The Regional Review

Volume II - No. 2

February, 1939


By Herbert E. Kahler, Coordinating Superintendent,
Fort Marion National Monument,
F. Hilton Crowe, Associate Editor,
Federal Writers Project in Florida.

Furnace at Fort Marion National Monument
Furnace at Fort Marion National Monument

Next to the dungeon no other feature at Fort Marion elicits so many questions as does the hot shot furnace located near the sea wall. Visitors are interested in knowing when this old structure was built, why and how it was used. The use of hot shot antedates the use of gunpowder. In 54 B.C. the Britons fired heated clay balls into the tents of the invading Romans with telling effectiveness. With the advent of gunpowder there was considerable hesitancy in using hot shot because of the great difficulty in controlling the time of the explosion. By experimenting, a clay was was devised that effectively separated the hot ball from the powder and in 1579 the King of Poland successfully carried on a siege using hot cannon balls in his guns. The use of hot shot became increasingly important in coast defense, especially in the destruction of wooden vessels. During the siege of Gibralter in 1782 a part of Spain's fleet was set on fire and destroyed by hot shot.

The heating of cannon balls was accomplished on open grates, a slow, wasteful and dangerous method. A great advance was made with the development of the hot shot furnace, which in 1794, was successfully used at the mouth of the Rhone River.

The hot shot furnace was brought to this country in the early part of the 19th century. One of the outstanding military engineers, Simon Bernard, Brigadier General for Napoleon Bonaparte, was employed by the United States to make a survey for coastal fortifications in the south eastern United States and his recommendations, which included the latest advances in coastal defenses, were presented to Congress in 1817 and adopted. At Fort Pike and Fort Macomb in Mississippi, Fort Morgan, Alabama, Fort Jefferson and Fort Marion in Florida, the hot shot furnaces are still in evidence. At Fort Pulaski, near Savannah, Georgia, only the foundation of the hot furnace remains.

The addition of this 19th century innovation to 17th century Fort Marion has an amusing history. The War Department in 1825 declared Fort Marion useless for defense purposes but in 1835, after the Second Seminole Indian War started, it declared the fort had defensive values and built a water battery and hot shot furnace. It also proposed the construction of shallow draft galleys as additional defense to the fort. Apparently, the Seminoles were expected to attack from the sea.

The hot shot furnaces varied somewhat in size, the one at Fort Jefferson being the largest. If the shot were placed into a cold furnace, it required one hour and fifteen minutes to heat them to a red heat and once the furnace was hot a 24-pounder shot could be brought to cherry-red color in twenty-five minutes, the 32 and 42-pounders requiring a few minutes longer. An unusual circumstance attending the heating was that the balls expanded under the heat but did not return to their normal size after cooling.

Once the balls were cherry-red or white hot, they were taken from the furnace with iron forks, scraped carefully with a rasp to remove scale, and carried in ladles to the cannon. The ladles were formed of an iron ring, the interior of which was bevelled to fit the ball, with two wooden handled arms inserted.

A number of other implements were needed at the furnace; pokers for stirring the fire, rasps, tongs with circular jaws for taking up shot, iron rake to remove cinders from the 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 were 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 preparation for loading the projectile, the gunners elevated the cannon muzzle; then they 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 sped across the billow.

The cartridges (powder charge minus shot) for hot shot were little different from 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 calibre 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 from 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 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 iron-clad Monitor and Merrimac, the days of wooden battleships were numbered and the hot shot oven quickly became obsolete, but although it is cold and useless today the hot shot furnace is still an object of curiosity and interest.

Old Oven at Fort Morgan
Old Oven at Fort Morgan, Mobile Point, Alabama.

This article was subsequently reprinted as NPS Popular Study Series #7.

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Date: 04-Jul-2002