Puddling (metallurgy)
Puddling was an Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

 means of making iron
Wrought iron
thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...

 and steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

. In the original puddling technique, molten iron in a reverberatory furnace
Reverberatory furnace
A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases...

 was stirred with rods, which were consumed in the process. It was one of the first processes for making bar iron
Wrought iron
thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...

 without charcoal in Europe, although much earlier coal-based process had existed in China.

Later, it was also used to produce a good-quality carbon steel; this was a highly skilled art, but both high-carbon and low-carbon steels were successfully produced on a small scale, particularly for sword
A sword is a bladed weapon used primarily for cutting or thrusting. The precise definition of the term varies with the historical epoch or the geographical region under consideration...

s, knives and other weapon
A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...



Puddling was one of several processes developed in the second half of the 18th century in Great Britain for producing bar iron from pig iron
Pig iron
Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel...

 without the use of charcoal. It gradually replaced the earlier charcoal-fuelled process, conducted in a finery forge
Finery forge
Iron tapped from the blast furnace is pig iron, and contains significant amounts of carbon and silicon. To produce malleable wrought iron, it needs to undergo a further process. In the early modern period, this was carried out in a finery forge....

It was invented by Henry Cort
Henry Cort
Henry Cort was an English ironmaster. During the Industrial Revolution in England, Cort began refining iron from pig iron to wrought iron using innovative production systems. In 1783 he patented the puddling process for refining iron ore...

 at Fontley in Hampshire in 1783–84 and patented in 1784. A superficially similar (but probably less effective) process was patented the previous year by Peter Onions. Cort's process consisted of stirring molten pig iron in a reverberatory furnace in an oxidising atmosphere, thus decarburising it. When the iron "came to nature", that is, to a pasty consistency, it was gathered into a puddled ball, shingled, and rolled (as described below). This application of the rolling mill was also Cort's invention.

Ninety years after Cort's invention, an American labor newspaper recalled the advantages of his system:

"When iron is simply melted and run into any mold, its texture is granular, and it is so brittle as to be quite unreliable for any use requiring much tensile strength. The process of puddling consisted in stirring the molten iron run out in a puddle, and had the effect of so changing its anotomic arrangement as to render the process of rolling more efficacious."

Cort's process (as patented) only worked for white cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...

, not grey cast iron
Gray iron
Gray iron, or grey iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite...

, which was the usual feedstock for forges of the period. This problem was resolved probably at Merthyr Tydfil
Merthyr Tydfil
Merthyr Tydfil is a town in Wales, with a population of about 30,000. Although once the largest town in Wales, it is now ranked as the 15th largest urban area in Wales. It also gives its name to a county borough, which has a population of around 55,000. It is located in the historic county of...

 by combining puddling with one element of a slightly earlier process. This involved another kind of hearth known as a 'refinery' or 'running out fire'. The pig iron was melted in this and run out into a trough. The slag separated, and floated on the molten iron, and was removed by lowering a dam at the end of the trough. The effect of this process was to desiliconise
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 the metal, leaving a white brittle metal, known as 'finers metal'. This was the ideal material to charge to the puddling furnace. This version of the process was known as 'dry puddling' and continued in use in some places as late as 1890.

The alternative to refining gray iron was known as 'wet puddling', also known as 'boiling' or 'pig boiling'. This was invented by a puddler named Joseph Hall
Joseph Hall (metallurgist)
Joseph Hall 1789 – 1862, the inventor of 'Wet Puddling', was born in 1789 and apprenticed in 1806 as a puddler to use Henry Cort's puddling process. He tried adding old iron to the charge of the puddling furnace and later puddler's bosh cinder to the charge. This caused the charge to boil...

 at Tipton
Tipton is a town in the Sandwell borough of the West Midlands, England, with a population of around 47,000. Tipton is located about halfway between Birmingham and Wolverhampton. It is a part of the West Midlands conurbation and is a part of the Black Country....

. He began adding scrap iron to the charge. Later he tried adding iron scale
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...

 (in effect, rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

). The result was spectacular in that the furnace boiled violently. This was a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 between the oxidised iron in the scale and the carbon dissolved in the pig iron. Again to his surprise, the resultant puddle ball produced good iron.

Hall subsequently became a partner in establishing the Bloomfield Iron Works at Tipton in 1830, the firm becoming Bradley, Barrows and Hall from 1834. This is the version of the process most commonly used in the mid to late 19th century. Wet puddling had the advantage that it was much more efficient than dry puddling (or any earlier process). The best yield of iron achievable from dry puddling is a ton of iron from 1.3 tons of pig iron, but the yield from wet puddling was close to 100%.

