Big mortars firing small projectiles: Stone Mortars

There’s one other type of mortar which was around during the Civil War. I’ve saved the stone mortars for the end since their employment and service was more trivial than practical. Like the other mortars, the evolution of stone mortars goes back to before the Revolution. Writing in the 1768, John Muller offered this description of stone mortar (or should I say “ftone mortar”) use:

There is another kind of mortar which serves to fling stones into an enemy’s works, when near at hand; such as from the town into the trenches in the covert way, or upon the glacis, and from these trenches into the town, or ravelins…. (Treatise of Artillery, page 91).

Muller offered this drawing of the mortar:

Stone mortars fired scattered projectiles for anti-personnel effect. The form of the mortar’s interior thus differed to better allow use of such projectiles. Turning again to Muller:

The form of the bore at the bottom being different from that in other mortars, is likewise adapted here to the bodies to be thrown out if it; baskets are made to fit the bottom of the bore, which, when filled, are let into the mortar by means of two handles, in order to load it quicker. The stones generally made use of upon this occasion are pebbles the bigness of a man’s fist, and as round as can be found. But as we said before, hand-grenades or small shells made for that purpose, of about two or two and a half inches diameter, will answer the purpose much better than stones.

The stone mortar used smaller charges than other mortars. The French 15-inch stone mortar of Muller’s time used a 3 pound charge, compared to 9 pounds for a contemporary 13-inch land service mortar. This low loading allowed the stone mortar to have 2/3rds the thickness of metal seen on other mortars. These were not weapons for long range firing, but rather to disrupt, maim, and kill enemy soldiers working within the confined spaces during the closing phases of siege operations – either to clear the breech of enemy defenders, or as a last ditch defense by the defenders to drive off the attackers in the breech.

The Army inherited stone mortars after the Revolution, either captured from the British or passed on from the French. A mortar matching Muller’s description is today on display at Yorktown.

10 July 11 320
French Stone Mortar

The mortar was made in 1756 by the Berenger foundry at Douay, France, as one can easily read from the markings. It’s extreme contours are due to the small chamber and very wide bore.

10 July 11 322
Bore of French Stone Mortar

Unless you have an excessively large ego, one can easily “get into” the stone mortar to view the chamber. Notice the thinness of the barrel.

There is some novelty to the stone mortar. But let’s be realistic. These are “one trick” weapons of little use outside siege operations. And even then only good for certain phases of the siege. As I’ve said with respect to other early American mortars, the quantity of serviceable European weapons in the armories meant (I think… but haven’t seen it written in as much words) the Army didn’t ask for any new stone mortars for some time. The Ordnance Board of 1818 recommended an established pattern of 15-inch stone mortar. But the Army ordered none.

The stone mortar came back around during the Ordnance Board meetings of 1838. The following year, the Army ordered one 16-inch caliber example from N.P. Ames Manufacturing. Another came from Ames in 1857. Although only two weapons in that pattern existed, the Ordnance Manuals of the period listed the type as standard. That pattern appears on the plate below, second from left.

The Ordnance Manual of 1850 indicated the 16-inch Stone Mortar had a 20 inch deep bore. The chamber was 6.75 inches long, narrowing from 5.3 inches to 3 inches in diameter. Overall length was 31.55 inches. The stone mortar weighed 1,500 pounds.

Eight-inch diameter trunnions matched that of contemporary 10-inch siege mortars. However the stone mortar required a different bed due to its unique operational needs. After loading the mortar with a basket filled with stones, the crew elevated the mortar to 60°. A more “uniform” option was to use solid shot, ranging from 6-pdr down to grapeshot size. A charge of just 1-½ pounds of powder was sufficient to loft the basket’s contents into the air. From there gravity would do the work, bringing the stones to a lethal velocity with the fall. Range was 150 to 250 yards. The stones or shot scattered over an area 50 yards by 90 yards.

The basket could also hold fifteen 6-pdr shells. The 1853 Instructions for Heavy Artillery suggested fifteen second fuses for the shells. For this projectile, with the mortar at 33° elevation and only a pound of powder, the payload landed about 150 yards down range. The shells scattered in a diameter of 20 to 30 yards. The low elevation and light powder charge were required to ensure the shells did not bury into the ground on impact, and instead burst on the surface. The manuals also indicated grenades could be fired in the same manner. But the instructions offered this warning:

As the shells are liable to burst on leaving the bore, the piece is fired by a slow match applied to a train of quick match, giving the men time to place themselves under cover.

Regardless of payload, the crew first loaded the powder charge. Over the chamber sat a wood plank with holes (to allow the flash of powder to ignite the shell fuses when used). The basket, filled with the required projectiles, sat on top of the plank.

All this sounds nice. But again, tactically these mortars had but a handful of situations to which they were applicable. Otherwise other weapons of the day filled the needs handily. One of the two 16-inch stone mortars survives today at West Point. So it was around during the Civil War. But there is no record of its operational use. The Ordnance Manual published in 1862 did not mention the stone mortar. The Confederates retained instructions for the use of stone mortars in the manuals they published during the war. I’ve seen one Confederate operational reference to a “stone mortar” used to cover the rear approaches of a fortification. This was likely an older type of European origin, impressed where nothing else better was available.

However limited the “niche” of the stone mortar, that tactical requirement still held. Instead of using these specialist weapons, the army opted for similar scatter projectiles fired from the conventional mortars in field service.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

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