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Crucible Steel-making

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Will customers losing family or friends please meet them by the bronze statues in Market Street?

A Glimpse of the Legacy

Thousands of visitors to Sheffield's Meadowhall shopping centre wonder at this curious PA system message. The statues themselves, though, are more wonderful by far.

There are three men in the group, half as big again as life size. Two of them are almost passive. The third seizes the attention of every onlooker. His back is braced, and sweat seems to pour from his glistening pate. His sopping neck-towel is clamped in his teeth, drizzling over the leather apron at his knees. The tongs flex across his thigh, bow-string taut under the weight of the crucible. A stream of dense and blinding fire springs from its lip.

Only a century ago, steel-making was a craft, and the scene recalls the zenith of that era. This burly bull of a man, straining against the cantilever of broiling metal, had the hardest job under Heaven. The 'teemer', as they called him, enacted the climax in a process so remarkable that it scarcely seems comprehensible today.

Crucible steel-making changed the world, marking an evolutionary step in the technical development of mankind. The man whose image is frozen in cast bronze at Meadowhall is its enduring symbol, though its true originator left us no likeness of himself. The Industrial Revolution in England had its full share of self-publicists, but Benjamin Huntsman is among its unsung heroes.

The Process Conceived

He was born in 1704 in North Lincolnshire, to Dutch parents. His interest in metalworking led to an apprenticeship with a Doncaster clockmaker. Young Benjamin soon appreciated the limitations of the watch springs of his time.

Thirty miles further west, another town was burgeoning, its economy fuelled by its knife-making acumen. Sheffield's steel, and indeed most of England's, was then made by the cementation process. Modern metallurgists marvel at the serendipity of this method for the making of cutlery blades; it could hardly have been better conceived had it been assisted by today's scientific understanding. Blister steel, as it was known, was produced in the cementation furnace by the carburisation1 of wrought iron bars, baked in stacks consisting of alternating layers of metal and charcoal. The hardness-inducing carbon diffused into the metal surface leaving the core soft, so that when bundles of bars were forged together they gave rise to thin, multiple layers of hard and soft steel. This laminated microstructure can be honed repeatedly to a fresh, keen edge, while retaining pliability and elasticity - perfect for a table knife.

Benjamin wasn't interested in knives, though, and this same type of steel made poor clock springs. In those artefacts, consistent and repeatable through-thickness mechanical properties are vital. Huntsman soon became determined to find a reliable way of making high-purity steel. Naturally enough, he moved to Sheffield to make his attempt.

In about 1740, Benjamin Huntsman bought a house at Handsworth, which was then a small village to the east of the city. Here, he established a forge in an adjoining building. There was already a crucible process for the refining of brass, by which impure metal was re-smelted in clay pots. It seemed possible that clean steel, free of entrained slag, could be made in the same way, but there were severe technical difficulties. The melting point of iron is far higher than that of copper-based alloys, and the first means of attaining it, through the combustion of incandescent coke, had only recently been discovered, by Darby at Coalbrookdale some twenty-five years before. Neither long-life furnace refractory nor crucible ceramics yet existed for use at these temperatures. Nonetheless, Huntsman began to experiment with the materials he knew and could procure.

Not much is known about how Benjamin Huntsman developed and refined his process, but by the time of his death in 1776 he had a thriving business and a worldwide reputation, to say nothing of a cohort of emulators. The excellence of his product and the diligence of his research assured his posterity. He made no attempt to secure personal fame, and as a devout Quaker he refused to allow anyone even to make a portrait of him.

His son William inherited the family's second factory, built at Attercliffe in 1772. The commercial process for crucible steel-making was by then around thirty years old, but it was still severely volume-limited at little more than a ton per month, even for this unusually large facility.

Over the next hundred years, the crucible process underwent considerable refinement and elaboration. By the 1880s, it had grown into a highly-skilled but labour-intensive sequence of tasks. The precision of the practice and the strangeness of the working environment may seem almost surreal to the modern observer. Fortunately, a few firms in Victorian Sheffield treated their responsibility to history with the same assiduousness as their manufacturing activity. We are fortunate to have their lucid descriptions of a crucible meltshop of the period.

