Iron Furnaces

Like the Mayan ruins of Caracol, the iron furnaces of the Appalachian uplands rise up from forested entanglements, vestigial relicts of a bygone age. The similarity extends beyond metaphor to architecture, as the pyramidal frustum shape is shared by both. The Mayan stepped pyramids were built to afford access to the temple at the summit in closer proximity to the sun deity; the weight-distributing stability of the pyramid was the only practicable option for this massive stone structure, as the Egyptians concurrently and independently determined. Like their Mayan predecessors, the truncated pyramids of the iron furnaces provided stability and ease of construction. There was, however, a more important feature to the design.  The functional interior oven chamber contained within was both necessary and sufficient for the concentration of heat to transform iron ore into relatively pure iron.

Smelting, the production of elemental iron from the reduction of the iron oxides that are the primary constituents of natural ore deposits, has been practiced since the dawn of recorded time; the Iron Age began circa 2,000 BCE with the discovery of the basic process somewhere in southern Eurasia. Iron is the fourth most common element in the earth’s crust (after oxygen, silica and aluminum) and its associated iron-rich ores are readily identifiable by their red pigmentation (due to the rubescence of ferric oxide - better known as rust) and by their density (they are very heavy for their size). The fundamental process of extracting a crude form of iron from its ore is fairly simple. In what is now known as a bloomer furnace or bloomery, which could be as simple as an open pit, charcoal (literally wood charred to coal) was ignited to achieve an elevated temperature into which small pieces of iron ore were inserted. Subsequent layers of a one-to-one ratio mix of charcoal and iron ore sustained the process until combustion was completed. The end result was known as a bloom, a porous mixture of pure iron and partially refracted impurities such as silica in an amorphous mass descriptively called slag. The bloom of sponge iron was removed from the dross and set upon a hard surface, there to be pounded or ‘wrought’ by an iron smith to hammer out all of the impurities. The resultant wrought iron, containing about .05 percent carbon absorbed from the charcoal, was the form of iron the use of which defined the Iron Age. The addition of carbon to iron in small quantities increases its strength due to the creation of interstitial iron-carbon compounds that impede the movement of dislocations, transforming malleable pure iron into toughened steel.

The iron blast furnace is an enhancement to the bloomery furnace in the application of higher levels of heating to create molten iron. The first blast furnaces were in use in China by the first century BCE (and possibly earlier) and there is some credence to the hypothesis that the Chinese skipped the bloomery process altogether. The first European blast furnaces appeared in different geographical areas in the late Middle Ages (476 – 1450 CE) including what is now Germany, Sweden and Switzerland.  The provenance of these multiple sites is not clear, and there is a possibility that the process was imported from China via the Silk Road. However, in that the blast furnace is a logical evolution of the bloomery, it is more likely that these were independent convergent technological innovations. By 1500 blast furnace technology migrated to France and England. The first iron furnace in the Americas was started by the Jamestown colonists in 1622 and spread with colonial America throughout the Appalachian region.

To understand the geographical requirements for the construction and operation of an iron blast furnace, it is necessary to review the fundamentals of the chemical process on which it is based.  The objective is to convert iron oxide (Fe₂O₃ or Fe₃O₄) into elemental iron (Fe); in the lexicon of electrochemistry, the iron must be reduced from a valence (charge) of either +2 or +3 to a valence of 0. All pure elements are at the 0 valence state, as they have neither an excess nor a dearth of electrons. Oxygen is very reactive; it has a strong propensity to take electrons from other elements to make up a more stable compound (one in which its outer electron shell is full).   In what are called oxidation and reduction reactions, frequently referred to by the acronym REDOX, there must be a balance between species that are oxidized (adding electrons or raising the valence number) and those that are reduced (losing electrons or lowering the valence number). Since we want to reduce iron oxide to make pure iron, we need something to oxidize with enough of a positive energy balance to drive the chemical reaction in the desired direction. It doesn’t take any energy to drive it the other way. If you leave a bar of iron outside, it will readily and rapidly return to its native iron oxide, or rusted, state. The reducing agent for the iron blast furnace is carbon monoxide (CO), which oxidizes to carbon dioxide (CO₂).

The main chemical reaction that is achieved in an iron furnace is then:

                                        Fe₂O₃    +    3CO      à   2Fe   +   3CO₂      

Carbon monoxide is produced by the injection of hot blast air into the chamber containing the charcoal (C for Carbon) according to the reaction:

                                                    2C + O₂  à  2CO

With carbon monoxide rising upward in the furnace, crushed iron ore was added to the top so that the reduction reaction would take place from top to bottom in several intermediate chemical reactions.  Starting from the cooler upper section of the furnace (~1000°F;   Fe₂O₃   à Fe₃O₄) through the hotter middle of the furnace (~1500°F;  Fe₃O₄ à FeO)  the final iron product emerged in the highest heat input at the  bottom (~2100°F;   FeO à Fe). Flux in the form of limestone was added to remove the impurities from the iron, most notably silica (the second most common element). Limestone is a sedimentary rock formed by the deposition of the exoskeletons of sea animals and is comprised primarily of calcium carbonate (CaCO₃).  Limestone decomposes to CaO and CO₂ with the heat input from the furnace; this in turn promotes the reaction of CaO with the undesirable silica contaminant according to the complimentary reaction SiO₂ + CaO  producing calcium silicate slag,  CaSiO₃.

