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Cannons in the 18th century were considered the kings of the battlefield. George Washington knew— as did virtually everyone on both sides of the conflict—that without artillery, the American cause would be seriously handicapped. Even before the smoke of Lexington and Concord had cleared, the American forces—at this point, little more than a ragged collection of colonial militias— had laid loose siege to Boston, effectively penning up Lieutenant General Thomas Gage and his British troops. The siege line ran around the city from Chelsea to Roxbury, but it left open Boston Harbor. By late May Gage’s forces had been reinforced through this crucial gateway, until there were some 6,000 redcoats in Boston, including Major Generals John Burgoyne, Henry Clinton, and William Howe. The stalemate at Boston continued for months, with the British numbers swelling, and General Howe replacing Gage as commander. With provisions in ever shorter supply for the British, their breakout began to seem like a possibility. Meanwhile, newly appointed commander in chief George Washington—facing the threat of mass desertions as winter approached and the initial wave of patriotic fervor waned—lacked the artillery necessary to drive the British from the city. “The inactive state we lie in,” he wrote in a letter to his brother, “is exceedingly disagreeable.” A 25-yearold Boston bookseller named Henry Knox, a veteran militiaman and avid self-taught student of military history and artillery, had attracted Washington’s attention with his work on the siege lines. Knox was a big man with a commanding presence, and in November Washington charged him with a critical mission: to make the 300-some-mile trek to Fort Ticonderoga, New York—which Benedict Arnold and Ethan Allen had captured from the British the previous May—and haul the dozens of cannons there back to Boston.

Knox would prove to be an artillery wizard and a godsend to Washington and the Continental Army. With a harsh New England winter looming, Knox—a man with boundless energy and determination—made it to the fort at the southern end of Lake Champlain and selected and disassembled 58 of the most serviceable cannons and mortars, ranging from 3-pounders to massive 24-pounders. After procuring 42 exceedingly strong sleds of nearly three tons each, 80 yoke of oxen, and a number of flat-bottom boats, he and his men proceeded to drag, float, and manhandle some 60 tons of artillery down Lake George, over mountains, and through trackless rugged forests. Outside Albany a huge 18-pounder fell through the Hudson River ice, and Knox spent an entire day retrieving it. At the end of January—two and a half months after setting out—he presented Washington with what he referred to as his “noble train of artillery.”

After a diversionary bombardment on the night of March 4–5, Washington had the guns mounted on Dorchester Heights, where they could command an unimpeded field of fire over the city and the British vessels in the harbor. General Howe and his officers were stunned by the sudden appearance of the artillery, and three days later Howe confirmed that the British would withdraw from Boston. Within the week the British forces, with thousands of loyalists in tow, embarked for Nova Scotia. Boston was back in patriot hands. Heroics aside, the most glaring lesson of Knox’s extraordinary feat was the indispensability of cannons for the patriot war effort.

In July 1775 Congress assigned a Commissary of Artillery Stores and authorized the raising of artillery regiments, supplemented by state artillery units. Still, Washington continued to be plagued by the lack of sufficient ordnance, ammunition, and gunpowder. On Christmas Day of the same year, he lamented to his military secretary, Joseph Reed, “Our want of powder is inconceivable.” So precarious was the situation that Washington’s troops stationed around Boston were issued spears to defend their lines.

At the outbreak of hostilities, the Americans had on hand only a small supply of old cannons formerly used by militia companies, in coastal fortifications, and on ships. The patriots toiled desperately to rectify the situation, and throughout the war, acquired artillery from three sources: British captures, foreign countries (mainly France), and colonial ironworks.

Initially, Washington’s fledgling force relied largely on what it could wrest from the British. Congress offered bounties for the capture of enemy cannons, on land or at sea. During the siege of Boston, an American privateer seized the British ordnance brig Nancy, carrying small arms, flints, bayonets, shot, cannons, and a 13-inch brass mortar. Other Yankee privateers soon followed suit. Continental Army surgeon Dr. James Thacher commented, “Before our privateers had fortunately captured some prizes with cannon and other ordnance, our army before Boston had…only four small brass cannon, and a few old honey-comb[ed] iron pieces, with their trunnions broken off.…Had the enemy been made acquainted with our situation, the consequences might have been exceedingly distressing.” Soon after, Commodore Esek Hopkins, commanding a small flotilla of converted American merchantmen, attacked British-held Nassau in the Bahamas, capturing two forts, along with dozens of cannons and mortars and an impressive store of shells, balls, and powder.

Besides the guns seized at sea and those hauled out of Ticonderoga, American forces also captured British cannons in the give-and-take of battle. When Burgoyne surrendered at Saratoga in 1777, the colonials got a windfall of nearly 50 pieces. But the Americans frequently lost their own guns in the same manner—in retreat or defeat. When Forts Washington and Lee were lost in the battles for New York and New Jersey in 1776–1777, the patriots left behind almost 150 cannons.

