In the early 17th century nations making claim to the Americas discovered an enormous wealth of natural resources. Raw materials bolster a country’s economy and increase its ability to dominate in trade and in war. Though the discovery of gold by the Spanish increased that nation’s wealth, iron ore from North America gave Great Britain the means to increase her power and influence over much of the globe by the end of the 18th century.
In 1607, the English settlers in Jamestown saw enormous potential in the abundance of iron ore they’d found. Vast forests allowed for making charcoal[1], necessary to smelt ore. Unlike gold, Iron is not found in nature in a pure state (except in meteorites). It is found in combination with other elements, iron oxide being the most common. Iron is reduced (smelted) in a forge by a process using carbon monoxide gases and high heat. In 1608, John Smith loaded several barrels of iron ore and shipped them back to England to be tested for viability. The East India Company discovered that this ore yielded top-quality iron. In 1619, the company invested in Virginia iron and by 1620 it began building iron furnaces at Falling Creek, east of present day Richmond. In 1644, John Winthrop constructed bog iron smelters[2] on two sites in Massachusetts: Braintree, south of Boston, and on the Saugus River, north of Boston. The forges in Virginia and Massachusetts were short-lived however. Native Americans massacred the twenty five workers at Falling Creek and destroyed the forge in 1622. The iron operations outside of Boston were closed in 1668 because of labor shortages. These were only temporary drawbacks due to events with England’s main supplier of iron ore at the time, Sweden.
England mined and smelted domestic iron ore. Over 80% of the iron produced was imported from Sweden and was needed to build the Royal Navy and merchant fleet, including armaments and household goods. By the late 1600’s, Sweden had created a Baltic empire that centered on the Gulf of Finland and extended into Germany. The Great Northern War (1700-1721) ultimately destroyed Sweden’s control over the Baltic Sea. An alliance of several nations including Russia (Peter the Great), Denmark, Norway, Prussia, and Poland declared war on Sweden. England remained neutral until Charles the XII of Sweden, in search of allies, negotiated with the Jacobites[3]. England declared war on Sweden in 1717. By war’s end and Sweden’s defeat, Sweden’s capacity to meet the demand of iron throughout Europe was destroyed. England was dependent on another source of iron and looked to her colonies in the New World.
By the late 1600’s and early 1700’s, iron ore was still being mined in Virginia and western Massachusetts. Small scale bloomeries[4], the most primitive form of smelting, reduced the ore into a porous mass of iron slag called bloom or sponge iron. The bloom was hammered into a low quality wrought iron by using hand held or large, heavy hammers powered by water. [More detail on this is given in later sections of this article]. The colonies lacked the iron forges and furnaces to create cast or pig iron which then could be converted into high-quality wrought iron products. Full scale forges with blast furnaces needed extensive capital, organization, trade networks, and technical expertise, to be built, which was not available in the colonies. That situation literally changed overnight. England, looking to her colonies in America to replace lost Swedish iron, invested large amounts of capital and expertise in a land where high grade ore was already proven to be common and accessible.
Prior to the 1720’s, only Virginia and Maryland exported pig iron to England. That changed dramatically as iron forge construction began in earnest and rapidly spread throughout the colonies. At the time, American industry was not allowed to refine raw materials. They were not to compete with manufacturers in England or take jobs from workers in the British Isles. The iron ore was to be smelted in America and shipped to Great Brittan. The Iron Act of 1750[5], spurred protests from colonial activists, was written to encourage the American manufacture of cast or pig iron and restrict the production of plate, sheet, and nail rods. In England, the iron was refined and manufactured into household products, wagon wheels, blacksmithing needs, firearms, steel, etc. These finished products were shipped back to America for purchase by the colonists.
As the colonies moved towards rebellion, iron forges expanded their capabilities beyond merely shipping smelted pig iron to England. Refinery forges were built. Blast furnaces made bar iron and steel that produced high quality products ranging from pots and pans to cannon and musket barrels.[6] By the start of the American Revolution, the colonies had a highly developed iron industry. If the colonies were taken as one country, they were within the top five iron producers of the world, and third in terms of exports.[7] .
History of Iron
Humans, since prehistoric times, as long as 6,000 years ago, discovered how to smelt metals – the process to separate the metal from the mineral ore. This discovery had such a great impact on the advancement of humanity that scholars have divided ancient history into the Stone Age, Bronze Age, and the Iron Age. Iron is one of the most common elements on earth. It is believed that iron was first fashioned into useful products and ornamental objects some 3,000 years ago. The first examples of iron mining and smelting is believed to have occurred in the ancient Hittite[8] culture in present Turkey and at Tell Hammeh[9], Jordon – approx. 900 BC.