The Process Enacted - Clay

The first job was to make the crucible itself. The detailed geometry changed little over the last century of its use, except for a progressive increase in overall size. From the outside, the crucible was vase-like in form, tapering out from its base and curving in again at the lip. At something like 600mm in ultimate height and 300mm in fullest outer diameter, its graceful shape would not have looked out of place in a domestic setting.

In one respect, though, a crucible was not at all vase-like. The wall thickness was considerable, at around 70mm. This vessel had to accommodate a significant weight of molten steel at temperatures close to the limit of infusibility, and to retain strength for two or three casts in spite of progressive erosion.

The clay recipe was a closely-guarded secret, and the preparation was fastidious. Stannington clay from the northwest of the city made the bulk, with additions of Stourbridge, Derby or sometimes China clay. Strength prior to firing was assured by incorporating coke breeze2 and 'grog' - the latter being fragments of used crucibles, smashed to dust.

A carefully-measured volume of water was mixed with the dry material, and the mortar was shovelled into floor-level trays. It was then trodden with bare feet for about three hours - no better way of driving out air bubbles was ever developed, and untrodden mortar frequently failed in firing or (worse) in use, weakened by these imperceptible voids.

The dense and homogeneous product was then weighed and hand-formed into roughly cylindrical blocks, before being placed in a lathe-turned wooden mould known as a 'flask'. The inside surface of the flask formed the outer profile of the crucible, and was usually lubricated with oil of creosote. The bottom of the flask consisted of an unattached but close-fitting circular 'plate' in which there was a central hole.

The initial profile had to taper all the way from the base (lacking as yet the narrowing at the top) as a result of the constraints of manufacture. There was a wooden 'plug', also turned and oiled, that was driven evenly into the flask using a mallet. Existent examples of this mallet are improbably small in size, presumably to enforce a gradual, and therefore repeatable, forming of the wall through a large number of progressive deformations. A spike on the end of the plug engaged with the hole in the plate, ensuring alignment and uniformity of wall thickness.

A strickle was used to trim off the excess clay, and the plug was drawn out. The flask was placed on the 'tree', a vertical post set in the ground (a surviving example is an iron billet). A spigot on top of the tree once again engaged with the hole in the plate, this time from the other side. The flask was carefully pulled off downwards, leaving the part-formed crucible standing on the plate atop the post.

Transferring the still-soft pot to a workbench was not a trivial task either. Tools for handling and supporting the 'green' crucible during transport were varied, but nonetheless specialised. Examples included spoon-like sheet steel shells, or shaped leather bags. Overhead rails and slings with pulleys were developed to negotiate progress round the workshop.

Once on the bench, the top was gently shaped using a bucket-like metal mould called a 'bonnet'. The curvature at the top of the crucible was vital for its strength. This was the final shaping operation in wet clay, and was thus a fraught point in the process. 'Turning in', even more than the other operations of the clay-shop, was regarded as a job for practised hands.

Crucibles were made a long time in advance of their use. After final shaping, they were left to air-dry for up to a week, before being transferred to racks adjacent to the furnace where a flow of warm air would complete the drying process. A common position for these racks was high on the stack wall directly above the melting holes. A typical duration for this final drying phase was one month.

There were two other components made in the clay-shop too. The simpler ones were the 'stands', flat circular discs about 70mm thick which would later be used to seal the crucible base. Apprentice 'potmakers' were given stands to fashion as their first task. Later they would make lids to fit the crucible mouth. These were also circular, and domed in profile.

The Process Enacted - Fire

A crucible melting-shop of the late nineteenth century was substantially an underground construction. A large cellar beneath the floor ensured a sufficient draught-volume. The steel was refined in a series of melting holes connected to the cellar-main by flues and accessible through a lid set into the shop floor. A top flue lead from each hole to a stack, and the base of each hole was made up with a grid of fire-bars with an ash-pit beneath. By this time, holes designed to take two crucibles each were the norm. A large melting shop might have thirty or more holes; there were probably more than a thousand holes altogether in the Sheffield area at this time.

