Bigger Iron Frogs: 10-inch Model 1861 Siege Mortars

Confederate Colonel Moses White probably wished he had a these in the armament of Fort Macon during March and April 1862:

Fort Macon 2 May 10 191

The pair are 10-inch Model 1861 Siege Mortars. Colonel White’s garrison lacked mortars when Federals laid siege to the fort. Instead the Confederates used six 32-pdr carronades (and eventually some columbiads) for high angle fire against the siege lines. On the other side, the Federals brought four 8-inch and four 10-inch siege mortars (exact model number unknown) to bear on the fort. The marker between the mortars indicates the pair were part of Fort Macon’s armament in the post-war period (and likely part of that shipped to the fort during the Civil War).

These mortars are part of an early batch produced by Cyrus Alger, of Boston, in 1862. Rather conveniently, the mortars are registry numbers 7 and 8, weighing 1,968 and 1,966 pounds respectively.

Fort Macon 2 May 10 193
10-inch Model 1861 Siege Mortar – Alger #7

Both have the inspectors initials of Thomas J. Rodman.

Fort Macon, NC - 10 In Siege Mortar Model 1861
10-inch Model 1861 Siege Mortar – Alger #8

For the most part, the 10-inch version differed only in dimensions from the contemporary 8-inch Model 1861. Both shared common external features – center mounted trunnions, lifting lug, and elevating sockets on the breech.

Fort Macon 2 May 10 195
Profile of Alger #7

Early examples from Alger had three elevating sockets on the breech. Later examples had eleven. Since the Fort Macon mortars are on an original bed, we also see the eye used for the elevating bar.

Fort Macon 2 May 10 194
Elevating socket and “eye” for the bar

Just like the other 1861 mortars, the 10-inch siege weapons had two vents. These lacked the divots, or recesses, of the larger seacoast mortars.

Fort Macon 2 May 10 196
Vents of 10inch siege mortar

One point I failed to mention yesterday in regard to the 8-inch mortars is the interior shape. The Model 1840 mortars used a conical or gomer chamber. But the Model 1861 changed to a “stretched hemispherical” chamber, similar to that used on Rodman guns. As the case with the big guns, the chamber profile reduced stress when fired. The shape also allowed the use of reduced charges without the need for additional wadding or modified sabots.

Turning back to the table comparing the 1840 and 1861 siege mortars, the 10-inch Model 1861 used the same charge as the earlier model of the same caliber, but it fired the shell over 100 yards further…. well again, according to the firing tables.

(Drat! I just noticed I put the wrong year model in there. Until I update this, read 1840 were the table has 1841.)

As indicated on the table, and just as with the 8-inch mortars, three vendors produced the 10-inch Model 1861. And as with the smaller mortars, the vendors used Rodman’s hollow-cast, water-cooled technique on at least those weapons produced after 1862, if not for all the production run.

Alger delivered the first batch of ten in April to July 1862. They added another sixteen from July 1863 to April 1864, bringing their total to 26.

Fort Pitt Foundry received credit for its first 10-inch Model 1861 in September 1862. The Pittsburgh foundry delivered 74 by the end of October 1864. Four of those stand on display today at the Army’s Fort McNair.

McNair 10 Apr 10 012
10inch Model 1861 Siege Mortar – Fort Pitt #18

This particular mortar is registry number 18 of Fort Pitt’s line, weighing 1940 pounds. It was inspected by Charles P. Kingsbury in 1863.

Unlike the Alger mortars, the Fort Pitt mortars had recesses at the vents. As with all Fort Pitt mortars, these have raised foundry numbers, but in this case above the left trunnion.

McNair 10 Apr 10 016
Lifting Lug, vents, and foundry number

Like the Alger mortar, the early Fort Pitt mortars had three elevating sockets.

McNair 10 Apr 10 017
Elevating sockets on Fort Pitt Mortar

All examples at Fort McNair date to 1863 and have three sockets.

As they are mounted on original mortar beds, the Fort McNair weapons tend to catch a lot of rain and other debris.

McNair 10 Apr 10 023
Bore of 10-inch Mortar

But this does allow examination of the rounded bore bottom.

Joining Alger and Fort Pitt, the Scott Foundry of Seyfert, McManus & Company, in Reading, Pennsylvania, produced 50 of the 10-inch Model 1861 starting in August 1864. I don’t have pictures the twenty-one survivors of that production run. One of them is in San Diego, so perhaps XBrad will stumble across it at some point.

Just as with the 8-inch siege mortars, the combat history of the 10-inch variety peaked at Petersburg. Roughly one-third of the total produced survive today.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

4 thoughts on “Bigger Iron Frogs: 10-inch Model 1861 Siege Mortars

  1. I can send you pictures of the two 10” mortars in Cohoes NY if you like. Really enjoy your work. Wish I had a computer instead of trying to view on a phone.

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