For centuries, these early forges throughout Asia, Africa, and Europe lacked methods to heat the iron to the melting point. Early forges used what has been called the direct process.[10] The iron ore was slowly burned with wood in a clay-lined oven. The iron separated from the surrounding rock without reaching a molten state. These ancient forges or bloomeries produced a spongy, crusty slag of iron. The impurities were squeezed out of the sponge iron by repeated heating and hammering to mix oxygen with the iron oxide while removing the carbon from the metal. The result was nearly pure iron that could be shaped with hammers and tongs, but too soft to keep a good edge. Because the metal was shaped, or wrought, by hammering, it came to be called wrought iron, howbeit low in quality.
It is theorized that because of the ravages of the plague upon the labor forces in Europe, by the late 1300’s, water began to replace humans or animals to power the bellows. Using a water wheel, a larger and more consistent volume of air became available to heat the ore in blast furnaces[11]. These higher temperatures completely melted the ore from which iron was made producing a far greater volume of iron at a higher grade. The melted iron was poured into casts and became known as the indirect process.
These cast iron forges used a branching structure that was formed in the sand. Many individual pockets formed right angles from a central channel or runner. The melted iron flowed down the runner forming ingots[12] in these smaller channels. The whole resembled a litter of piglets suckling a sow. When the metal had cooled and hardened, the smaller ingots (pigs) were broken from the runner (sow) hence the name pig iron. Because pig iron was intended for re-melting during the wrought iron process, the uneven size of the ingots and the small amounts of sand mixed within caused insignificant problems when compared to the ease of casting and handling them.
Pig iron has a very high carbon content along with silica and other dross (unwanted materials removed from metals during smelting). This makes the iron very brittle and not useful as is except for very limited applications. A finery forge is needed to refine the pig iron into bar iron from which finished iron products were cast and hammered.
Smelting iron ore
As mentioned, iron does not occur pure. Iron ores are mainly iron oxides and include magnetite, hematite, limonite, and an assortment of other rocks in smaller quantities. The iron content of these ores ranges from 70% down to 20% or less. Coke is a substance made by heating coal until it becomes almost pure carbon. Sinter[13] is made of a lesser grade. The oxides are reduced as the ore is roasted with coke and lime to remove a large amount of the impurities for a more pure iron product. This is done through smelting. Oxygen causes fire to burn hotter and when managed properly, conditions are ripe for carbon monoxide to remove oxygen from the iron ore. Air rises upward and passes through a layer of charcoal (or coke). As the air burns the charcoal, the air is converted to carbon monoxide. The carbon monoxide (CO) continues to rise upward and latches onto oxygen atoms in the ore. The oxygen from the ore, now attached to the carbon monoxide, is drawn from the ore as carbon dioxide (CO2), and continues upward, eventually escaping through the furnace stack.
An additional layer of lime[14] and sometimes gabbro[15] is lined above the iron ore. High heat generated by the fire causes the lime to melt and form a flux[16]. Since it melts at a lower temperature than the iron in the ore, it helps facilitate the flow of silicates and other impurities from the ore. The glassy flux also coats the iron as it melts. It forms a protective barrier between the liquid iron and the oxygen in the furnace gasses, preventing the iron from oxidizing away. Charcoal, iron ore, and lime are called the burden[17] and is managed by the founder who also regulates the bellows and the supply of air flow to the fire.
The burden is held in place above the crucible (the lowest level of the furnace where the melted iron is collected) by a narrowing of the furnace lining called the boshes. Air was pumped into the furnace above the crucible, but below the boshes.
Grades of Cast Iron
Iron was classified as gray[18], white[19], or mottled[20] (pig iron). After the ore was smelted, cast iron was graded by fracture testing; breaking the iron to visually inspect how the carbon was interspersed through the cast. The crystallization that produced the various grades was deliberately controlled by the founder. With great knowledge and skill, he regulated ore, fuel, air, flux, and even cooling rate to create desired grades. The casting shed at the base of the furnace collected the cast iron (in the crucible) and slag waste which in turn were removed from the furnace. Molds were specially prepared to await the molten metal. Slag waste[21] solidified when cooled, was disposed of usually at the waterfront by dumping it over the bulkhead[22].
