thumb|Bog ore

Bog iron is a form of impure iron deposit that develops in bogs or swamps by the chemical or biochemical oxidation of iron carried in solution. In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite (FeO(OH)), and related limonite.

Iron-bearing groundwater typically emerges as a spring and the iron in it forms ferric hydroxide upon encountering the oxidizing environment of the surface. Bog ore often combines goethite and magnetite, and may include vugs and stained quartz. Oxidation may occur through enzyme catalysis by iron bacteria. It is not clear whether the magnetite precipitates upon the first contact with oxygen, then oxidizes to ferric compounds, or whether the ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment surface and reoxidized upon exhumation at the surface.

Bog iron, like other hydrous iron oxides, has a specific affinity for heavy metals. This affinity combined with the porous structure and high specific surface area of bog iron make it a good natural sorbent. These properties combined with the fact that bog iron is cheap to obtain are incentives for its utilization in environmental protection technologies. The iron in the water is then oxidized by dissolved oxygen or, through enzyme catalysis by iron bacteria (e.g., Thiobacillus ferrooxidans and Thiobacillus thiooxidans) that concentrate the iron as part of their life processes. Presence of these bacteria can be detected by the oily film they leave on the surface of the water. Due to its easy accessibility and reducibility, bog iron was commonly used for early iron production. Early metallurgists identified bog-iron deposits by indicators such as withered grass, a wet environment, hygrophilous grass-dominated vegetation, and reddish-brown solutions or depositions in nearby waters. Smelting with a bloomery furnace often results in between 10 and 20 mass percent Fe being reduced to iron bloom, while the rest is transferred into the slag. The bloom must then be consolidated with a hammer to make usable wrought iron. There is some archaeological evidence that lime was added to furnaces to treat silica-rich ores that were difficult to smelt by the bloomery process. The technology then spread throughout Europe in the following two millennia, reaching Poland in the 2nd century BCE. In Russia, bog ore was the principal source of iron until the 16th century, when the superior ores of the Ural Mountains became available.

North America

Pre-Columbian

Iron was produced by the Vikings on Newfoundland around 1021 CE. Excavations at L'Anse aux Meadows have found considerable evidence for the processing of bog iron and the production of iron ore.

Colonial North America

Bog iron was widely sought in colonial North America. The earliest known iron mines in North America are the mines from St. John's, Newfoundland, reportedly in operation by Anthony Parkhurst in 1578. The first mining efforts in Virginia occurred as early as 1608. In 1619 Falling Creek Ironworks was established in Chesterfield County, Virginia. It was the location of the first blast furnace facility in North America.

Lake Massapoag in Massachusetts was drawn down by deepening the outlet channel in a search for bog iron. The Saugus Iron Works National Historic Site, on the Saugus River in Saugus, Massachusetts, operated between 1646 and 1668. The site contains a museum and several reconstructed buildings. The success of the Saugus Iron Works, and the rapid depletion of the region's natural bog iron, led the owners to send prospectors into the surrounding countryside. In 1658 the company bought of land which covered areas that are now Concord, Acton, and Sudbury. They set up a large production facility in Concord, Massachusetts, along the Assabet River with dams, ponds, watercourses, and hearths, but by 1694 the natural bog iron there had also been exhausted, and the land was sold for farming.

In Central and Southern New Jersey, bog ore was mined and refined for the production of naturally rust-resistant tools and wrought iron rails, many of which still grace staircases in Trenton and Camden. During the American Revolution, bog iron cannonballs were cast for the colonial forces.

19th century United States

Bog iron was also found on the Eastern Shore of Maryland. The remains of a commercial smelting operation near Snow Hill, Maryland, are now a state and national historic site. Known as Furnace Town, it was called the Nassawango Iron Furnace after the nearby creek. The commercial furnace ran from about 1825 to 1850.

The Shapleigh Iron Company constructed a smelter at North Shapleigh, Maine, in 1836 to exploit a small bog iron deposit in Little Ossipee Pond. The plant commenced operation in 1837, but according to an 1854 history of Shapleigh "the business [proved] unprofitable, therefore after a few years it was abandoned."

See also

  • (iron-oxidizing bacteria)

References

  • ASME PDF file with detailed reconstruction drawings of the furnace and surroundings.
  • at the Maryland Historic Trust.

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