Puddling process

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Puddle oven

The puddle process (also known as flame furnace refining ) was a process widespread in the 19th century for converting the pig iron produced in the blast furnace into wrought iron (then also called puddle iron ), later also into hardenable forged steel . In the puddling process, freshening resulted in a low-carbon pellet interspersed with slag particles , which was further processed in several operations by cutting and forging . Wrought iron has been made since ancient times, but the puddling process used cheap hard coal for the first time instead of the increasingly expensive charcoal due to deforestation . This made wrought iron significantly cheaper. The puddling process is therefore a key technology of the industrial revolution , alongside the steam engine , the spinning machine and other inventions.

The Englishman Henry Cort invented the puddling process in 1784 . He had noticed that the carbon contained in hot pig iron evaporates when air brushes over it. While before this invention carbon and other iron companions could only be removed by the fan-assisted combustion of charcoal, his method was based on the combustion of cheap hard coal without the use of fans or crucibles.

It was replaced in the 1860s by the Bessemer process and the Siemens-Martin process , which for the first time enabled mass production of significantly cheaper and purer malleable iron, which was then referred to as steel .


According to the beginning of the 20th century usual classification of iron in carbon-rich, not plastically formable , not malleable, cast iron and carbon, formable, malleable steel , wrought iron since the term steel belongs. However, due to a slightly different alloy and in particular due to the residual slag it contains, it is not identical to modern steel.

Steel used to describe only a small group of special products with a carbon content of 0.4% to 1.2%, which could be forged, welded and, above all, hardened. All other products made from refined pig iron were referred to as malleable iron, wrought iron, or fresh iron . When the Bessemer process spread, with which fluent iron was produced, malleable iron was called welding iron and the steels made from it were called welding steel .

Puddle oven

Schematic outline

The puddle furnace, made of refractory bricks, had a separate combustion chamber in which coal burned on a grate. Coal could be introduced through appropriate flaps and the ashes removed. The hot combustion gases moved under a vaulted ceiling through the adjacent work room and only then into the chimney. A small wall, the so-called fire bridge , between the combustion and work space prevented contact between coal and iron. The heat of combustion was also reflected through the furnace ceiling into the work area. In the work room, the iron to be worked on lay in a shallow trough ( English puddle = "puddle"), which was just big enough for the puddler to reach every part of the trough with his long poles. As is customary in foundries, the hollow was lined with sand. The work area had a flap through which the pig iron ingots were brought in and, at the end of the process, the blow molds were removed, which otherwise remained closed except for a small opening through which the puddler could insert his work rods and work the iron. This opening could also be closed. The supply of air into the work area should be avoided as far as possible because it would have oxidized the iron. A flap on the fireplace was used to regulate the temperature.

The principle of the puddle oven remained unchanged. Minor changes were primarily made to reduce the cost of building and maintaining the ovens. Originally, the sand lining and part of the masonry had to be renewed weekly. For example, cavities for air cooling were inserted in the fire bridge and under the work area, or the puddle furnace was reinforced with external cast iron plates. There were puddle ovens with a kind of cast-iron frame structure into which the fire-resistant wall parts were inserted. The floor under the sand bed of the trough was designed as a cast iron plate. A slag lining was sometimes used in place of the sand bed. A so-called fox bridge was often installed opposite the fire bridge, also a small wall that prevented the slag from flowing off together with the iron. Later, the proportions were adapted to other fuels or iron qualities.

The puddler couldn't be enlarged because its cavity was limited by the reach of the puddler's work bars - and its physical capabilities. Occasionally built double ovens were actually only two ovens attached to each other. Attempts with mechanical puddles have not been successful.


Puddler at work

Sufficient heat was generated in the puddle furnace to melt the pig iron , which was usually introduced as ingots, and then to freshen it, i.e. H. to reduce its content of carbon and iron components such as silicon , manganese , phosphorus and sulfur in an oxidation process . As the liquid iron gradually decarburized, its carbon content-dependent melting point rose. While the temperature in the furnace remained largely the same, the metal became more and more rigid until it could be pushed together into lobes and removed from the furnace.