The production of mild steel in the puddling furnace was only achieved in about 1850 in Westphalia in Germany and was patented in Great Britain on behalf of Lohage, Bremme and Lehrkind. It worked only with pig iron made from certain kinds of ore. The cast iron had to be melted quickly and the slag to be rich in manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

. When the metal came to nature, it had to be removed quickly and shingled before further carburisation occurred. The process was taken up at the Low Moor Ironworks at Bradford
Bradford lies at the heart of the City of Bradford, a metropolitan borough of West Yorkshire, in Northern England. It is situated in the foothills of the Pennines, west of Leeds, and northwest of Wakefield. Bradford became a municipal borough in 1847, and received its charter as a city in 1897...

 in Yorkshire
Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

) in 1851 and in the Loire
Loire is an administrative department in the east-central part of France occupying the River Loire's upper reaches.-History:Loire was created in 1793 when after just 3½ years the young Rhône-et-Loire department was split into two. This was a response to counter-Revolutionary activities in Lyon...

 valley in France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 in 1855. It was widely used.

The puddling process began to be displaced with the introduction of the Bessemer process
Bessemer process
The Bessemer process was the first inexpensive industrial process for the mass-production of steel from molten pig iron. The process is named after its inventor, Henry Bessemer, who took out a patent on the process in 1855. The process was independently discovered in 1851 by William Kelly...

, which produced steel. This could be converted into wrought iron using the Aston process for a fraction of the cost and time. For comparison, an average size charge for a puddling furnace was 800 – while a Bessemer converter charge was (13,600 kg). The puddling process could not be scaled up, being limited by the amount that the puddler could handle. It could only be expanded by building more furnaces.


The process begins by preparing the puddling furnace; this involves bringing the furnace to a low temperature and then fettling it. Fettling is the process of painting the grate and walls around it with iron oxides, typically hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

; this acts as a protective coating keeping the melted metal from burning through the furnace. Sometimes finely pounded cinders from a charcoal forge, puddling furnace, reheating furnace, or blast furnace is used instead of hematite. In this case the furnace must be heated for 4–5 hours to melt the cinder and then cooled before charging. Either white cast iron or refined iron is then charged into hearth of the furnace. For wet puddling, scrap iron and/or iron oxide is also charged. This mixture is then heated until the top melts, allowing for the oxides to begin mixing; this usually takes 30 minutes. This mixture is subjected to a strong current of air and stirred by long bars with hooks on one end, called puddling bars or rabbles, through working doors. This helps the oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 from the oxides react with impurities in the pig iron, notably silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

, manganese (to form slag) and to some degree sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 and phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

, which form gases and escape with the exhaust of the furnace.

More fuel is then added and the temperature raised. The iron completely melts and the carbon starts to burn off as well. When wet puddling the mixture will begin to "boil" due to the added iron oxide. The carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 formed in this process causes the slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

 to "puff up" on top, giving the rabbler a visual indication of the progress of the combustion. As the carbon burns off the melting temperature of the mixture rises from 1150 –, so the furnace has to be continually fed during this process. Eventually the carbon is mostly burned off and the iron "comes to nature", forming into a spongy material, indicating that the process is complete and the material can be removed. The hook on the end of the bar is then used to pull out large puddle balls of the material, about 35–40 kilograms (70–80 pounds) each, and 30–38 centimeters (12–15 inches) in diameter. Sometimes a large pair of tongs are used to remove the puddle balls.

These puddle balls are then transported to the hammer or squeezer by dragging them along iron slopes built between the furnace and the shingling
Shingling (metallurgy)
Shingling was a stage in the production of bar iron or steel, in the finery and puddling processes. As with many ironmaking terms, this is derived from the French - cinglage....

 equipment or, more commonly, the puddle balls are loaded into iron wheelbarrows and transported to their destination. Shingling expels slag and welds shut internal cracks, while breaking off chunks of impurities. The iron is then re-heated and rolled out into flat bars or round rods. For this, grooved rollers were used, the grooves being of successively decreasing size so that the bar was progressively reduced to the desired dimensions. Some of the iron oxide is from the scales that form in the later steps of shingling and rolling. The quality of this may be improved by faggoting
Faggoting (metalworking)
Faggoting or faggoting and folding is a metalworking technique used in the smelting and forging of wrought iron, damascus steel, and other steel. Faggoting is a process in which rods or bars of iron and/or steel are gathered and forge welded together. The faggot would then be drawn out lengthwise...


Puddling furnace

The puddling furnace is a metalmaking technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

 used to create wrought iron
Wrought iron
thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...

 or steel from the pig iron
Pig iron
Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel...

 produced in a blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...

. The furnace is constructed to pull the hot air over the iron without it coming into direct contact with the fuel, a system generally known as a reverberatory furnace or open hearth furnace
Open hearth furnace
Open hearth furnaces are one of a number of kinds of furnace where excess carbon and other impurities are burnt out of the pig iron to produce steel. Since steel is difficult to manufacture due to its high melting point, normal fuels and furnaces were insufficient and the open hearth furnace was...