Finery Forges
Finery forges, by the mid-1700’s emerged throughout the colonies as colonial demand increased for refined iron. Cast iron forges began refining processes to meet this demand. They pressed the 1750 iron act by not waiting for the iron to be shipped to England and sent back to the colonies as finished products. Bar iron, wrought iron, and steel was refined at colonial forges and made available for blacksmiths, castings for family furnishings, and steel products for armories and gunsmiths.
The Saugus Forge home web site best explains the process of finery forges: Workers in the forge converted brittle cast iron “pigs” and “sows” into malleable wrought iron by carefully removing excess carbon in two separate processes, fining and hammering. These forges were specially constructed with stone and lined with cast iron plates. A charcoal fire was built large enough to cover the end of a sow.
To refine cast iron into wrought or bar iron, heavy pigs and sows were dragged from the furnace to the forge by oxen. They were positioned in the finery hearth through an aperture in the side of the chimney. Rollers guided the sows into the fire where they were slowly melted. Long iron bars or ringers were used to manipulate the melted iron, lifting it up into the air blast over and over again until the carbon was sufficiently reduced. As the carbon content went down, the melting temperature went up. This may have been an indicator to the finer (foreman) that the iron had reached the desired carbon content. The process produced more slag and it is possible that some slag may have been deliberately added to assist in the carbon reduction process.
The iron was removed from the finery hearth as a loop[23]. Excess charcoal was removed from the outside surface of the loop then the hammering began. The initial hammering was done with long-handled sledge hammers. Then it was dragged to the 500 pound helve-hammer[24] for heavier blows. Hammermen completed the wrought iron bars by forging them between the hammer and the anvil. The loop was hammered into a block or bloom. From there the bloom was hammered systematically from its middle out towards one end. The bar would be re-heated numerous times in the chafery hearth[25] to maintain a welding heat. The bar would be turned end-for-end in the tongs and the hammerman would draw out the other end of the bar, again from the middle, outwards.
In the loop stage, the iron was in the form of spongy mass of iron crystals with pockets of slag throughout. The hammering process welded and elongated the iron crystals. As in the blast furnace, slag acted as a flux to reduce oxidization while the iron was welded together. By working from the center outward, excess slag was squeezed to the ends of the bars. The result was the main product of the iron works, wrought iron merchant bars.
The majority of merchant bars were shipped to merchants or blacksmiths. Eventually it was the blacksmiths off site that would hammer the wrought iron into serviceable tools and hardware. There were three types of ferrous metal in use in the colonial period: cast iron, high in carbon and brittle, but excellent for making frying pans, Dutch ovens, cannon balls, and cheaper grades of cannon; wrought or bar iron, low in carbon and very tough so excellent for anchor chains, nails, and musket barrels; and steel, carefully controlled amounts of carbon allowing the steel to be hardened for specific uses such as knife blades, files, saws, springs, musket ramrods, and swords. The first two methods have been discussed in this article leaving steel.
Steel
Samuel Johnson’s 1755 dictionary defines steel as: “… is a kind of iron, refined and purified by the fire with other ingredients, which renders it white and its grain closer and finer than common iron. Steel, of all other metals, is that susceptible of the greatest degree of hardness, when well tempered; whence its great use in the making of tools and instruments of all kinds… purest and softest iron, by keeping it red-hot, stratified with coal-dust and wood-ashes, or other substances that abound in phlogiston, for several hours in a close furnace.”
The stratified process produced a blister steel. Thin strips of highly refined wrought iron are layered (stratified) with charcoal (mixed with bone, leather, and wood charcoal) in a box that could be sealed to keep air out. The box is heated to about 1,500 degrees and kept hot for many hours. The carbon soaks into the iron and converts it to steel. The name blister steel comes from the blistered blue-gray scale that forms on the surface of the bars. Due to the long soak, the carbon is distributed fairly uniformly. However, there is slightly more carbon toward the outside of the bar.
Cast steel can be attributed to an English watchmaker, Benjamin Huntsman. Around 1739[26], Huntsman, in his pursuit to find a better and more durable metal for springs in watch construction, discovered that under the right conditions, blister steel could be melted in a crucible and stirred while liquid. He used a blast furnace and melted the wrought iron in a clay crucible. He added carefully measured amounts of pure charcoal. This caused a very uniform distribution of the carbon. The resulting alloy was both strong and flexible when cast into springs. His invention was a windfall for global navigation, leading to the development of nautical chronometers that showed a ship’s precise longitude (east/west) position. Prior to this, ships’ captains could only determine their exact location in latitude (north/south). By the late 18th century, cast steel was being used for cutlery and chisels. At the same time, Huntsman inadvertently invented modern metallurgy.