One of Sheffield's principal natural advantages was the availability of ganister sandstone in the Upper Don Valley, which was almost unique in its refractory properties in the days before synthetic ceramics. It was the lack of a substitute material for the bricking-out of the hole that did more than anything to protect the city's market ascendancy and to prevent successful emulation.

The refractory wall of the furnace hole was permanent. A moulded lining was made up by ramming a ground ganister paste into the space between the wall and a wooden former. This took about 36 hours to dry out, first by forced convection (which could be achieved by actuating dampers in cross-flues connecting adjacent working stacks) and latterly by making a slow coke fire in the cavity. The same coke-fire technique was used to bring the furnace up to temperature ready for use, whether or not it had been freshly lined. In fact a lining would last for several weeks, and different contours were developed as steelmakers sought the most uniform heating possible for their crucibles.

On the opposite side of the shop to the holes and their stacks were the annealing furnaces, in which the crucibles were fired prior to use. A sufficient complement of crucibles, stands and lids would be set inside on the day before their intended use, and the hearth beneath filled with coke. A charge of burning coke would then be shovelled into this hearth and the furnace closed up. By regulating the draught, the clay-ware within was slowly brought up to red heat by the following morning.

On the day of the 'heat' itself, work around the holes would start at around six in the morning. The fire-bars were 'fettled' using rods to scrape off any clinker adhering from previous use, and the red-hot stands were carried over and set down on the bars. Next came the precious crucibles, transported with tongs. After placing each one on its stand, a scoopful of silica sand was poured into its mouth, followed by the laying on of the lid. Burning coke was then shovelled onto the fire-bars, with the remaining space up to the level of the lids being filled by dry coke. The furnace lid would then be closed and the dampers opened, creating a full draught and full combustion conditions. After about half an hour, the crucible would be incandescent and fused to its stand by the melting of the silica, and about half of the coke in the hole would have been consumed.

The Process Enacted - Steel

While this was going on, the charge of blister steel would be prepared and brought to hole-side. Originally manufactured in a cementation furnace (often on the same site), this low-grade re-carburised iron was the usual feedstock of the crucible process. It could be broken up by hammering, and it was weighed in lump-form before being moderately preheated in a pan.

Back at the hole, the lid was taken off the first crucible and the 'funnel' inserted, a tubular wrought-iron chute used to direct the charge into the mouth. An iron bar with a spooned end was threaded down the funnel and rested on the stand at the bottom, with the purpose of protecting the clay-ware from the full impact of the charge. The steel was carefully poured in by tippling the pan, the bar and funnel were withdrawn and the lid replaced, before repeating the process for the second crucible. Finally, the hole was topped up with fresh coke and the furnace covered.

At intervals of approximately an hour, the process would be taken off full draught to charge more coke. After about three hours, the steel would be fully melted. This would be checked by displacing the lid and probing the contents of the crucible with a rod. In the original crucible process the steel was teemed in this state, leading to a honeycombed structure in the ingot, which was difficult to consolidate by hot-working and limited the application of the product for drawing3 applications. The problem lies in the release of carbon monoxide, forming blowholes. A means of preventing this was soon realised and became known as 'killing by fire'. It entailed a final coke charge and a further three-quarters of an hour or so all with vents open and the furnace at maximum temperature. The ganister came into its own and this superior feature of crucible steel, gas-free during its solidification phase, was effectively exclusive to Sheffield.

After this final phase of the furnace process, the scene from Meadowhall was enacted. The first man up was the 'puller-out', whose job it was to raise the crucible up from the bars at the bottom of the hole and onto the shop floor. This lift of around a metre and a weight of up to twenty kilograms was accomplished in one swing. The 'puller-out' needed great strength and was exposed to intense heat, but the physical task was brief and did not demand the same control as the next job in line, that of the teemer. Nonetheless, the 'puller-out' had to set the crucibles down gently - a broken pot at this time meant catastrophic waste. He wore the same water-drenched leathers, gauntlets and towels as the teemer.

