Iron Bloomery – The most basic process used from the iron-age to medieval times.  

  • Charcoal, iron ore and air are combined to smelt an iron “bloom”
  • The iron bloom is forged and worked to the final “Wrought Iron” shape

During the iron-age, bloomery furnaces rapidly replaced open charcoal fires as an effective way to forge.  These furnaces or pits were made of clay and stone and were designed to be heat-resistant, built with pipes referred to as tuyeres.  These tuyeres were used to force air into the furnace using a bellows system to heat up the charcoal and increase furnace temperatures.  The yield of this primitive furnace is a Bloom, which has a spongy consistency (not molten).  While the bloom is still at high temperatures, the hot iron bloom is worked and refined by hand forging – creating “wrought iron”.

Bloomery Furnace
Iron Bloom being Forged
Iron Bloom being Forged by hand

The Iron Age

Within Western cultures, iron began to be used as a productive material around 1200 BC.  With early metals being produced from Copper and Bronze during the Bronze Age, the earliest known samples of iron were probably produced by accidentally introducing iron ore into a bronze smelting furnace.  The iron age was a global-scale development, that took place over the span of 1500 years because various cultures were all at different levels of development.  With respect to the Western hemisphere, iron appears to have been smelted as early as 3000 BC, but bronze smiths, not being familiar with iron, did not put it to use until much later. 


Bloomery Smelting Middle Ages
Bloomery smelting during the Middle Ages, as depicted in the De Re Metallica by Georgius Agricola, 1556

In the art work displayed to the left you can see a rendering of a 16th Century bloomery smelting furnace during the Middle Ages

Bloomery Furnace
Bloomery Furnace Cross Section to illustrate how the process works.

Medieval Europe

Early European bloomeries were relatively small, smelting less than 1 kg (2.2 lb) of iron with any single furnace firing. As time continued, men organized to build progressively larger bloomeries in the late 14th century, with an average capacity of about 15 kg (33 lb), though exceptions did exist.  Harnessing the power of flowing water, men created waterwheels to power the bellows apparatus, which allowed the bloomery to become larger and hotter.  European average bloom sizes quickly rose to 300 kg (660 lb), the point where the bloomery scale stayed until their demise.

As the bloomery scale increased, the iron ore was exposed to burning charcoal for a longer time.  When combined with a strong air blast required to penetrate these larger stacks of  ore and charcoal, the iron starts to melt and become saturated with carbon in the process, producing a material referred to as pig iron that can’t be forged.  Therefore, pig iron was considered an unfortunate waste product detracting from the yield of the larger bloomeries.  It wasn’t until the 14th century that early blast furnaces, identical in construction but dedicated to the production of molten iron, were built.  These new furnaces provided the oxidation to reduce the melt into cast iron and steel.

In Europe, these Bloomery type furnaces typically produced a range of iron products from very low carbon iron to steel containing approximately 0.2% to 1.5% carbon.  The master black smith had to select bits of low carbon iron, carburize them, and pattern-weld them together to make larger steel sheets.  Even when applied to a non-carburized bloom, this pound, fold and weld process resulted in a more homogeneous product and removed much of the slag.  For weapons grade steel material, the process had to be repeated up to 15 times to achieve this high quality steel.  Each welding’s heat oxidises some carbon, so the black smith had to make sure there was enough carbon in the starting mixture.  The alternative was to carburize the surface of a finished product. 

In England and Wales, the blast furnace arrived in the Weald region of South-East England in about 1491.  Iron making in the Weald used ironstone from various clay beds and heat generated from charcoal made from trees in the heavily wooded landscape.  The Weald region produced a large proportion of the wrought-bar iron made in England in the 16th century and most of the British cannon until about 1770.  When iron making began to use the higher heat energy from coke (separately produced from coal), the Weald region industry declined because the coal necessary to produce the coke wasn’t available in the area.

One of the oldest known blast furnaces in Europe was found in Lapphyttan in Sweden, where carbon-14 testing dated the artifacts to be from the 12th century.  Interestingly, the oldest iron bloomery in Sweden was also found in the same area.  Similar to the pre-Christian Roman occupation period in English history, the Swedish bloomery was carbon-14 dated to the pre-Christ era (700 BC).