Rolling Mill
The rolling mill and slitting mill, unlike the forge and blast furnace which were chemical and metallurgical engineered, these represented mechanical technology. It is believed that in the 1580s, the use of gears (similar to what might be seen in grist mills or saw mills) was applied to rolling mills for the purposes of flattening iron. The rolling mill consisted of a pair of cast iron rollers supported in a heavy-duty wrought iron framework. The machine was linked to waterwheels with iron couplings. The top and bottom rollers turned in opposite directions so that the bar iron could be pulled into the machine.
Wrought iron merchant bars were preheated in a cord-wood fired reverberatory furnace[27] to bring the iron to a red/orange heat. When the iron was malleable, it was fed into the rollers. The torque of the waterwheels on the rollers created a high pressure and flattened the iron bars. There was likely a mechanism for adjusting the distance between the rollers so that flats of varying thicknesses could be made. Flat bar was shipped out so that blacksmiths would have the wrought iron stock to make wagon tires, axes, saw blades, and hinges
Slitting Mill
Some flat bar might also be processed through the slitting machinery. Archaeological finds provide evidence that the slitting mill made ¼” x ¼” iron rod for the purpose of making nails. The slitter machinery was made up of two square iron bars with cylindrical bearings stacked upon the square shaft and bolted together. A similar but interlocking set of cutters and spacers was assembled upon the other square shaft. These were also coupled to waterwheels and turned in opposite directions. Water was fed over the slitters to keep the precision cutters cool and properly heat treated. Iron flats were heated to a red/orange heat and fed into the slitters. The flat bars were pulled through the slitters and sliced lengthwise. Flat bar and slit bar would have been pounded into dimension by using a series of water-powered hammers in a battery or perhaps more commonly, using hand held hammers.
Nails
Colonists needed nails. Wide expanses of timber allowed for most construction to be made with wood – unlike Europe which used to a large extent stone and brick. Blacksmiths milled most of their wrought iron into thin strips. They slit those strips into small square rods and sold them to householders. It was up to the user to cut the square rods into short lengths and use small dies to shape points and heads.
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RESOURCES
Heite, Edward F. The Pioneer Phase of the Chesapeake Iron Industry… “Quarterly Buletin,” Volume 38, Number 3 (September 1983), Archeological Society of Virginia, p. 133.
Johnson, Samuel. A Dictionary of the English Language… 1755: Third Edition 1768: Printed by W. G. Jones, Dublin, Ireland.
Lambert, Mark. Spotlight on Iron and Steel. 1988: Rourke Enterprises, Vero Beach, FL.
Gordon, Robert B. American Iron (1607 – 1900). 1996: John Hopkins University Press, Baltimore, Maryland.
Lloyd, John. The Early History of the Old South Wales 1760 – 1840. 1906: The Bedford Press, London, UK.
Swank, James M. History of the Manufacture of Iron in All Ages and Particularly in the United States from Colonial Times to 1891. 1892: The American Iron and Steel Association, Philadelphia, PA.
Web:
Lienhard, John H., Iron in America.
FOOTNOTES
[1] Charcoal production was very labor intensive and required the work of many woodcutters, carters, and the colliers who oversaw the conversion of seasoned wood into charcoal.
[2] Bog ore is an iron-rich sedimentary rock that was harvested locally from bogs and similar bodies of water. It was also found in fields and meadows that used to be bogs. Bog ore is often considerably less than 50% iron. The rest of the rock was made up of impurities that the workers had to remove.
[3]The Jacobites were the supporters of King James VII (of Scotland) and the II (of England) and his heirs. Bonnie Prince Charlie as he was known ruled Britain from 1685 – 1689, but because he was a Roman Catholic, he was replaced by his daughter Mary and her husband, the Dutch Prince William of Orange.
[4] A bloomer is a type of furnace once widely used for smelting iron from its oxides. The bloomer was the earliest form of smelter capable of smelting iron. A bloomery’s product is a porous mass of iron and slag called a boom.
[5]The iron act of 1750 was passed by Parliament to encourage iron production in the colonies. It provided for duty-free importation of colonial pig iron and (by a later extension of the law) bar iron into any English port. Among its restrictions that did not allow colonists to produce finished products from iron was the decree that no mill or engine for slitting or rolling iron or any plating forge to work with a tilt hammer or any furnace for making steel should be erected in America. Activists for self-government and self-alliance used this act, among others, to showcase their resentment to aspects of British law.