The teemer now picked up the crucible in his own tongs and swung it towards the mould. At this point, he would brace the tongs across his outer leg and lean out to tipple the crucible, progressively angling it by rolling the tong-arms over the fulcrum of his thigh. A colleague (also represented in the Meadowhall group) would come alongside with a rabble on the end of a rod. His job was to dam the slag floating on the steel within the crucible, and to prevent it from following the metal into the mould.

A typical mould was square in section, comprising two halves bound together by wedged rings. It was between half a metre and a metre in length, set at an angle in a pit in the floor. The aperture at the top was seldom more than 75mm wide. Pouring twenty kilograms of molten steel into a target area this size, with a minimum of splashing and inside two minutes is a highly-skilled action, but one that also requires considerable physical strength and endurance.

Teemers were trained 'cold' by tipping lead-shot into moulds. It is still presumably possible to try this - a nice lifetime ambition for a steel-mad Researcher. Reproducing the full experience, with the addition of intense heat and extreme danger, is no longer possible anywhere in the western world.

The teemer now stood back, the crucible empty except for slag, and the third member of the statue-group stepped forward to cap the mould with a plug to limit oxidation of the cooling metal. It would take no more than five minutes to solidify. If the ingot was to be sold on, full cooling in the mould would normally be allowed. If the shop served a forge, though, the wedges would be knocked out and the mould split immediately, and the ingot would be carried to the hammer. Heat is an investment, a literal case of striking while the iron is hot.

The working day would be far from over at this point. The puller-out would already have returned the crucible to its hole, complete with its now permanently-attached stand. The process would start again, albeit with a slightly reduced charge-weight. This was because of the crucible's progressive weakening by erosion - such limitations were learned empirically. Quite often, a third melt in the same pot was accomplished too. Attending holes in rotation, a team of six to eight men working a twelve-hour shift could manage sixty heats or even more. By the end of it, the annealing furnace would already have been loaded with tomorrow's fresh crucibles. The last job of the day was to dispose of the spent ones. Now completely useless, they were often broken with some relish.

A Truer Legacy

The modern city of Sheffield has good reason to be thankful to Benjamin Huntsman. The local worthies have belatedly begun to acknowledge him, though for a very long time a remarkable man was forgotten. They demolished his little house in the 1930s, along with its maze of runners and hearths, and its scorched masonry. Although the city's Crucible Theatre is incongruously famous for snooker, the origins of the name will now be clear. Play-lovers might have suspected Miller's witches, but the explanation is close to home.

Sheffield's modern-day economy is founded in high-purity metals. Quality counts for more than quantity these days, all over the western world. Huntsman's patient perfectionism seems almost prophetic.

Here, too, is the founding of something more than blank ingots. The character of the city was made this way, in these dark places, playing with fire. Men and women with steel in their eye, chins stuck out as if still biting on that sodden neck-towel. Proud and straightforward, frank and true.

And there is a global postscript to the story too. These were the beginnings of industrial materials science, the empiricism of the chemists transposed from laboratory to factory. Huntsman and his peers in different crafts (Wedgwood for another) took the vital steps from knowledge to utility. Quite literally, they made the world what it is today.

If you would like to learn more about the history of industrial metallurgy, an excellent source is The Industrial Revolution in Metals - Ed. Day and Tylecote, Inst. of Metals (now IoMMM) 1991, ISBN 0 901462 82 9.

Alternatively, the general writings of Ken Barraclough and Ron Tylecote are recommended, which capture and preserve the achievements of this marvellous age.

1The deliberate and controlled combination of carbon with iron in a diffusion process. Carbon increases the strength of steel, at the expense of its ductility.2Small flakes of crushed coke. Charcoal or graphite were also sometimes used.3Applications in which the cross-section of the steel is greatly reduced, thus demanding high ductility. Wire-making (by drawing of a rod through a succession of dies) is an example.

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