The coal, which was abundant in England and therefore cheap, was burned in the fire chamber, while the work area was hermetically sealed as far as possible. As a result, the 200 to 250 kg pig iron ingots lying in the hollow of the work area melted. The fire bridge prevented the iron from absorbing unwanted components of the coal, especially sulfur, which would have made it unusable.

The oxygen required for the oxidation came mainly from the slag, rich in iron-oxygen compounds and other oxides , which the puddler had to bring into constant contact with the viscous to pasty iron. The puddler had to constantly stir the pig iron with the slag floating on it with its long stirring and scraping rods ( English puddle ) in order to burn (oxidize) the carbon and the iron components . Bluish flames (CO) indicated the process. The puddler had to change his rods several times before they became soft. As a result of the carbon loss, lumps of iron formed, which the puddler squeezed into 4 to 6 equally large piles weighing around 40 kg, from which cauliflower-shaped lumps were formed. In the event of insufficient processing, the lobes could fall apart, so that the puddler, who worked in chord , had to process them again. The rugs were pushed to the fire bridge, the hottest place in the hollow, where they were exposed to the greatest possible heat for 4 to 6 minutes with the opening closed.

Then the flap of the workroom was opened, the rams removed with pliers and dragged across the hall floor of the ironworks to the water-powered drop hammer , and since the invention of James Nasmyth in the 1840s, also to the steam hammer. They were later transported in carts. Finally, the puddler let the liquid slag drain from the furnace.

The quantity and quality of the lobes produced essentially depended on the skill and strength of the puddler, which he was able to use in a mostly 12-hour and often 16-hour working day.

Three different procedures emerged:

Puddle on tendon with a maximum of 0.3% carbon content;
Puddling on grain or fine grain with 0.3% - 0.6% carbon content;
Puddling on steel or hardenable iron with 1.0% - 1.3% carbon content.

The puddling on steel had been described in the patent of Henry Cort as a possibility, but after previous attempts in Carinthia only by Franz Anton Lohage been and Gustav Bremme from Unna in the late 1840s brought to production. Essentially, the process consisted of avoiding the final phase of decarburization and allowing the flakes to be formed faster and forged immediately. It was strongly dependent on the temperature control and on the ability of the puddler to stir (only) long enough under the slag layer, which protected the iron from oxidation with the air, until the correct degree of decarburization was achieved. Only very experienced puddlers could be entrusted with this. In 1853 approx. 2600 tons of welding steel were produced in the Rhenish-Westphalian mountain district, which corresponded to 40% of the total Prussian crude steel production.

Like the furnace, the process remained basically unchanged. Other fuels were later used in other countries, such as B. charcoal, wood, lignite or dried peat, whereby the dimensions of the puddle ovens had to be adapted to the properties of the fuels. Liquid pig iron was also used, but this mostly failed due to the lack of space in the workshop. Faber du Faur put blast furnace gas one from the blast furnace as a fuel, but this had the consequence that a failure had an impact in one of the ovens directly on the other furnace.

Further processing

The shell with a slag content of up to 4% was shaped under a hammer into porous flatbreads, which were pressed into flat bars by a grooved roller and cut. Several such bars from different productions with different iron qualities were then bundled with wire, heated in the welding furnace and again rolled into bars until after several work steps the finished semi- finished product in the form of bars, sheets or plates was created. The slag content was reduced to 0.6%, but could never be completely removed. Welding iron always contained slag, the resulting grain was visible to the naked eye on the broken structure. Welding iron was therefore more resistant to corrosion than river iron.