. The major advantage of this system is keeping the impurities of the fuel separated from the charge.

There were two major types of puddling furnaces used in the United States. The first is the single puddling furnace, which is based on the same design used in England and, thus, the most common. The second kind is the double puddling furnace, which was most often found on the east of the Allegheny Mountains.

The general design of a single puddling furnace is as follows. The footprint of the furnace was 3.3–3.6 meters (11–12 feet) long, 1.5 – wide (depending on hearth size) and 1.5 m (4.9 ft) tall. The outer walls were 23 centimeters (9 inches) thick and made of typical brick and then covered by cast iron plates. Wrought iron square bars, called cross binders, are run through the roof of the furnace and bolted to the cast iron plates to keep the roof from collapsing. The chimney was 9 – tall and 40 cm (15.7 in) square. There would be a small work hole allowing access to the fire, and a work door allowing access to the hearth. The average work door was 55 cm (21.7 in) wide by 68 cm (26.8 in) tall, lined with firebricks on the inside and with a small square work hole for tools.
The hearth is where the iron is charged, melted and puddled. The hearth's shape is usually elliptical; 1.5 – in length and 1 – wide. If the furnace is designed to puddle white iron then the hearth depth is never more than 50 cm (19.7 in). If the furnace is designed to boil gray iron then the average hearth depth is 50 –. Due to the great heat required to melt the charge the grate had to be cooled, else it would melt with the charge. This was done by running a constant charge of cool air on it, or by throwing water on the bottom of the grate.

The fireplace, where the fuel is burned, used a cast iron grate which varied in size depending on the fuel used. If bituminous coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 is used then an average grate size is 60 by 90 cm (2 by 3 ft) and is loaded with 25 – of coal. If anthracite coal is used then the grate is 1.5 by 1.2 m (4.9 by 3.9 ft) and is loaded with 50 – of coal.

A double puddling furnace is very similar to that of the single puddling furnace, with the major difference being there are two work doors allowing two puddlers to work the furnace at the same time. The biggest advantage of this setup is that it produces twice as much wrought iron. It's also more economical and fuel efficient as compared to a single furnace.


Though it was not the first process to produce bar iron without charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...

, puddling was by far the most successful, and replaced the earlier potting and stamping
Potting and stamping
Potting and stamping is a modern name for one of the 18th century processes for refining pig iron without the use of charcoal.-Inventors:The process was devised by Charles Wood of Lowmill, Egremont in Cumberland and his brother John Wood of Wednesbury and patented by them...

 processes, as well as the much older charcoal finery process. This enabled a great expansion of iron production in Great Britain to take place. That expansion constitutes the Industrial Revolution so far as the iron industry is concerned. Most 19th century applications of wrought iron, including the Eiffel Tower
Eiffel Tower
The Eiffel Tower is a puddle iron lattice tower located on the Champ de Mars in Paris. Built in 1889, it has become both a global icon of France and one of the most recognizable structures in the world...

 and the original framework of the Statue of Liberty
Statue of Liberty
The Statue of Liberty is a colossal neoclassical sculpture on Liberty Island in New York Harbor, designed by Frédéric Bartholdi and dedicated on October 28, 1886...

 used puddled iron.

See also

  • Crucible steel
    Crucible steel
    Crucible steel describes a number of different techniques for making steel in a crucible. Its manufacture is essentially a refining process which is dependent on preexisting furnace products...

  • Decarburization
    Decarburization is the process opposite to carburization, namely aimed at decreasing the content of carbon in metals . Decarburization occurs when Carbon in the metal reacts with gasses present in the atmosphere...

  • Double hammer
    Double hammer
    A double hammer is a forging implement used in metallurgy. It operates on puddle balls and blooms by hitting both sides at the same time. Double hammers are made of two blocks attached to rollers which facilitate opposing movement along a set of rails...

  • Steelmaking
    Steelmaking is the second step in producing steel from iron ore. In this stage, impurities such as sulfur, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required.-Older...

Further reading

  • W. K. V. Gale, Iron and Steel (Longmans, London 1969), 55ff.
  • W. K. V. Gale, The British Iron and Steel Industry: a technical history (David & Charles, Newton Abbot 1967), 62-66.
  • R. A. Mott, 'Dry and Wet Puddling' Trans. Newcomen Soc. 49 (1977-8), 153-8.
  • R. A. Mott (ed. P. Singer), Henry Cort: the great finer (The Metals Society, London 1983).
  • K. Barraclough, Steelmaking: 1850-1900 (Institute of Materials, London 1990), 27-35.
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