The process of converting iron ore to molten iron took about 12 hours. At the conclusion of this period, the furnace tenders would first draw off the calcium silicate slag impurities that were floating on top of the molten iron. The liquid iron was then tapped by shattering a clay plug to allow the iron to flow into channels that had been dug in the sand and clay of the casting floor.  To facilitate the handling of the iron when cooled, the main flow channel fed smaller branch channels to contain smaller, more manageable, ingots; the arrangement, reminiscent of a sow feeding her piglets, is the origin of the term ‘pig iron.’ Due to the absorption of extra carbon in the fabrication process (about 4%), the end product was very hard and therefore very brittle; it is the form of iron called cast iron. Cast iron was widely used for applications such as stoves and cookware where ductility and toughness were not important. For more critical applications such as wagon wheel rims, weaponry and machinery components, the cast pig iron was transported from the furnace to a foundry for where the carbon levels were reduced to between 0.2 and 1.5% to produce high strength, low carbon steel.

An operating iron furnace required close proximity to a source of iron ore, limestone, and, most importantly, to be surrounded by extensive wooded tracts for the manufacture of charcoal. Iron furnace production ranged from 3 to 10 tons of iron per day, depending on the size of the structure. A 5 ton per day iron furnace required about 50 cords of wood (a cord is a pile 4 feet x 4 feet x 8 feet) or, if converted to standing forest, an almost unfathomable one hectare (2.5 acres) of trees hacked down for every day of operation.  The charcoal was made from the felled trees by erecting a stacked-wood pile about 40 feet in diameter and 25 feet high (about 250 cords) and covering it with dirt so that it could be slowly and anaerobically charred over the course of about 30 days. Since the average wood cutter could produce about 2 cords of wood per day, this meant that there would need to be about 25 wood cutters in addition to about 5 full time colliers to tend the charcoal fires. Iron ore and limestone were quarried from nearby deposits and brought to the furnace by wagon. A 5 ton furnace would require about 15 tons of iron ore and 2 tons of limestone every 24 hours.  The entire operation was obviously a major industrial and coordinated enterprise that operated around the clock with an on-site staff of 20 and a supporting network of dozens of colliers, cutters and quarrymen. In 1840, there were 804 furnaces operating in the United States (42 in Virginia and 12 in Maryland) that produced a total of 286,903 tons of iron.

The only iron furnace in Shenandoah National Park was the Mount Vernon Furnace at the bottom of the Madison Run Fire Road in an area surrounded by 28,000 acres of woods, necessary for the gargantuan charcoal operation. It is hard to imagine the cacophony of the furnace operation. According to Carolyn and Jack Reeder in the PATC Publication Shenandoah Secrets, “Noise filled the forest in the mid-nineteenth century: the clanging and clanking of picks and shovels as iron was mined from surface cuts a mile from the furnace, the ring of axes as trees were felled and hewed to supply the charcoal hearths, the clatter of the ore cars on the narrow-gauge railway line leading to the furnace….Activity in this now peaceful place went on day and night. Once the furnace was fired, it operated around the clock for months. From the embankment behind the furnace, workers charged its blasting chamber with raw materials, emptying wagonloads of ore, limestone, and charcoal into an opening at the base of the stack.” The furnace was destroyed by Union forces in 1864 and rebuilt ten years later, only to succumb to the competition of the iron furnaces of Pennsylvania, where the availability of anthracite coal obviated the costly and time consuming process of charcoal manufacture. By the late nineteenth century, the iron furnaces of Virginia and Maryland were silent.

Further west, the extensive iron deposits in Massanutten Mountain were exploited to an even greater extent, the region home to three major furnaces within a relatively small geographical area. In an unpublished tract entitled The History of the Fort by S. H. Munch of Seven Fountains, Virginia in December 1925, the author discusses the iron furnaces at length. Note that the “Fort” refers to “Fort Valley,” the area surrounded by the northern Massanutten Mountain elongated highland enclosure. He notes that Caroline Furnace and Elizabeth Furnace were named, along with Catherine Furnace in Page County, for the three daughters by a man named Blackford. They produced 3 tons of iron and consumed 600 bushels of charcoal a day (about 1 acre of trees). The pig iron, which sold for $58.00 to $60.00 per ton, was used for the manufacture of car wheels and boiler plate. The iron ore of the Fort Valley, known as hematite, was extensive and easily mined, yielding “better than 50% iron.”  The iron furnaces of Massanutten were also burned by the Federals in the latter days of the Civil War, never to regain their antebellum prominence as the engines of the local economy. Their silent sentinels remain.