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In pulling off some broken mortar on the dirt foundation (where the actual brick footers rest) of a 1920's house crawl space, I discovered a layer of black coal just under the mortar. There are chunks of it, and some smaller particles.

Was coal ever used as an insulator? Why would the original builder install a layer of coal below the mortar?

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I've never heard of a dirt footer. This entire foundation might be a bit of a home made 'use whatever we have laying around' thing. Back in the 20's lots of builders WERE the homeowners and used what they had. –  DA01 Jun 18 '12 at 18:51
    
@DA01 -- the dirt footer is the mound that the actual brick footer sits on. I may need to rephrase my question to avoid confusion. When the house was built, or some time after, they dug out the area under the house and made it a walk in crawl space. The shoulders of clay were left and covered with a thin layer of mortar. –  a coder Jun 18 '12 at 19:19
    
Where do you live? I've never heard of such a foundation before. I'd be a bit wary of the entire thing, but perhaps it makes sense given your region. –  DA01 Jun 18 '12 at 19:26
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Maybe the builder was trying to make diamonds. –  Tester101 Jun 18 '12 at 20:26
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My grandfather used to feed pigs coal to get rid of parasites - and coal tar (creosote) has historically been used as a pesticide on wood poles. Perhaps that is somehow related... –  Mark Schultheiss Jun 18 '12 at 20:31
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Coal (especially in cities above a certain latitude) was stored under houses, often delivered down a chute to be handled as little a possible as it is messy & dusty.The cost of removing the leftover coal (an unpleasant job) far exceeded its value, so it stayed, often getting covered up later to stop the dust. Heating oil & tanks today face a similar $ dynamic in houses changing to gas, geothermal, electric, etc.

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