Iron bloomeries survived in Spain and southern France as Catalan forges into the mid-19th century, and in Austria as the Stückofen to 1775.

East Asia

China has long been considered the exception to the general use of bloomeries.  It was thought that the Chinese skipped the bloomery process completely, starting with the blast furnace and the finery forge to produce wrought iron.  However, more recent evidence shows that bloomeries were used earlier in ancient China, migrating in from the west as early as 800 BC, before being supplanted by the locally developed blast furnace. 

Highly advanced compared to the European time line, by the 5th century BC the metalworkers in the southern state of Wu had invented the blast furnace and developed the means to both cast iron and then decarburize the carbon-rich pig iron produced in a blast furnace to a low-carbon, wrought iron-like material. 

Sub-Saharan Africa

The historical sub-Saharan African iron smelting processes are documented to be variants of the bloomery processes found in European cultures.  The earliest records of bloomery-type furnaces in East Africa are discoveries of smelted iron and carbon in Nubia in ancient Sudan dated at least to the 7th to the 6th century BC.  It is known that the ancient bloomeries that produced metal tools for the Nubians and Kushites produced a surplus for sale.  The site of Gbabiri, in the Central African Republic, has also yielded evidence of iron metallurgy, from a reduction furnace and blacksmith workshop; with earliest dates of 896-773 BC and 907-796 BC respectively.


Though to have migrated from the Mediterranean basin, iron production was established in ancient India during the latter half of the 1st millennium BC.  Wrought iron was used in the construction of monuments like the Iron pillar of Delhi, built in the 3rd century AD during the Gupta Empire.  The pillar was systematically constructed using a towering series of disc-shaped iron blooms.  Unlike other parts of the world, the culture in India did not use cast iron for architectural structures until more modern times.  

The Americas

The oldest iron bloomery in the Western part of the USA is located in Califorinia.  A view of the bloomeries (‘Catalan forges’) at Mission San Juan Capistrano, the oldest (circa 1790s) existing facilities of their kind in California.  In the Spanish colonization of the Americas, bloomeries or “Catalan forges” were part of ‘self sufficiency’ at some of the missions, encomiendas, and pueblos. As part of the Franciscan Spanish missions in Alta California, the “Catalan forges” at Mission San Juan Capistrano from the 1790s are the oldest existing facilities of their kind in the present day state of California. The bloomeries’ sign proclaims the site as being “…part of Orange County’s first industrial complex.”

Catalan Furnace
Cross Section of a Catalan Furnace, illustrating the bloom section, charcoal and air port.
Remains of Catalan Furnaces in Mission San Juan Capistrano CA
Remains of Catalan Furnaces in Mission San Juan Capistrano CA

New England Iron Working – Armory to the Colonial States

The English settlers of the 13 colonies did bring technical know how from England, but the British sought to situate most of the skilled iron masters at colonial locations to provide iron to produce essential hardware for cooking, buildings, farming and ship repairs.  Starting with Bloomery technology, the early settlers were able to make these essential pieces pieces from the small yields provided in a bloomery.  However, when the colonial army needed cannon and large ammunition to drive the better armed British away.  The iron masters of New England began using Blast Furnace process to extract larger melts of iron from earthen ores.  During the American Revolution, 80% of the cannon produced in the colonies were made at the Salisbury Furnace. For years, local historians have dubbed the site the “Arsenal of the Revolution.”

Colonial Era Blast Furnace
Depiction of Colonial Era Blast Furnace
Salisbury Iron Ore
Limonite, Litchfield County – Yale Peabody Museum of Natural History


Salisbury Iron – the “Iron Rush”

In 1728, during a move to open the western lands for settlement, surveyors were sent to the area from Hartford, Connecticut to determine borders and establish townships. While working in the field, surveyors found their compasses behaved erratically because of massive iron deposits underfoot; they also found traces of iron around animal holes and diggings. By 1731, John Pell and Ezekiel Ashley were exploring the areas in the western section of Salisbury, where they discovered at Old Hill (later named Ore Hill) the largest and richest of the deposits that became renowned as Salisbury Iron Ore.

