Saturday, April 25, 2015

Oyster Shell Tabby


Colonial Fort Dorchester, circa 1757
One of the oldest, most enduring forms of Spanish and British colonial architecture is oyster shell Tabby. I'll briefly share what I've learned about the history, the materials and how to go about building traditional Tabby walls.

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

4 comments:

  1. Hi Patrick,

    Simply and awesome post...and project to share!!

    I have worked with this a few time when in my much younger days. I think it is one of the greatest forms of "natural building" still available today to those interested in sustainable architecture. Tapia, (or tabia) and some of the forms of Bousillage, Bajareque, 小舞壁 (Komakikabe-wattle and daub) also incorporate this medium. I am sure there are others...

    Thanks for sharing this!! I would love to see more about your work in this medium, and what else you have learned/think.

    Regards,

    j

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  2. Good and informative post.

    Several times on the recent visit to Savannah I was told that the sidewalks that I was looking at with an oyster shell aggregate were tabby. My impression though that what I was looking at, and photographing, much to the curiosity of other tourists, were actually a Portland cement based concrete with oyster shell as an exposed aggregate.

    I had a client who a few years back asked me to select a color to paint their building to match to concrete, which set me off on an exploration of the differences in color of concrete. As well, they expressed what I thought was a rather narrow perception of the look of concrete as being a monotonous gray as in relatively new Los Angeles bridge abutments. Since then I have been collecting photographs of concrete, in particular when it is of worn or intentionally exposed aggregate.

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  3. can something similar be done with mussel shells?

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  4. In our Tabby mix it was predominantly oyster; however, there was some mussel and clam shells also. They all produce shells out of calcium carbonate and work just fine. As far as what makes a 'real' Tabby, I would say it's mostly about using shells as the principal aggregate and tamping the mix inside a form to create a wall. Historically lime was used as a binder. Today a little white cement is commonly used to provide added compressive strength. Although they can be quite attractive, I wouldn't classify driveways or stuccoes that utilize crushed oyster shells as Tabby per se, rather perhaps a 'Tabby effect'.

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