As it became apparent to Britain’s political and maritime rivals in Europe that the American rebellion was more than a brief flare-up, they began to make munitions available to the Continentals. The governing authorities in the various American colonies commissioned private vessels to sail to the French, Spanish, and Dutch Caribbean colonies of Martinique, Santo Domingo, and St. Eustatius, where they purchased European munitions often at usurious markups. By the spring of 1776 France—Britain’s greatest rival—was sending increasing amounts of artillery, muskets, and ammunition to the Americans. Many French guns served well as fieldpieces, while others were found to be so cumbersome that Knox—who by then had been made a colonel and named the Continental Army’s chief of artillery—ordered them melted down and recast as lighter ordnance of larger caliber. Still, the importance of French artillery in the American struggle for independence cannot be underestimated. Historian James M. Potts declared: “Without help from the French, the colonists could not have stood up to the British army.”

The continuing and growing need for cannons, however, far outstripped the supply available through either capture or foreign purchases, and the Continental Congress looked to local sources for the casting of America’s engines of war.

For centuries cannons had been forged of either bronze or iron. Many of the guns that the British had abandoned at Ticonderoga were bronze, as were nearly all the cannons provided by the French during the war. Bronze cannons failed less frequently than their iron counterparts and when they did fail, they didn’t burst into fragments, as did cast-iron guns. However, iron guns could be made heavier and with bigger bores and longer barrels producing greater range. And more to the point, iron ore was more readily available in the colonies and therefore cheaper than copper, the base metal for bronze.

Ironworks had constituted one of the first viable types of industry in the colonies, from the early 18th century on, when furnaces began to operate from the New England colonies to the Carolinas and Georgia. By 1775 the American colonies were producing some 15 percent of the world’s iron, exporting about 7,500 tons a year.

While Britain’s foundries had been perfecting the art of gunmaking for centuries, until the Revolution very few American founders had ever cast cannons. And as tensions between the mother country and the colonies had grown, Britain had banned manufacture of artillery in America, declaring it both illegal and disloyal to cast guns. Nonetheless, with the outbreak of hostilities, the Continental Congress and the governing bodies of the colonies called upon America’s ironmasters to manufacture cannons—and to do so with all haste. At the risk of their lives and property, many complied.

For an iron furnace to operate successfully—that is, to smelt raw ore into usable metals—four environmental components were required: a nearby deposit of iron ore, a reliable source of moving water to power the blowing engines, a large forest nearby for the production of charcoal for fuel, and a ready source of lime to act as flux.

Iron could be found in either direct deposits or as bog ore, the latter being less pure. Even the best iron ore deposits, however, possessed nonferrous impurities. Iron composition differed from site to site, and with- out the benefit of modern understanding of chemistry and technology, the smelting of raw ore into usable iron was an imprecise science. According to Robert Brooks, editor in chief of Foundry Management and Technology magazine, “Any imperfections in the molten iron threatened the finished gun’s structural integrity, making it susceptible to distortion by force and heat. That’s one of the reasons so many guns shattered and split.”

Iron cannons were manufactured in four steps: Making a cast, or mold; pouring molten iron into the mold; boring the barrel; and “proving,” or testing, the cannon. Deceptively simple though this might now appear, it was a highly specialized and complex process, demanding great skill and care at every juncture. Iron-making was then a traditional craft; there were no how-to manuals. And considering that most of the colonial foundries had not turned out a single cannon prior to the war, it was very much a matter of trial and error. “There was no prescription at the time for manufacturing cannon,” says Edie Shean-Hammond, National Park superintendent at Pennsylvania’s historic Hopewell Furnace. “They had to experiment.”

The cannons were generally cast vertically. The mold was lowered into a deep pit, the breech end bottom-most, and packed firmly with wet sand. The pouring of the molten iron had to be performed at the proper speed, to ensure that it filled the cavity evenly and cooled properly. At some foundries, guns were cast solid, to be drilled and bored to the desired caliber after cooling. Other foundries placed a solid core as symmetrically as possible down the center of the mold, to form an undersize bore to be enlarged in a later step.

The pouring process was not without its pitfalls. Early in the war, an experienced Pennsylvania foundry man invited several local ironmasters to observe a pouring. Immediately after introducing the molten iron, there came a rumbling from inside the mold, and the solid core suddenly shot upward and through the roof, spilling molten metal onto the floor, where, according to one witness, it “slithered around…like snakes.”

The final phase of the cannon manufacture was the proofing stage. It was here that the gun would be tested for serviceability. It was general practice to “double-charge” and “double-shot” a piece, to test its strength and stability. Failures were common. In May 1776 ironmaster Daniel Joy, acting as agent for the Board of War, tested nearly 200 guns at two Pennsylvania foundries and reported that none had “proved” successfully. Four months later, writing from Pennsylvania’s Hopewell Furnace, Joy noted that of 41 cannons cast and proved, 28 were shipped to Philadelphia for service while the remaining 13 were rejected.

