Wood charcoal fueled the forges in the nailery on Mulberry Row and heated the stoves in the kitchen. It was stored under lock and key in wooden sheds built about 1794 that likely resembled temporary lean-tos and served as secure holding areas for fuel that could be easily transported to the nailery, smith's shop, or the main house. As fuel needs increased on Mulberry Row, additional sheds may have been built, each holding 8,000 bushels of charcoal. The nail-making operation required large amounts of fuel; Jefferson calculated that, on average, 666 bushels of charcoal were needed to make 172,480 nails in the nailery. In the spring of 1800, Frank, an enslaved charcoal-burner, produced most of the charcoal in coal-kilns on the mountaintop after learning the trade from the German charcoal-burner Jacob Silknitter. David Hern, Lewis, and six hired slaves cut 200 cords of coal wood for Frank's charcoal production. Charcoal-burners like Frank earned a half dime for every bushel to the cord of wood that their kilns yielded, and Jefferson used cash incentives to maximize the efficiency of work performed by Frank and other enslaved charcoal burners, including James Hubbard, Cary, and David Hern.