While the 1731 discovery of iron ore suggested the potential for an industry, processing the ore into iron required money and iron-making technology.  In 1732 entrepreneur Thomas Lamb arrived from Massachusetts and began purchasing property and water rights for power.  He eventually controlled more than 5,000 acres.  In September 1734, Lamb was granted water privileges on the Salmon Fell Kill in the Lime Rock section of Salisbury.  There he began processing ore at his bloomery, a forge used for smelting ore into wrought iron.  Lamb’s efforts thus initiated almost two centuries of northwestern Connecticut iron production.  Richard Seymour’s East Canaan bloomery followed Lamb’s Forge in 1739 and was constructed adjacent to the Blackberry River, a tributary of the Housatonic. Joseph Skinner’s forge in Sharon first produced iron in 1740, and in Kent iron was first made in 1744.  In Salisbury another forge at the outlet of Twin Lakes opened in 1748, and another by John Gray on Sharon Mountain began operation in 1750.  By this time, an “iron rush” materialized in the northwest corner.

Click Here for more information on the Salisbury Iron Region

Blast Furnaces – Ethan Allen & Cannon Making

While the ealry iron bloomery forges manufactured very good quality wrought iron, each had a maximum production capacity limited to about 400 pounds per day.  With the demand for iron continuing to grow, the Forbes brothers of East Canaan along with John Hazeltine of Massachusetts and a 23-year-old from Cornwall named Ethan Allen built the region’s first blast furnace in the Lakeville section of Salisbury in 1762.  While Ethan Allen was in the Salisbury he had learned of the rich lode of iron ore that had been found there, an entire hill of almost pure hematite, virtually free of impurities.  Ethan realized that there was a great opportunity awaiting the person who could build a charcoal blast furnace in Salisbury to melt the iron ore so it could be cast into useful products and into iron bars to be hammered in the forges.

Everything that was needed for a blast furnace was right there in Salisbury: a large lake fed by springs with a steady outflow of water that could operate a water wheel to produce compressed air; a large supply of limestone that could be dug out of the hills at Lime Rock, midway between Cornwall and Salisbury; hills covered with hardwood trees which could be harvested to make charcoal; and finally, Ore Hill itself, with its fabulous lode of high quality iron ore.

Ethan fortunately met a man with a similar desire, Paul Hazeltine, who with his father and brothers operated several iron works in Eastern Massachusetts. Paul’s father, John, on hearing of the potential in Salisbury, committed himself to build a blast furnace if the necessary property and mineral rights could be obtained. Ethan promptly took care of this, working with the Forbes brothers, and in January 1762 the four men entered into a partnership to construct the furnace. For his contribution in making the arrangements and his continuing tie to the operation, Ethan received a one-eighth interest in the furnace.

Soon the furnace was in full operation, with a large crew of local workmen under Allen’s direction, producing potash kettles, pig iron and other needed products. (A section of one of the pieces of pig iron produced by Ethan Allen in 1764 was recently discovered buried not far from the furnace site, and is now on display in the Salisbury Cannon Museum).  The furnace continued in operation for over eighty years, until the year 1844, when it was torn down to make way for a factory producing pocket knives. During the American Revolution the furnace was operated by the Connecticut Committee on Safety to produce over 800 iron cannons. It was a major industrial installation for its time. Before long the section of Salisbury where the furnace was located became known as “Furnace Village”, a name which remained until 1846, when it was changed to Lakeville.

Salisbury Iron Works
Site of the Revolutionary War Foundry, Salisbury - Connecticut Historical Society
Salisbury Cannon
This four-pounder cannon was cast in Salisbury and buried in Danbury in 1777 to save it from the British.