Daniel Hughes, owner of a furnace in what is now Washington County, was commissioned by the Maryland Council of Safety to cast cannons for the army. In a subsequent letter to Congress, the council stated, “The cannon of his first casting did not stand proof, but he has his furnace in such order that the cannon they cast are very good.” Apparently, this qualified recommendation convinced Congress, which allocated an $8,000 advance for the manufacture of “1,000 tons of cannon for use of the United States.” John Cox, owner of New Jersey’s Batsto Furnace, complained to his manager in late 1776, “Out of the 12 howitzers in the first load three burst in proving and out of the last twelve 5 burst so that there are 8 guns gone out of the 24.”

Various flaws could render a cannon unacceptable: unwanted inclusions, air pockets or bubbles in the casting, “honeycombing” (porosity) in the metal, uneven cooling, an improperly aligned core, and off-center or uneven boring. Even proofed guns had their issues. One chronicler of Connecticut’s Salisbury Foundry noted, “Guns of this period were seldom ‘true.’ ” Compounding the problems were the cannonballs themselves; they were sometimes undersize or “out of round,” often causing slow, erratic flight once they’d skipped and bounded their way down the barrel. It is a credit to the patriot gunners that they achieved effective use of their guns.

From almost the beginning of the war, foundries throughout the colonies began to cast guns to meet the patriots’ demand. Hopewell Furnace produced some 115 cannons for the war effort and helped keep both the army and navy in shot and shell for the duration of the conflict. Just a short distance from Hopewell, other Pennsylvania furnaces at Reading, Warwick, and Cornwall regularly turned out guns and ammunition as well. In 1776 alone the Warwick Furnace shipped 60 guns—12- and 18-pounders—to the Continental Army. Meanwhile, Pennsylvania furnaces like Berkshire, Oley, Elizabeth, Pine Grove, and Durham focused mainly on casting cannonballs.

Before the war New Jersey’s Batsto Furnace, like most other colonial-era furnaces, produced pig iron, stoves, water pipes, pots, kettles, salt pans, firedogs, and firebacks. Shortly after the fighting at Lexington and Concord, it began casting cannonballs and hollow shot, and later, howitzers. It transported finished munitions overland to Philadelphia under cover of night through dense woodland rather than risk encountering British patrols along the Mullica or Delaware rivers.

In the final year of the war the Springfield Arsenal in western Massachusetts went from making cartridges and accouterments, storing war matériel, and building carriages for imported French 4-pound guns, to casting their own bronze cannons. “Initially, we weren’t very good at it,” commented Richard Colton, park ranger and historian at the Springfield Armory National Historic Site. “According to the records, 50 percent of the guns we tested failed.” But the arsenal went on to refine its casting technique and for the next 20 years served as the only government-owned foundry making guns.

Rhode Island was initially more fortunate than most of the colonies, in that its Hope Furnace had been casting ordnance for England since 1739. In early 1775, the colonial government ordered 60 heavy cannons and followed up the next year with a request for six 6-pounders, six 12-pounders, four 8-inch howitzers, four 6-inch howitzers, and six mortars.

When the conflict began, the loyalist owner of the Salisbury Furnace in the northwest corner of Connecticut fled to Boston. Connecticut governor Jonathan Trumbull confiscated the works for the patriots. According to some historians, the Salisbury Works supplied a significant percentage of the cannons produced for the Continental forces, including the guns that eventually armed Old Ironsides when it joined the U.S. Navy in 1797. According to foundry historian B. W. Powell, when in full operation, Salisbury employed some 60 men of varying skills and consisted of a blast furnace, molding house, boring mill, furnace barn, and a guard house “to maintain order and decorum.”

The Continental Congress depended on Salisbury for a steady supply of guns. A resolution of the Connecticut State Assembly dated December 18, 1776, stated, “Whereas, the article of cannon is of great importance…and much of ours has…fallen into the hands of our enemies…the Continental Congress [has requested] for a large number to be sent to the northern fronts and for our ships….There appears no way to obtain them but by our foundry at Salisbury.”

It is impossible to determine the exact number of ironworks in all colonies that produced guns and ammunition for the American cause during the Revolution or to ascertain with precision where the guns were sent when completed. Many of the foundries were working in the shadows, essentially “out in the middle of nowhere,” as Batsto Furnace’s John Morsa puts it.

There can be no question, however, of the contribution made by the wartime colonial ironworks. “It took incredible courage,” says Morsa, “for these men to maintain operations in the face of the daily threat of a British or Indian raid.” For major ironworks in exposed locations, the dangers were real: Within a matter of months, Pennsylvania’s Valley Forge and New Jersey’s Mount Holly ironworks were reduced to ashes by the British.

Sadly, a number of foundry owners paid a heavy price for their loyalty. Thousands of dollars’ worth of guns were ordered on credit by state governments and by the Continental Congress, which paid late in depreciating currency or not at all. Without the money to buy supplies or pay their workers, many foundries shut down. Owners such as Hopewell’s Mark Bird went heavily into debt and eventually lost everything. As Hopewell superintendent Shean-Hammond observes, “Confidence in the new nation’s future led to the ruin of many of those responsible for its independence.”


Ron Soodalter has written more than 150 articles for publications, including the New York Times, Military History, Wild West, and Smithsonian. His most recent book is The Slave Next Door.

Originally published in the January 2015 issue of Military History Quarterly. To subscribe, click here.