The Castillo de San Marcos was built by the Spanish from 1672 to 1696. This massive chunk of colonial power existed both to oversee their Floridian lands and deflect the wicked British, who were ever attempting to encroach upon Spain's idyllic collection of palm trees.

Heated cannon shot was pioneered in the late 16th century, but the hot shot furnace wasn't perfected until the end of the 18th century, in France. The hot shot furnace in question on this page has no connection to the Castillo's original inhabitants.

Spain ceded Florida to the United States in 1819, as part of the Adams-Onís Treaty. The US Army took official possession of the Castillo de San Marcos on July 10, 1821, and promptly renamed it Fort Marion, after US Revolutionary War hero Francis "The Swamp Fox" Marion (1732-1795). As pleased as the US may have been with its new massive coquina-stone acquisition, it obviously didn't consider Fort Marion to be of any particular military use, because the War Department declared the fort "useless for defensive purposes" in 1825.

Ten years later, however, the US was embroiled in the Second Seminole War (1835-1842). Chiefly an effort to remove the Seminole Indians from Florida to reservations west of the Mississippi, this was a hugely expensive conflict in both human life and resources. During this period there must have been some concern about a waterborne Seminole attack on Fort Marion (which sounds ridiculous now, but angry dudes paddling towards you in canoes would have been scary), so the decision was made to add a water battery to the fort.

A water battery adds firepower to a fort's wall-mounted guns, allowing a defender to fire balls at water level, often "skipping" shot into an attacking ship (or canoe)'s hull. Counterintuitively, heated shot could be utilized in such a manner. Even after skipping merrily across the water, a heated solid iron ball retained enough heat to set a ship's hull aflame, if properly lodged therein.

The inspiration for a water battery at Fort Marion was probably not the threat of amphibious Seminoles. Florida would become the 27th United State on March 3, 1845, and Fort Marion's water battery was built from 1844 to 1846: Impending statehood and the potential need to defend itself from actual naval powers, particularly with the Mexican-American War (1846-1848) rapidly approaching, seems a more likely reason.

But the subject of the water battery finally brings us to the Castillo de San Marcos' hot shot furnace, which was built by US Army engineers in 1844. The Castillo's hot shot furnace is unique for two reasons: Its construction, and its placement.

Most of the hot shot furnaces at America's fortifications were straight-up brick affairs, built from a standard kit provided by the US government. The Castillo's shot furnace, meanwhile, is basically a brick structure, but with a layer of coquina stone (the material from which the Castillo itself was constructed), and then a coating of plaster. When some restoration work was done on the Castillo's hot shot furnace a number of years ago, it was discovered that its chimney is completely made of coquina!

Why coquina, other than the obvious fact that it's just so darned cool? By the time Fort Marion's water battery was being built, the coquina stone-constructed Castillo had been standing for a century and a half, proving that, at least under local conditions, this building material was an effective one indeed. So, why not coquina? It was a locally plentiful, hardy and easy-to-work substance.

The placement of the Castillo de San Marcos' hot shot furnace is also unique. A shot furnace's location was chosen solely by its proximity to the guns it would need to serve. In all other cases, this led to a fort's hot shot furnace being placed practically as an afterthought, often rather awkwardly as viewed today in a satellite photo.

The Castillo's hot shot furnace, conversely, fits perfectly with the symmetric nature of the fort and its water battery. This may seem a trivial observation, but it's obvious that someone was paying attention to such details in 1844, wishing to maintain the historic fort's pleasing aesthetic...and this is something that didn't appear to happen anywhere else.

Infinite thanks to Volunteer Coordinator of the Castillo de San Marcos National Monument Jill Leverett, for providing much information for this page, and one of the pictures!

Please click on any of the images on this page to see their full-sized versions, plus read more of my half-baked observations!