Looking straight down at the one of Fort Pike's two shot furnaces that is at least recognizable as a shot furnace (they're both along the rear of the fort's eastern-facing, curved wall, and this is the southernmost of the two), it seems pretty clear that this was little more than a grill, atop which cannonballs would be placed until they were red-hot and ready for action.

The earliest means by which shot was heated, to be gingerly placed into cannon and flung at enemy ships in the hope that they would lodge in that ship's timbers and cause an unquenchable fire, was to simply place the balls into an oven, or to heat them on just such a grill as this. Taking into account how close the rear of this furnace is to the fort's wall, we can also tell that the system by which balls would be heated in later furnaces, dropping them into one end and fetching them red-hot out the other, wasn't how this one worked. If it ever worked. Evidence that hot shot was ever used in battle in the United States is pretty sparse, with the only reliable tale that I've heard coming from Fort McAllister in Georgia, during the US Civil War.