Castillo de San Marcos. Photo by the author via Flickr
The oven is that small structure with the chimney you see in the distance. I always wondered why it was located outside the fort’s walls. The article below explains that. Note the semi-circular structures in the grass to the left side of the photo. Those were gun emplacements also built by the U.S. Army after Florida became part of the United States.
The following article was compiled by workers of the Writers’ Program of the Work Projects Administration in the State of Florida in 1940. You’ll find a copy of the actual manuscript at Florida Memory.
The substantial little furnace for heating cannon balls on the seaward side of Fort Marion (Castillo De San Marcus) has been an object of interest since its erection in 1843. These auxiliaries of the artillery have been common to forts erected on the seacoast since revolutionary times, and the shot heated in these ovens were very effective against wooden ships.
The hot shot furnaces were all about the same size and held 60 or more shot, according to the caliber. The shot being placed in the furnace cold, it required one hour and fifteen minutes to heat them to a red heat, but once the furnace was hot, a 24-pounder shot could be brought to red heat in 25 minutes; the 32 and 42-pounders requiring a few minutes longer. An unusual circumstance concerning the heating was that the balls expanded under the heat but did not
return to their normal size after cooling.
A look into the shot oven at the Castillo. From the author’s collection at Flickr.
Once the balls were cherry-red or white hot, they were taken from the furnace with iron forks, scraped carefully with rasp to remove scale, and carried in ladles to the cannon. The ladles were formed of an iron ring, the interior of which was beveled to fit the ball, with two wooden-handled arms inserted.
Several other implements were attached to the furnace also; pokers for stirring the fire, rasps, tongs with circular jaws for taking up shot, iron rake to remove cinders from ash pit, tub for cooling implements, rammer with head covered by a circular plate of sheet iron of larger diameter than the ball to remove clay from bore when clay wads are used, and a bucket. Many of the implements were furnished in twos so that one set could be cooling in the tub while the others were in use. When the battery was in action it took three men to serve the furnace, and handle the tools.
In siege batteries, or in other situations where there were no furnaces available, a grate was used for heating shot. In a painting of the Battle of Niagara, during the War of 1812, an American woman is shown heating shot on a grating of this sort, while another rushes the cherry-red balls to waiting gunners. In loading the projectile, gunners elevated the cannon’s muzzle sufficiently to allow the ball to roll in, and rammed the cartridge or powder bag home. After the powder was seated, a dry hay wad was rammed against it, then a wet hay or clay wad. Next the powder bag was pricked open and primed through the vent, and a wet sponge passed through the gun. Finally, the hot shot was rolled in packed with another wet hay or clay wad, the match was applied to the touch- hole, and the meteoric projectile bounded across the billow.
The cartridges (powder charge minus shot) for hot shot were little different than those used for ordinary projectiles, being made of cannon cartridge-paper, or parchment well pasted to prevent the powder from sifting out. Sometimes two bags were used, one within the other. When clay wads were used they were cylindrical in form, about one caliber long, and were well moistened. Wet hay wads were preferable, however, and these were soaked in water for about 15 minutes then allowed to drip.
When the wet hay was used, steam was often seen to issue from the touch- hole or vent as soon as the ball was rammed home, but as this was the effect of the heat of the ball against the water contained in the wad no danger resulted from it. It is said that the ball could cool in the gun without the charge taking fire, but shots were usually fired as quickly as possible to prevent the steam dampening and injuring the powder.
It has been argued by some that the cannon ball would cool in its passage through the air towards its objective, but the contrary is true; the temperature of the ball was increased by friction with the air. According to the Ordnance Manual of 1861, a red-hot shot retained sufficient heat to set fire to wood after having struck the water several times!
The penetrations of cold and hot shot into wood were equal under the same circumstances. Charges for hot shot were reduced, however, to one quarter or one-sixth the weight of the shot in order that the ball might remain in the wood and not penetrate too deeply as it was found that the fire was communicated more rapidly and certainly to the wood when the ball did not penetrate more than 10 or 12 inches. At a greater depth the shot would be less effective, as the communication with the external air was not sufficient for combustion.
With the invention of the ironclad Merrimac and Monitor during the later days of the War between the States the days of wooden battleships were over, and the hot shot furnace became obsolete also. During its heyday, however, the arrival of some of the furnace’s cookery rolling along pitch-oozing decks littered with fragments of power bags, very likely terrified the seamen. Often the ship would go up in flames from ignited rigging, or blow up from a shot to the magazine. Somewhat slower, but just as effective, were shots placed “‘twixt wind and water’,” which smouldered [sic] away in the oaken sides until quenching the blaze was impossible.
Although cold and useless today, the hot shot furnace at Fort Marion still stands on a humble monument to the ingenuity of artillerists who have established the Coast Artillery branch of the Army.