Cornwall Iron Furnace

Cornwall Iron Furnace was one of many ironworks that were built in Pennsylvania over a sixty-year period, from 1716 to 1776. There were at least 21 blast furnaces, 45 forges, four bloomeries, six steel furnaces, three slitting mills, two plate mills, and one wire mill in operation in Colonial Pennsylvania.

The furnaces at Cornwall Furnace went through two stages of technology. Peter Grubb was born in Delaware about 1702 and settled in what is now Lebanon County in 1734. He bought about 300 acres (1.2 km²) of magnetite rich land. Grubb also noticed that his land had the other natural resources needed to produce iron. Namely, vast stands of timber for the production of charcoal, running water to operate the bellows, and an ample supply of limestone needed to add flux to the smelting furnaces. Grubb's plans were further helped by the fact that the magnetite at Cornwall was either very close to or on the surface of his land. He was ready to venture into the iron business and set about the task of building an iron "plantation". These centers of iron production were usually located well away from the heavily cleared farmlands and were nestled in the Ridge and Valley section of Pennsylvania. Grubb constructed his furnaces, first a bloomery and later the more modern charcoal-fired blast furnace and the support buildings and mill village that was needed to house his workers. He named his operation Cornwall because his father, John Grubb had come from Cornwall, UK in 1677. Cornwall Iron Furnace was an excellent fit for the agricultural based economy of the Thirteen Colonies. Iron was needed to make into tools, nails and weapons. The official policy of Great Britain frowned on manufacturing in the colonies, but England was no longer able to produce the needed iron for its needs let alone the needs of the colonists. In fact England had become dependent on importing iron from Sweden.

Peter Grubb was not really an ironmaster, but a builder. In 1745 he leased the ironworks to a consortium, Cury and Company, for 25 years and returned to Wilmington. The consortium continued the operation, with ownership passing to Peter's sons, Curtis and Peter Jr., after his death in 1754. The brothers took over the operation in 1765 and ran it quite successfully until the late 1780s. Curtis operated the Cornwall Furnace and lived on site; c1773 he built the original 19 rooms of the mansion that still stands prominently next to the property. Peter Jr. ran a forge at Hopewell, refining the pig iron produced by the furnace into more valuable bar iron. The ironworks were major suppliers to the Revolutionary War effort, and George Washington once visited to inspect the operation. Unfortunately for the Grubb family, as described in Curtis Grubb's biography, they were unable to retain control of the operation after Curtis' marriage in 1783. Most of the Grubb's holdings gradually fell into the hands of Robert Coleman, culminating in 1798. Coleman's son, William, was named manager of Cornwall Furnace and lived in the mansion; in 1865 the Colemans remodeled it into the 29 room structure known today as Buckingham Mansion.

In American Colonial history, the Iron Act, passed in 1750, was part of the British legislation designed to encourage the production of raw materials (including pig iron) in colonial America, but to restrict their manufacture there into finished iron goods. Existing manufacturing works could continue, but new ones for certain processes were prohibited.

The first furnace built by Peter Grubb at Cornwall Iron Furnace was a bloomery. Grubb built this in 1737 to test the market value of his ore. It was an economical way to test the market without having to invest in building the much more efficient and profitable blast furnace.

A bloomery is basically an enlarged blacksmith's hearth. It consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. (Sandstone was used at Cornwall.) Near the bottom, one or more clay pipes enter through the side walls. These pipes, called tuyeres, allow air to enter the furnace, either by natural draft or by forced with a bellows. An opening at the bottom of the bloomery may be used to remove the bloom, or the bloomery can be tipped over and the bloom removed from the bottom.

The first step taken before the bloomery can be used is the preparation of the charcoal and the iron ore. The charcoal is produced by heating wood to produce the nearly pure carbon fuel needed for the refining process. The ore is broken into small pieces and roasted in a fire to remove any moisture in the ore. Any large impurities in the ore can be crushed and removed. Since slag from previous blooms may have a high iron content, slag from previous blooms can be broken up and recycled into the bloomery with the new ore.

In operation, the bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top, in a roughly one-to-one ratio. Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron, without melting the ore; this allows the bloomery to operate at lower temperatures than the melting temperature of the ore. Since the desired product of a bloomery is easily forgeable, nearly pure iron, with a low carbon content, the temperature and ratio of charcoal to iron ore must be carefully controlled to keep the iron from absorbing the carbon and becoming unforgeable. Limestone could also be added to the bloomery, about 10% of the ore weight, which would act as flux and help carry away impurities.

The small particles of iron produced in this way fall to the bottom of the furnace and become welded together to form a spongy mass of the bloom. The bottom of the furnace also fills with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. Because the bloom is highly porous, and its open spaces are full of slag, the bloom must later be reheated and beaten with a hammer to drive the molten slag out of it. Iron treated this way is said to be wrought, and the resulting nearly pure iron wrought iron.