With the spread of wrought iron, the processing methods developed: In 1820 John Birkinshaw received a patent for the manufacture of rolled railroad tracks , which became the basis for the construction of the railroad network. T-beams were initially riveted from angle iron and sheet metal. The first T-beams were rolled in England in 1830. In 1839 James Nasmyth invented the steam hammer. The largest and strongest plates that could be rolled for the construction of the Britannia Bridge in 1847 measured 3.65 m × 0.71 m and were up to 2.44 cm thick. After the invention of the universal rolling mill in 1849, double-T beams could be rolled as a profile. In 1851 Alfred Krupp rolled a seamless wheel tire for the first time , for which he received a patent in 1853. In 1861 he put the steam hammer "Fritz" into operation, which had a hammer weight of 30 t, which was later increased to 50 t.


The puddling process was developed by Henry Cort in 1784 and first spread from South Wales to England and Scotland, and from 1820 to the continent. In Germany, the process was introduced in 1826 by Friedrich Harkort in his factory in Wetter . During the beginning of industrialization , it became increasingly important (e.g. for the manufacture of steam engines and railroad tracks). More than 94 puddle ovens are said to have existed around 1810. At the beginning of the 1860s there were 3,600 ovens. Gutehoffnungshütte (GHH) operated 60 puddle furnaces in the Oberhausen rolling mill in 1863. The peak was reached in 1873 with 7,264 puddle ovens in 287 iron works. The amount produced using the puddle method rose in Great Britain and France until 1882, in Germany and Belgium until 1889. At the same time, however, the percentage of total iron and steel production fell steadily, as the total amount was initially due to the Bessemer method and a little later also to Siemens -Martin process was increased.

Initially, the Bessemer process only caught on slowly because it relied on phosphorus-free iron, while the puddle ovens could process different types of iron into a wide range of products. The displacement of the puddle process by the Bessemer process began in the lucrative rail market, but the puddle process was long preferred for many products made by forging, including the British Navy, which insisted on its use in shipbuilding because of its better corrosion resistance. Some entrepreneurs were also waiting for their patents to expire.

It was only when the Thomas process was introduced in the course of the 1880s , its teething troubles were overcome and the production of different steel grades was mastered in large quantities that wrought iron produced using the puddle process began to be displaced. The first steel bridges in bridge construction were not erected until the 1890s. In 1913 the proportion of wrought iron had fallen to a few percent, only in Great Britain it was 13.6%. The converters produced steel much faster and more cheaply.


Wrought iron from the puddle furnace had a wide range of uses. From him were u. a. Bars and profiles, sheets, wires and cutlery are manufactured. Countless bridges were built from wrought iron, such as B. the former cathedral bridge in Cologne, the Rhine bridge Waldshut-Koblenz , the Griethausen railway bridge , the Garabit viaduct in France or the Ponte Maria Pia and the Ponte Dom Luís I in Porto . The tallest building in the world at the time, the Eiffel Tower , was also made of wrought iron.

See also


  • Oscar Stylish; H. Steudel, ironworks. A monograph , R. Voigtländer Verlag in Leipzig [approx. 1908], pages 90 to 100
  • Handbuch der Eisenhüttenkunde Volume 3 by A. Ledebur , 5th edition, published by Arthur Felix, Leipzig 1908, page 195 ff.
  • Akoš Paulinyi : Puddling: a chapter in the history of iron in the industrial revolution. Munich, Oldenbourg-Verlag 1987 (together with the Deutsches Museum), ISBN 3-486-26200-9 .
  • Adolf Ledebur: The wood gas puddle company at Zorger-Hütte in the Harz Mountains in 1860

Web links

Commons : Puddle Stoves  - Collection of images, videos and audio files

Individual evidence

  1. The information in this article is based essentially on the book by Akoš Paulinyi: Puddling: a chapter from the history of iron in the industrial revolution. Munich, Oldenbourg-Verlag 1987 (together with the Deutsches Museum), ISBN 3-486-26200-9 .
  2. ^ A Resident Assistant: General description of the Britannia and Conway tubular bridges on the Chester & Holyhead Railway. Chapman & Hall, London 1849, p. 16 f. ( Digitized on Google Books )
  3. Philipp Stein: 100 years of GHH bridge construction . Gutehoffnungshütte Oberhausen, Sterkrade plant, Oberhausen 1951, p. 78 .