Civil War Era and Beyond

The Gun Foundry
“The Gun Foundry,” a painting by John Ferguson Weir (1866), illustrates the interior of a period foundry depicting innovative production methods of that era used for armaments, e.g., the Civil War Parrott gun. - West Point, New York

Post World War II

Development of Ductile Iron

During World War II, there as a large effort to search for hard cast iron material compositions high strength and high levels of wear resistance, but without the dependence of critical elements, such as Chrome (Cr) or Nickel (Ni), considered essential to higher priority materials such as steel.  One of the private companies conducting research in this area was the International Nickel Company (INCO), who held the patent on “Ni-Hard” iron.  INCO gave a young team of engineers (Keith Dwight
Millis, Albert Paul Gagnebin) the job of assessing the cast iron material behavior with various other elements to produce hard surface carbides.  One of the elements these materials research engineers were interested in studying was Magnesium (Mg), however their Lab Manager, Norman Pilling, was hesitant to allow this because he new it was a reactive mixture.  None the less, the engineers carefully experimented and discovered both carbides and spheroidal graphite in the resulting samples.  In parallel, the British Cast Iron Research Association (BCIRA) was also working in developing a new iron material capable of achieving similar mechanical properties to malleable iron, without having the cost of a long heat treat cycle for the iron casting.  To accomplish this task, Henton Morrogh experimented with rare earth elements to achieve a similar spheroidal graphite structure with Cerium (Ce) that the INCO metallurgists had documented with their process.  The results from both INCO and BCIRA research efforts were published at the 1948 AFS Casting Congress in Philadelphia. 

Gagnebin and Millis
Gagnebin (l) and Millis congratulate each other upon receiving AFS Gold Medals in 1952 for their invention.
Micro Sample of original DI
The microsamples of the heats tapped on April 12, 1943 showed that the graphite had taken on a spheroidal shape…and ductile iron was born.


Prior to the publication of their results, both inventions had been disclosed and separate patents assigned to both organizations.  While the INCO conversion process was discovered in 1943, the company continued to develop the process, giving them a wider claim and successfully defend the patent against future infringements.  Therefore, the lab team of Millis, Gagnebin and Pilling took measures to the develope the magnesium conversion process secretly, effective measures that prevented the leakage of information outside of their small group of metallurgists.  The metallurgical lab experimentation continued for five years before an application was filed by INCO in Washington, D.C. on November 21, 1947.

In order to establish, for the record, on an earlier date an INCO application was filed in Great Britain, March 22, 1947.  As the patent publication process seems to move slowly, it appeared that the U.S. Patent Office was thoroughly investigating this one, possibly because Henton Morrogh from BCIRA filed January 25, 1949 for a patent on his Ce process in hypereutectic irons.  The INCO main patent and the patent on improved cast iron 2,485,760 and 2,485,761, respectively, were granted October 25, 1949.  The Ce patent was granted November 15, 1949 to BCIRA.  Incidentally, INCO subsequently purchased the Ce patents from BCIRA.

Licensing and Patent Infringement 

On August 14, 1952 the International Nickel Company alleged that Ford Motor Company had infringed on their 1949 Patent No. 2485760.  The patent in issue covers a cast ferrous alloy, variously referred to as nodular iron, ductile iron, spheroidal iron and S. G. iron.  The defendant, Ford Motor Company, was charged with willful and deliberate infringement by manufacture, use and sale of nodular iron crankshafts.

The Honorable Simon H. Rifkind as assigned as Special Master to take evidence and report his findings of fact and conclusions of law.  On February 11, 1958, the Master filed his report holding all of the contested claims valid and infringed. 

At the time of the hearings before the Master, INCO was very successful in securing 113 active licensees in the United States and 236 in foreign countries.  The value of the technology was obvious:  from 1949 to 1955 the annual world-wide production of nodular iron had increased at an accelerating rate. 

While Ford argued over the technical claims associated with the ductility and the minimum percentage of magnesium (they cast crankshafts below 0.035% Mg), the court ruled in favor on INCO based on the novelty of the patent.  “The novelty in the patent was not improved ductility, but the achievement of spheroidal graphite in the as cast iron by the retention of magnesium, thereby enabling each particular type of iron to exhibit the true properties of its matrix, whatever the matrix properties might be, with a minimum of interference from the graphite.”


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