Heated shot or hot shot is round shot that is heated before firing from muzzle-loading cannons, for the purpose of setting fire to enemy warships, buildings, or equipment. The use of hot shot dates back centuries and only ceased when vessels armored with iron replaced wooden warships in the world's navies. It was a powerful weapon against wooden warships, where fire was always a hazard. Its use was mainly confined to shore batteries and forts, due to the need for a special furnace to heat the shot, and their use from a ship was in fact against Royal Navy regulations because they were so dangerous, although the American ship USS Constitution had a shot furnace installed for hot shot to be fired from her carronades. The French Romaine-class frigates originally also featured the device, but they proved impractical, dangerous to the ships themselves, and were later discarded.
The idea of setting fire to enemy warships can be traced back to the ancient world, where fire arrows and incendiary materials such as Greek fire were used. In 54 BC, heated clay balls were used by the Britons to attack Roman encampments, while in medieval siege warfare, catapults were used to hurl fire balls and other incendiaries into besieged castles and settlements.
The original method of heating round shot was to cover them in the coals of a large wood fire, or heat them on metal grates placed over a fire pit. These time-consuming methods were improved by the French, who used specially-constructed furnaces to heat shot in their artillery batteries at the mouth of the Rhône River in 1794. The United States incorporated hot-shot furnaces into the design of coastal fortifications during the construction of the Second System of seacoast defenses just prior to the War of 1812. Colonel Jonathan Williams left his post as Commandant at the US Military Academy to build hot-shot furnace fortifications such as Castle Clinton and Castle Williams in New York Harbor during this period. When French engineer General Simon Bernard came to the US in 1816 to head the Board of Fortifications, for the construction of permanent forts to defend the US coastline, he introduced the idea of hot shot furnaces of the French pattern. The chain of US seacoast forts built between 1817 and the American Civil War, such as Fort Macon, subsequently had one or more hot shot furnaces included as part of their standard defenses.
A hot shot furnace was typically a free-standing brick structure with special iron racks and grates, varying in size according to the number of round shot they were to heat and the number of cannon they served - a large furnace might hold 60 or more round shot. They were commonly 6 to 8 feet (1.8 to 2.4 m) wide, and anything from 8 to 30 feet (2.4 to 9.1 m) in length. A brick chimney was situated at one end with a firebox located in the front or side of the opposite end. The interior of the furnace was lined with fire brick and had sloping iron rails sized to hold round shot. Cold round shots were placed in the furnace and allowed to roll down the inclined rails in rows. The first shots halted over the firebox at the low end and were heated "cherry red", approximately between 800 and 900 °C (1,470 and 1,650 °F). When they were removed, the next shots rolled down to take their place and be similarly heated. Care had to be taken not to overheat the shot, as any that were hotter than "cherry red" were likely to become misshapen, and jam in the bore of the gun.
Three men were required to manage a furnace. One maintained the fire and added cold shot, a second man removed heated shot from the furnace, and the third man cleaned them. Special tools were required to handle heated shot. An iron fork was used to remove heated shot from the furnace, then the shot was placed on a stand and cleaned by rubbing off loose surface scale with a rasp. A pair of tongs with circular jaws were used to handle the shot at the furnace. To carry the shot to the cannons, hot shot ladles were used. The ladles had an iron cup for the shot with one or three handles. Round shot less than 24 lb (11 kg) weight size could be carried by one man with a single-handle ladle, while larger shot needed a three-handle ladle, carried between two men like a stretcher.
Great care had to be taken loading heated shot into a cannon to ensure that the red-hot shot did not prematurely ignite the cannon's charge of gunpowder.
A cartridge bag of gunpowder was loaded first. A double bag was used with heated shot to prevent leakage of grains of gunpowder as the bag was rammed down the cannon. Once the bag was in place, a wad of moistened clay or cloth was rammed down against the bag to shield it from the heated shot, which was loaded next. If the cannon was to be fired at a downward angle, another wet wad was rammed against the ball to prevent it from rolling out.
A common practice with heated shot was to fire it with a reduced charge of gunpowder. This would cause the shot to lodge in the wood of the target ship rather than penetrating it, and also cause greater splitting and splintering of the wood.
In 1860, the Martin molten iron shell was introduced to Royal Navy service. These shells were filled with iron melted in a cupola furnace and were intended to break up on impact, splashing molten iron on the target and setting fire to any combustible material present. The shells were named after their designer, an employee of the Royal Laboratory at the Royal Arsenal, the interior was lined with mixture of horse-hair and loam for insulation.
The furnace installation, known as Anderson's Cupola. burned coke and used a steam-powered fan to produce a forced draft. From the time of lighting, around an hour was required to bring 7 hundredweight (320 kg) of pig iron to its melting point of 1,150 to 1,200 °C (2,100 to 2,190 °F) - this amount could fill thirty 8-inch shells. After filling, the shells were left for a few minutes before firing, which allowed the metal in the filling hole to solidify and seal the hole. The shells remained effective even if an hour elapsed between filling and firing as, by this time, the filling would have solidified and the shell casing heated, making them equivalent to conventional heated shot. This included shells that had failed to break up on impact and had remained embedded in the timbers of the target.
Various sizes of shells were tested, but it was found that only the largest shells had a useful incendiary effect. Experiments were carried out in 1859 using the aged, redundant frigate HMS Undaunted as a target. The first three shells were ineffectual, but after the fourth and fifth more had been fired, a fire had been started on Undaunted's lower deck that could not be put out with her fire fighting equipment. The ship was finally sunk with conventional shells.
Molten-iron shells were easier to handle and somewhat more effective than the red-hot shot they replaced. A cupola furnace for melting iron was installed on HMS Warrior. The system was declared obsolete in 1869.