Puddling furnace
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Schematic drawing of a puddling furnace |
The
puddling furnace is a metalmaking
technology to create
wrought iron from the
pig iron produced in a
blast furnace. Pig iron contains high amounts of carbon and other impurities, making it brittle. The puddling furnace burns off these impurities to produce a malleable low-carbon
steel or wrought iron.
The furnace was 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 process. After lighting and being brought to a low temperature, the furnace is prepared for use by "fettling"; painting the grate and walls around it with iron oxides, typically
hematite. Iron is then placed on the grate, normally about 600
lbs, and allowed to melt on top, mixing with the oxides. The mixture is then stirred vigorously with a "rabbling-bar", a long iron rod with a hook formed into one end. This causes the
oxygen from the oxides to react with impurities in the pig iron, notably
silicon,
manganese (to form slag) and to some degree
sulfur and
phosphorus, which form gases and are removed out the
chimney.
More fuel is then added and the temperature raised. The iron completely melts and the
carbon starts to burn off as well. The
carbon dioxide formed in this process causes the
slag 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, 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 plastic 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 40
kg each. These are then hammered ('
shingled') using a powered hammer, latterly
steam hammer, to expell slag and weld 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 descreasing size so that the bar was progressively reduced to the desired dimensions. The quality of this may be improved by
faggoting.
The puddling furnace began to be displaced with the introduction of the
Bessemer Process, which produced mild
steel or wrought iron for a fraction of the cost and time. For comparison, an average size charge for a puddling furnace was 600 lb, for a Bessemer converter it is 15
short tons. 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 is one of several that were developed in the second half of the
18th century for producing bar iron from pig iron without the use of charcoal. It was invented by
Henry Cort at
Fontley in Hampshire in 1783â€"4 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 above). This application of the rolling mill was also Cort's invention.
Unfortunately, Cort's process (as patented) only worked for white cast iron, not grey cast iron, which was the usual feedstock for forges of the period. This problem was resolved probably at
Merthyr Tydfil 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 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 was known as 'wet puddling'. This was invented by a puddler called
Joseph Hall at
Tipton. He began adding scrap iron to the charge. Later he tried adding iron scale (in effect, rust). The result was spectacular in that the furnace boiled violently. This was in fact a
chemical reaction 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 ironworks 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 and that described above under 'process'. 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 1.0.
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 only worked 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. 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 in
Yorkshire (
England) in
1851 and in the
Loire valley in
France in
1855. It was widely used.
Though it was not the first process to produce bar iron without
charcoal, puddling was by far the most successful, and replaced the earlier
potting and stamping 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.
*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.