In 1742, Grubb replaced his bloomery with a 30-foot (9.1 m) high charcoal-fired cold blast furnace. The blast furnace burned hotter than the bloomery and was able to render molten pig iron ("charcoal iron") from the ore.

A blast furnace relies on the fact that the unwanted silicon and other impurities are lighter than the molten iron that is the main product. Grubb's furnace was built in the form of a tall chimney-like structure lined with refractory brick. Charcoal, limestone and iron ore (iron oxide) were poured in at the top, and air was blown in through tuyeres near the base. The resulting "blast" promotes combustion of the charcoal (more modern furnaces use coke or even anthracite), creating a chemical reaction that reduces the iron oxide to the base metal which sinks to the bottom of the furnace. The exact nature of the reaction is:

Fe₂O₃ + 3 CO → 2Fe + 3CO₂

More precisely, the compressed air blown into the furnace reacts with the carbon in the fuel to produce carbon monoxide, which then mixes with the iron oxide, reacting chemically to produce iron and carbon dioxide, which leaks out of the furnace at the top. In the beginning of the reaction cycle, the hot blast, also called "wind", containing pre-heated gas from Cowper stoves and air, is blasted into the furnace through tuyeres. The wind will ignite the coke and the Boudouard reaction will take place:

C + O₂ → CO₂
CO₂ + C → 2 CO

The temperature in the furnace typically runs at about 1500 °C, which is enough to also decompose limestone (calcium carbonate) into calcium oxide and additional carbon dioxide:

CaCO₃ → CaO + CO₂

The calcium oxide reacts with various acidic impurities in the iron (notably silica), forming a slag containing calcium silicate, CaSiO₃ which floats on the iron.

The pig iron produced by the blast furnace is not useful for most purposes due to its high carbon content, around 4-5%, making it very brittle. Some pig iron is used to make cast iron goods, often being remelted in a foundry cupola.

For other purposes further processing is needed to reduce the carbon content to enable iron to be used for tools or as a construction material. There have been various processes for this. The earliest process was conducted in the finery forge. In the late 18th century, this began to be displaced by 'potting and stamping', but the most successful new process of the industrial revolution period was puddling.

This is now done by forcing a jet of high-pressure oxygen into a special rotating container containing the pig iron. Some of the carbon is oxidised into carbon monoxide, CO, and carbon dioxide, CO₂. This also oxidizes impurities in the pig iron. The container is rotated and the processed pig iron can be separated from the oxidised impurities. Before the mid 19th century, pig iron from the blast furnace was made into wrought iron, which is commercially pure iron. At that period, if steel was needed, particularly pure varieties of iron were heated with charcoal in a cementation furnace to produce blister steel (with about 1-2% carbon). This might be further purified using the crucible technique, but steel was too expensive to use on a large scale. However, with the introduction of the Bessemer process in the late 1850s and then other processes, the production of steel was dramatically increased. By the late 19th century most iron was being converted to steel before use.

Cornwall Iron Furnace became obsolete by the 1880s. The Bessemer and open-hearth processes of creating steel, the replacement of charcoal with coke and anthracite coal, the discovery of iron deposits at the Iron Range in Minnesota near Lake Superior, and the building of modern factories in Pittsburgh, Steelton and Bethlehem brought about the end of iron production in Cornwall. Cornwall Furnace no longer earned a profit in its last ten years of operation and the last owner, Robert Habersham Coleman, had it shut down on February 11, 1883. In 1932, the furnace and ancillary buildings were deeded by Margaret Coleman Buckingham and have since been restored and open to the public.

• Cast Iron
• Iron
• Ironworks
• List of National Historic Landmarks in Pennsylvania
• National Register of Historic Places listings in Lebanon County, Pennsylvania

• Sharon Hernes Silverman. "A Blast from the Past: Cornwall Iron Furnace". Pennsylvania Heritage Magazine. Retrieved February 14, 2007.
• "Cornwall Furnace, USGS Lebanon (PA) Topo Map". TopoQuest. Retrieved July 10, 2008.

• H. R. Schubert, History of the British Iron and Steel Industry (1957).
• R. F. Tylecote, History of Metallurgy (1991).
• Gordon C. Pollard, 'Experimentation in 19th century bloomery production: evidence from the Adirondacks of New York' Historical Metallurgy 32(1) (1998), 33-40.

• Cornwall Iron Furnace official site
• Cornwall Iron Furnace Photos
• Science Aid: Blast Furnace How iron is extracted, for high school level
• Blast Furnace animation
• How a Blast Furnace works Illustrated.
• Precursors of the Blast Furnace
• Extensive picture gallery about all methods of making and shaping of iron and steel in North America and Europe. In German and English.
• Blast Furnace Museum Radwerk IV
• Schematic diagram of blast furnace and Cowper stove
• ironfurnaces.com - a free wiki dedicated to preserving the history and location of historic blast iron furnaces
• Period pro and con debate on the Iron Act