Video
The Science Behind Tabby
Transcript
Hello and welcome today we'll be talking a little bit about the building technique that was used to build these slave quarters, barn and other buildings and structures here on the Kingsley Plantation that building technique is simply called tabby. Tabby's been around for hundreds if not thousands of years maybe even as far back as the Roman Empire but it was introduced to the southeast coast of the united states in about the 1580s to the St. Augustine area to make tabby you need four ingredients you need oyster shells you need sand you need water and you need lime lime being the key ingredient is the bonding agent or it's basically the glue that kind of holds everything together so lime was hard to get during that time period it had to be imported from Cuba and the cost of it and the availability of it left the settlers wanting for something different so what they come up with was making something called quick lime from oyster shells. That process takes a lot of time and it takes a lot of manpower but chemically speaking it's pretty simple what you have to do you take the oyster shells and you have to fire them but before you do that you have to build something that you can fire with so what they did they built a kiln and it's simply called a rick kiln because you're alternating layers of logs with layers of oyster shells until you get the quantity that you need it and you put logs around the outside of it and before you set it on fire you got to think a little bit ahead what are you going to do with it once you make the quick line are you going to use it right then if so you'll have to make yourself some brick molds like these if you're going to make bricks if you're going to pour it like tabby then you'll have to make something like this which is called a cradle mold if you're not going to use it as soon as you make it then you're going to have to build something to store it in and it's basically you're building barrels. So what you do you make your rick kiln you got it all put together and you set it on fire and it burns for a couple of days and hopefully what comes out the other end is something called quick lime the whole oyster shells the key component of it is something called calcium carbonate and when you subject it to an intense heat what comes out the other end is something called calcium oxide or quick lime is what uh what I have right here in this this wheelbarrow so we know that we're going to use them immediately so we're mixing it up make sure it's time fixed. (sounds of metal against wheel barrel)
and since we're going to be making bricks with it the only thing i have to add to it is sand if I was going to pour it like concrete then i would have to add whole uninspired oyster shells to it as in that grate or simply as a volume monster so since I know I'm going to be making bricks I'll mix it up give me some sand preferably river sand because beach sand is much too salty and it'll start breaking down your building in a very short time so I'll put the sand at it
mix it (sounds of scrapping metal)
a little too wet so add some more sand to it
I'm sure there's a formula for this but basically it just comes from working with it until you get the right consistency of it
can't be too thick it shouldn't be too thin
when you add water to it like I've done it changes the calcium oxide into what's called calcium hydroxide or slake lime and you know you got the mixture right if your slake line puts off heat and it starts bubbling
and it did both
and it looks like it's about mixed enough
so what I'm going to do is take my trowel and I'm going to put the slight slight lime mixture into my brick molds and you can really feel the heat coming off of it now fill it up so it's got a level at the top that you're satisfied that will make a nice brick out of.
Drying is also part of the chemical reaction now remember when we first burned it when we heated it up it burns off a carbon dioxide element which changes it from calcium carbonate into calcium oxide we added water into it and changed it to calcium hydroxide now it's absorbing carbon dioxide from the air back into the mixture and going back to the original form which is calcium carbonate, which is what we started with in the first place in the form of oyster shells. So to start with you have calcium carbonate in the form of oyster shells you have calcium carbonate in the in the building of tabby and that's the process itself. So thanks for stopping by but if you'll excuse me I got some more bricks to make.
Description
Volunteer Eddie demonstrates the process of making tabby and explains the chemistry lesson behind it. This skill practiced by those enslaved at Kingsley Plantation required hard work, knowledge of the process, and building prowess. Tabby buildings tell the stories of those who built them, and the people who lived there. The largest collection of standing tabby slave cabins is at Kingsley Plantation.
Duration
6 minutes, 34 seconds
Credit
NPS
Date Created
02/11/2021
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