Fining and puddling in Europe

After the introduction of the blast furnace in Medieval Europe in the 13th or 14th century, a means of converting the high-carbon cast iron from the blast furnace to low-carbon wrought iron was necessary. At the site of the early ironworks at Lapphyttan in Norberg, central Sweden, some features are believed to be remains of small fining furnaces, but very little is known about their original structure and how they were operated. They seem to have a certain similarity to the remains of ancient Chinese fining furnaces.

The fining hearths that we know more about are from later centuries. These are generally much larger than the Chinese fining hearths, and use water power for both the blast and the associated hammer. There were numerous variations on one basic plan: we shall see below two types as they were used in parts of Sweden well into the 20th century. Fining was used here much longer than elsewhere because Swedish coal was not suitable for the puddling process, which replaced fining in most of Europe from the 18th century.

Fining used enormous quantities of charcoal. Many attempts were made, especially in Britain, to substitute mineral coal for charcoal, but the sulphur content of the coal ruined the quality of the wrought iron produced. The final result of many attempts by many persons was the puddling process, in which the fuel was separated from the iron in a reverberatory furnace. We shall see further below the operation of a puddling furnace in Britain that was still in operation in 1950, almost a century after puddling was made obsolete by the introduction of the Bessemer process.

The Swedish Walloon hearth

Below is a painting of a Walloon fining hearth in Sweden in 1786. (The Walloons are an ethnic group in Belgium. Many Walloons emigrated to Sweden in the 17th century and brought this technology with them.) The sketch here on the right shows how it functioned. Charcoal burns in a shallow hearth, intensified by a blast of air. Cast iron is placed on the charcoal bed, and more charcoal is piled over it. The iron melts in droplets which fall through the oxidizing flame of the air blast, losing some of their carbon, and coalesce in a bloom (or ‘loupe’) at the bottom of the hearth. When a suitably large bloom has formed, the finer uses an iron bar to lift it up above the flame again, where it again melts, losing more of its carbon. This lifting operation is called rabbling.

Some of the iron also burns, and the finer adds some sand (quartz, SiO₂) to form a low-melting slag (FeO·SiO₂). Passing through this liquid slag in the hearth also aids in the decarburization of the iron droplets. When the rabbling operation has been repeated a number of times, the carbon content of the bloom is so low that it does not melt at the temperature of the fire. The finer then removes it to the water-powered hammer to squeeze out as much as possible of the slag that it contains.

The bloom produced by fining is very similar to the bloom produced in the process of direct iron smelting – this is surely why the same word is used for both. Each must be hammered to remove slag, and distinguishing between bloomery slag and finery slag is an ongoing challenge in archaeometallurgy both East and West.

The Swedish Lancashire hearth

The Walloon forge is a rather open structure from which much heat is lost, wasting charcoal. In 1828 the Swedish ironmaster Gustaf Ekman visited a number of ironworks in Britain. He found that the design of a fining hearth he observed in Lancashire was the best for his purposes, and he built a copy of it in his ironworks. This proved a great success, and it was soon imitated in many other Swedish ironworks.

The Lancashire hearth is a much more closed structure than the Walloon forge, and therefore more fuel-efficient. It also includes other improvements, such as a heated air blast. It operates at a much higher temperature, so that the conversion of cast iron to wrought iron occurs more quickly, saving both fuel and labour.

In time larger and larger Lancashire hearths were built in order to produce larger blooms. This made the work of the finer harder, and in the 1890’s the ‘Lägerwall machine’ was invented, a water-powered aid in the operation of rabbling.

The short clip below from a Swedish film shows the operation of a Lancashire hearth with a Lägerwall machine.

Puddling

Fining used great quantities of charcoal. That was aceptable in Sweden, with its rich forest resources, but in Britain, by the early 18th century, the supply of charcoal became a severe limiting factor for the expansion of iron and steel production. Coke-fueled blast-furnace iron smelting became common by about 1750, but in fining, charcoal continued to be necessary because of the sulphur content of coal and coke. Many attempts were made to produce wrought iron from cast iron using coke, but the first commercially successful invention was Henry Cort’s puddling process, patented in 1784 (Mott 1983).

‘Puddle’, used as a verb, means ‘stir about’. In the iron puddling process, cast iron is melted in a hearth heated by a separate coke fire. The puddler stirs the molten iron with an iron rod so that the carbon in the iron burns away. As the carbon content of the iron falls, the melting point rises, and in time the iron solidifies as a ‘ball’ of low-carbon iron mixed with slag, very similar to the ‘bloom’ produced in bloomery smelting or in fining. In the same way as in those processes the puddled ball is moved to a water- or steam-powered hammer (or sometimes a rolling mill) to squeeze out most of the slag.

Far into the 20th century, old-fashioned wrought iron, with ‘fibrous’ slag inclusions, was considered in some applications to be superior to modern mild steel without this ‘fibre’. The demand for this material was supplied by puddling long after the technology was made obsolete by the Bessemer process and later developments. The photographs and film further below show puddling as it was practised at three British ironworks in the 1950’s and 1960’s.

John Percy, a medical doctor as well as a metallurgist, described the puddling process in great detail from actual observation in 1859 (1864: 654–657). His description concludes with a note on the health of the puddler:

Puddlers vary considerably in skill, and between two puddlers in a fortnight there may be a difference of 10 cwts. [500 kg] in the yield of puddled bars. Most puddlers work until 50 years of age, and many even afterwards. Puddling is probably the severest kind of labour in the world; yet many puddlers attain the ripe age of 70 years, or more. The majority die between the ages of 45 and 50 years; and, according to the returns of medical men to the Registrar, pneumonia, or inflammation of the lungs, is the most fre­quent cause of their death. This is what might have been anticipated from the fact of their exposure to great alternations of temperature under the condition of physical exhaustion. Mr. Field, optician, Bir­mingham, informs me that puddlers are moreover liable to cataract, induced by the intensely bright light of the furnace; that he has seen a great number of such cases, and supplied the patients with glasses.

Puddling, photographed by W. K. V. Gale in 1950–1951. Taken from Gale 1973, figs. 38–43; see that book for a more detailed explanation.

Puddling at Thomas Walmsley & Sons Ltd., Bolton, England, 1960’s. Kindly supplied by Ironbridge Gorge Museum Trust (Sarah Roberts, archivist).

And here is a video of one of the very last puddling runs anywhere in the world, at the Butterley Ironworks in Britain in 1965:

References

Last edited by DBW 25 February 2023
Stylistic changes 5 June 2024