The Colonial Art of Making Steel

Making steel in the 18th century was a challenge, especially in America. Steel was essential for making edge tools, such as axes and scythes, and other items like bayonets, knives and the mechanisms for guns and clocks; yet the science of making steel was poorly understood. Most steel used in the American colonies was imported from England. Only a few Americans attempted to make steel, often with little success. The Trenton Steel Works, located at this site, was a rare example of colonial American steel manufacturing. From the late 1740s to the mid-1780s, the furnace produced steel of variable and sometimes questionable quality at irregular intervals.

Making Steel by the Cementation ProcessThe Trenton Steel Works employed a cementation furnace. The brick-lined furnace enclosed a chest in which lengths of bar iron were layered and packed with powdered charcoal and capped with sand. A fire was then lit beneath the chest, gradually heating the furnace to between 1560°F and 1830°F. The hot gases from the fire circulated around the chest, heading the iron and charcoal, and then passing up a flue and out a stack.

Inside the cementation chest, the red-hot bar iron absorbed carbon from the charcoal. Once the furnace reached the desired temperature, the fire had to be tended night and day to maintain the necessary. After five days or more, the converted iron was removed.

The product of the cementation furnace was called "blister" steel, so called because of its raised blistery appearance. Blister steel fresh from the furnace as of no use: it had to be hot worked and reduced in sections by forging under a water-powered hammer, then rolled into bars that could be sold to artisans who made tools and machines.

The Trenton steel furnace may have been similar to this example recorded in Sheffield in the mid-1760s. A. Furnace wall, B. Fire Chest, C. Cementation Chest, D. Hot Gas Chamber, E. Chamber Arch, F. Flue, G. Stack (G. Jars, Voyages Metallurgiques, 1774).

The Furnace House and BaseThe cementation furnace was contained within a building that measured 31 feet by at least 36 feet, roughly matching the dimensions of the "house" given in a sale advertisement of 1765. Archaeologists uncovered the remains of the outer walls of the furnace house, standing in places up to two feet high and composed of locally quarried rock.

In the northeast corner of the furnace house was discovered the stone and brick base of the furnace itself. Walls of mortared stone linked the furnace base to the north wall of the furnace house. These most likely buttressed the furnace structure, offsetting the risk of collapse from repeated firings. Compared to examples found in England, the furnace was rather small, measuring only 10.5 feet by 9 feet in plan. The Trenton furnace is thought to have resembled a type of single-chest furnace documented by French metallurgist Gabriel Jars in Sheffield, England in 1765-66.

Archaeologists found no steel on the site. This was too precious a commodity not to have made its way to market. Furnace brick was recovered in abundance, along with pieces of the cast-iron furnace grate that would have covered the ash pit at the base of the structure.

The furnace house probable resembled the structure shown in this sketch of the Blackhall Mill furnace in Derwent Valley, England, produced in the 1750s (R.R. Angerstein's Illustrated Travel Diary, 1753-1755).

Trenton Steel in the Colonial PeriodThe first steel furnace was erected at this site by Trenton blacksmith Benjamin Yard sometime between 1745 and 1750. It was one of only five steel works in the American colonies in this latter year, when the British parliament inventoried the American iron and steel industry, seeking to control its growth and protect English metalworking interests.

Yard and subsequent owners received their bar iron from the furnaces and forges of northern New Jersey and eastern Pennsylvania. Andover iron was especially well suited to steel manufacture. The bars were shipped down the Delaware in Durham boats and offloaded at the falls just a few hundred feet from the steel works site. The manufactured steel was then transported to Philadelphia and other eastern seaboard markets, by boat or by wagon.

In 1762, Yard sold the furnace to a pair of young Philadelphia Quaker merchants, Timothy Marlack and Owen Biddle. Both Matlack and Biddle are best known for their support of the American Revolution. Matlack, as Secretary to the Continental Congress, inscribed the copy of the Declaration of Independence that is on display at the Nation Archives. Biddle, a trained clockmaker and founding member of the American Philosophical Society, would have had an appreciation of the use of steel in precision instruments.

In 1770, John Pemberton, another Philadelphia Quaker merchant, acquired a half share in the steel works and retained his brother-in-law, John Zane, a member of a well-known Pennsylvania iron-making family, to run the works. Zane sold steel behind Pemberton's back, ran up debts and eventually disappeared, turning up a few years later in the Caribbean as a penniless carpenter. During this period, despite being claimed as "quite equal if not better in quality than what is imported from England," Trenton steel was of questionable caliber and sold with some difficulty.

Trenton Steel During the War YearsDuring the American Revolution, the Trenton Steel Works operated intermittently. In March 1776, a half ton of Trenton steel was supplied to the Continental Army in Albany. The furnace may have been damaged in the fall of 1777 at the same time that American troops rendered the nearby plating mill inoperable in order to keep it from falling into the hands of the British.

In July 1781, following Trenton merchant Stacy Potts's successful attempt to revive the furnace, the Trenton firm of Potts & Downing secured a contract with the U.S. government to convert Andover iron into steel. Potts & Downing failed to deliver fully on this contract, and by 1783 were in debt to one of their main suppliers of bar iron. The furnace managed to stay in operation through at least mid-1783, but several years of legal wrangling led to Stacy Potts losing most of his assets and moving west to Harrisburg to start a new life.

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Petty's Run
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Petty's Run
Changing Landscapes Along Petty's Run
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