[6] Thomas Jefferson writes of iron production in Virginia, exclaiming the quality and toughness: “The indications of iron… are numerous, and dispersed through all the middle country. The toughness of cast iron… is very remarkable. Pots and other utensils, cast thinner than usual, of this iron, may be safely thrown into, or out of the wagons in which they are transported. Salt-pans made of the same, and no longer wanted for that purpose, cannot be broken up, to be melted again, unless previously drilled in many parts.”
[7] Customs Report for exports to London and other British ports survive for most years from 1707 – 1776. A sample record of Virginia and Maryland (lumped together) for 1770 and 1771 are as follows: 1770: 512 tons of bar (or wrought) iron at 9 ½ to 10 ½ £ per ton and 1125 tons of pig (cast) iron at 18 to 20 shillings per ton. 1771: 654 tons of bar iron at 9 – 11 £ and 1857 tons of pig iron at 18 – 20 shillings per ton.
[8]The Hittite Empire (c1700 BC – 1200 BC) occupied the region of Anatolia (Asia minor and present day Turkey) prior to 1700 BC. They expanded their territories into an empire which rivaled, and threatened, the established nation of Egypt. After repeated attacks, they eventually succumbed to the Assyrians.
[9] Tell Hammeh is a small artificial hill located near the point where the Zarqa River Valley opens into the Jordan Valley. The Arabic word hammeh means hot spring, and this name reflects the thermal springs that lie next to the Tell. Hammeh is close to several features that are generally seen as prerogatives for site selection in iron production: iron ore, water to cool the metal, clay for the forge construction, and wind to supply air to the forge.
[10] Direct process or Direct Reduction reduces iron oxides to metallic iron below the melting point of iron. The reducing gas is a mixture of gasses, primarily hydrogen and carbon monoxide. The process temperature is typically 800 to 1,050 degrees C.
[11]Blast Furnace is a smelting furnace in the form of a tower into which a blast of hot compressed air can be introduced from below. Such furnaces are used chiefly to make iron from a mixture of iron ore, coke, and limestone.
[12] A block of metal typically oblong in shape.
[13]Sinter is a hard siliceous or calcareous deposit precipitated from mineral springs.
[14] Calcium oxide, in the form of lime, was often used for the purpose of flux since it could react with the carbon dioxide and sulfur dioxide produced during roasting anhd smelting to keep them out of the working environment.
[15]Gabbro is a dark, coarse-grained plutonic rock of crystalline texture, consisting mainly of pyroxene, plagioclase feldspar, and often olivine.
[16] Fluxes are used in smelting for several purposes, chief among them catalyzing the desired reactions and chemically binding to unwanted impurities or reaction products. Calcium oxide, in the form of lime, was often used for this purpose, since it could react with the carbon dioxide and sulfur dioxide produced during roasting and smelting to keep them out of the working environment.
[17] Burden is the name given to the layers of iron ore, flux, and charcoal or coke which heated air flows through during smelting of metal ores.
[18]Gray iron is characterized by its graphitic microstructure, which causes fractures of the material to have a gray appearance. It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2.5 – 4% carbon, 1-3% silicon, and the remainder iron.
[19] White iron displays white fractured surfaces due to the presence of an iron carbide precipitate called cementite They offer hardness at the expense of toughness.
[20]Malleable iron starts as a white iron casting that is then heat treated for a day or two at about 950 °C (1,740 °F) and then cooled over a day or two. As a result, the carbon in iron carbide transforms into graphite
[21] Slag is the glass-like by-product left over after a desired metal has been separated (smelting) from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals.
[22]A partition built to prevent the passage of air, water, or mud.
[23]In the production of bar iron or steel, during the finery process, a bloom or loop of iron was then hammered into bars. Loop is from the old Frankish luppa or lopp, meaning a shapeless mass.
[24]A power hammer consisting essentially of a heavy head at one nd of a lever lifted by power (water) and dropping by its own weight on work that rests on an anvil. Typical weight of the hammer head during pre-colonial and colonial times was approximately 500 pounds.
[25] Chafery Hearth was used in the Walloon process forge that uses two hearths (whereas the German process uses a single forge or hearth. The finery forge refines the pig iron into wrought iron and the second chafery hearth to shape the wrought iron into bars.
[26]In 1740, Huntsman took out a patent for the process of cast steel. It is therefore assumed that his experiments occurred a year or so prior to then.
[27]A reverberatory furnace is a furnace in which the roof and walls are heated by flames and radiate heat onto material in the center of the furnace.