From The Feud: The Hatfields and McCoys: The True Story, by Dean King (Little, Brown, 2013), Kindle pp. 64-66:
The still sat on a flat bald stretching about fifty feet across the side of the mountain. Devil Anse used a sixty-gallon boiler that he had bought from the owner of a steamer on the Big Sandy. The deal had taken place at dusk one evening near Louisa, Kentucky. They rolled the heavy boiler onto a flatboat, covered it with a tarp, and disguised it with barrels. Then Devil Anse and three men—possibly his sons, and possibly Big Jim, Randall’s son, who worked for Devil Anse making moonshine (though it is hard to know for sure since the business was clandestine)—had poled it up the river. Finally, it, like everything else, had been lugged the mile up the creek to the bald on a corn sled—a wooden crate on runners for hauling corn out of sloped, rocky fields. They cut a door in the bottom of the boiler and placed it on a big square slab of sandstone that was balanced with rocks underneath its corners.
Devil Anse and his sons built a dry stone wall around the still with a roof of split boards over it. They left a hole in the wall to allow them to reach in and build a fire beneath the sandstone slab. Fresh ice-cold water was funneled to the operation via wooden troughs from an uphill spring. The wood they needed for making buckets and barrels and for fires was plentiful around the bald. All they had to haul up was the main ingredient. When they were making apple brandy, or applejack, Devil Anse’s specialty, they needed three hundred bushels for a large batch, and lugging those apples up to the still on the corn sled was a major task. Up top, the men took turns mashing the apples a bushel at a time in a solid tub, using the butt of a small buckeye tree. They shoveled the apple pulp into 125-gallon vats and stirred in water to create what looked like a thin applesauce. They made about 1,300 gallons of apple mash at a time and then let it sit for ten days while it soured.
On the eleventh day, they began filling the still with the fermented apple mash. The cap was screwed onto the still, and the worm—a copper coil—onto the cap. They built an intense but low-smoke hickory-wood fire beneath the stone. By heating the stone instead of directly heating the boiler, they never burned the mash. Once the stone and still were hot, it took just a small fire to keep the batch at a low boil, just right for making moonshine. Alcohol vaporizes at 173 degrees F, and they kept it as close to that temperature as possible to avoid scalding it.
As steam rose from the simmering mash, it passed through the copper coil, which ran through a wooden barrel filled with cold spring water, and condensed. The resulting liquid trickled out into a wooden bucket. Each full bucket was emptied into a barrel. As long as the stream of liquid coming from the barrel tasted like brandy, they kept it coming, usually for about four hours. Once it got watery, they snuffed the fire, emptied the still through the door in the bottom, and started over again. This way they made six singlings—the amount of whiskey from a full still—in a twenty-four-hour period. Each singling amounted to about ten gallons. It was intense work, and when it was finished, they were only halfway there; a man could get very drunk and very sick off singlings, but this was not the product they were after.
Once enough singlings were collected to fill the still twice, the men gave the still a thorough cleaning, then filled it with the singlings and lit the fire; the steam ran through the worm and was condensed again, this time producing an even purer whiskey, the doublings. It was about 98 percent pure alcohol. Around ten gallons were produced before it began to weaken. Then the men put the fire out, topped off the remaining liquid with more singlings, and lit the fire again.
In this way, six gallons of mash produced a gallon of singlings, and a hundred and twenty gallons of singlings yielded forty gallons of top-quality Hatfield applejack.