The term “petre cave” refers to the presence of calcium nitrate, or nitre, in the soil of the cave, the by-product of nitrogen-fixing bacteria. No record exists to tell who first discovered nitre in the cave, but from the start men valued the cave for this commodity. Nitre, put through a simple process, could be converted into potassium nitrate, or “saltpetre.” Saltpetre, combined with sulfur and charcoal in the proper quantity, renders a good grade of black gunpowder – as dear as gold to men and women taming a wilderness.
So dear, indeed, that 1812, the cave’s worth increased over 25 fold in a single day. Hyman Gratz had purchased a half-interest in the cave from Fleming Gatewood for $10,000. Gatewood, along with Charles Wilkins, had bought the cave from Leonard and John McClean, who had bought it from John Flatt, who had purchased the land and the cave from Valentine Simon for $116.67 only a few weeks before. Then, on July 9, all parties cleared title, determining the cave’s exact worth.
There was money to be made from the cave, and Gratz and Wilkins set about making it. They had the capital and physical assets – in the form of nearly 70 slaves – to mount a considerable enterprise underground. The slaves began to construct the apparatus which would leach the nitre out of the soil, under the watchful eye of overseers, by the light of dim and smoky lanterns.
Bare from the waist up, the slaves began their work by hollowing long logs of tulip poplar, using a long iron tool called a “spoon-bit” auger. They would position the tool at the end of the log to turn out the softer heartwood, and turn the auger by hand for hours to ream out half the log; then several more hours were spent hollowing from the other end to finish the job. Dozens of such logs were hollowed to make a pair of wooden pipelines extending over a third of a mile into the darkness.
Square leaching vats were then built, with split logs overlapping to form a filter below. The slaves set collection troughs below the vats, and a pump tower that rose in the center of the site. When the entire apparatus was complete, the slaves were made to set it into motion.
The slaves, by smoky lantern, led oxen deep along the broad passages to gather cave soil into oxcarts. Once gathered, the soil was returned to the vats and shoveled in. Then water, perpetually falling from the lip of the cave mouth, flowed down the first wooden pipeline to pour over the soil in the vats. The water would seep through the soil, leaching out the nitre; the result, pouring into the collection troughs, was an amber-colored, frothy liquid called a “nitre beer.” A slave standing atop the pump tower then pumped to elevate the nitre beer so that it could flow, by gravity, back to the surface by the second, elevated, pipeline. Slaves outside the cave would then combine the nitre liquid with potassium-rich ingredients like potash, oxblood and turnips, and boil the mixture over an open fire until all the water had evaporated, leaving crystals of saltpetre behind. They then packaged these crystals to be delivered to the newly-formed DuPont chemical company in Delaware to be made into gunpowder.
And that was a good thing.
Because war was brewing, a war we now call the War of 1812. The young United States found itself again at war with the British, and one British strategy was to choke off America’s supplies of foreign gunpowder. The British success in this could have proven disastrous, had it not been for the Mammoth Cave. It, and at least one other local cave, Dixon Cave, supplied ingredients to help tilt the conflict in favor of the U.S. Yet there is an irony – the nation’s continued independence was bought, in part, by the labor of those who had none.
Last updated: May 23, 2018