabby- Historic Oyster Shell Concrete

  1. Tom’s usual enthusiasm for gathering the essentials toward understanding something interesting, and his always superb writing.

    I’m going to send this on to the Coast Defense Study Group, itself another scholarly gathering of very serious followers of the long history of our national coastal defenses, such as somehow prevented the attacks that might have come along … and therein this useful information will serve others … now I’ll keep a sharp eye out for mention in and around derelict coastal gasworks down South !

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