Thursday, June 13, 2019

The Crucible

Can you guess what chemical process is being demonstrated here at a "living history" event we presented a number of years ago? Hint: It is very common and can be found all around us at any time. It has been used since ancient times as a weapon of war, in bread, root canals, the theatrical stage, grits, making paper and to give pickles an extra crunch. Oh yeah and sewage treatment too.

It was in fact, recently referred to in an article about hiking by Ed Parsons in this past Saturday’s Conway Daily Sun giving evidence that it was produced in New Hampshire.

While the production process to make this chemical compound is technically known as "thermal decomposition and re-hydrating" I call it burning rocks and then getting them wet.

In the picture above, the rock closest to the edge of the board is a little smaller and lighter in color and weighs a little bit less that the one next to it. Originally they were both about the same, but the lighter colored one was burned in a fire.

There was a third rock here, but we added some drops of water on top of it. At that point, that rock expanded its volume two or three times and reacted violently with hissing and steam. Then the water started to boil and the rock crumbled into a fine powder. It really is magical stuff to watch.

Maybe this diagram of the chemical process will make it easier to figure out.

Did you get it yet? It should be crystal clear now right? 

Let's try it another way... back to the Conway Daily Sun.

Ed's article talked about a hike that started on a dirt road named Lime Kiln Road. Opps, I just gave it away.

Yes, the chemical compound is lime. Near this trail are two stone lime kilns built in 1838 and 1842 where limestone was mined, heated in this kilns, powdered into lime, packed into barrels and shipped throughout New England. One of the kilns was featured on the cover of the 1956 Haverhill Town Report.

And here is a more recent photograph and below that a diagram that helps explain what went on inside (click on images to enlarge them).

After being quarried nearby, the limestone was loaded in the kiln from a wooden ramp (now missing) through a hole in the top (like the top of a chimney) and mixed with wood or charcoal. A fire was started at the bottom and stoked as needed through those brick holes on the left side (click on image to enlarge it). As the limestone crumbled it released the burnt lime which was collected through the opening at the base of the kiln (the opening looks like a large fireplace). It was then packed into casks to keep it from clumping or catching fire.

According to James Garvin's A Building History of Northern New England (available at a number of local libraries including Conway through our new Northern New Hampshire Library Cooperative) until the early 1700s, when limestone quarries and large masonry kilns, such as those seen above, began to be developed "lime as frequently obtained by calcining (heating) oyster shells."

Here is an example of one of these simpler, open air, lime ricks for burning oyster shells recreated at George Washington's Ferry Farm.

So what is actually going on here on a chemical level? Well it starts with burning calcium carbonate - both limestone and seashells are the same chemical compound - calcium carbonate.

When heat is applied the carbon dioxide is driven off into the air leaving calcium oxide also known at quick lime.

The quick lime was packaged, shipped and stored until needed. When water is added a chemical reaction occurs, heat is generated and the resulting material is calcium hydroxide also known as slaked lime.

In 1677 Joseph Moxon wrote poetically and mysteriously of the process.

Now to the uses. According to Garvin plastering ceilings and walls consumed most of the lime produced in New England. While extolling the gifts of the ox, Thomas Randall also known as the Eaton Poet, explained how the plaster was made.

(Note: I never said it was good poetry). For more on the Eaton Poet, see our previous blog.

Lime has many other uses in architecture including mortar for bricks, paints, whitewash, glass and the putty to set the glass panes into the window frames.

See this previous blog for information on burning mud to make bricks. 

Finally (and I literally mean finally) as Moxon writes lime was used in burials as it "consumes dead Bodies."

For more details on this or any other historical subject in the White Mountains, contact us at the Conway Public Library's Henney History Room.

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