Crucible steel

Crucible steel describes a number of different techniques for making steel alloy by slowly heating and cooling iron and carbon (typically in the form of charcoal) in a crucible.

Key to the formation of any steel is the conversion of iron oxide, into iron. Iron is not found in its elemental state due to free oxygen in the atmosphere, so some method must be used to remove the oxygen again and return the iron to a pure state. The most basic way to do this is to heat it in the presence of carbon, which will then combine with the oxygen to form carbon dioxide, leaving pure iron.

Luckily one common source of carbon is charcoal, which happens to be one of the few common fuels that also burns with enough heat to cause the reaction to occur. Simply heating the iron directly leaves the resulting alloy with a very high carbon content, often 4 to 5%. In this condition it is rather brittle, and not considered steel, but rather pig iron. Lower carbon steels can be produced, but only if the mixing of the carbon and iron is more carefully controlled. Crucible steels use this process, controlling the mixture rates by confining the iron in high-temperature containers instead of placing it on the charcoal directly. Modern methods instead remove carbon from the pig iron, due to its low cost.

The first form of crucible steel was wootz, developed in India some time around 300 AD. In this system the iron was mixed with glass and then slowly heated and then cooled. As the mixture cooled the glass would bond to impurities in the steel and then float to the surface, leaving the steel considerably more pure. Carbon could enter the iron by diffusing in through the porous walls of the crucibles. Carbon dioxide would not react with the iron, but the small amounts of carbon monoxide could, adding carbon to the mix with some level of control. Wootz was widely exported throughout the Middle East, where it was combined with a local production technique around 1000 AD to produce Damascus steel, famed throughout the world.

As early as the 17th century, Europeans knew of India's ability to make crucible steel from reports brought back by travellers who had observed the process at several places in southern India. Several attempts were made to import the process, but failed because the exact technique remained a mystery. Studies of wootz were made in an attempt to understand its secrets, including a major effort by the famous scientist, Michael Faraday, son of a blacksmith. Working with a local cutlery manufacturer he wrongly concluded that it was the addition of aluminium oxide and silica from the glass that gave wootz its unique properties.

Nevertheless it was possible to produce quality steel in Europe, by importing the highly valued Swedish ore. Although it was not understood at the time, the Swedish ore contained very low levels of common impurities, leading to higher quality irons and steels from otherwise identical techniques applied to other ores. Swedish iron was packed into stone boxes in layers with charcoal in between them and heated in a furnace for an entire week. The result was an bar of metal known as blister steel - the surface of the bars became uneven from a multitude of blisters (or blebs) - which varied in quality from one bar to the next and within each bar. A number of blister rods were then wrapped into a larger bundle and re-heated and hammer-forged to mix together and even out the carbon content, resulting in the final product, shear steel. Germany was particularly well invested in this process, largely due to being physically close to Sweden, and became a major steel exporter in the 18th century. The technique was later improved as the cementation process.

Meanwhile a new technique was developed in England by Benjamin Huntsman, a clockmaker in search of a better steel for clock springs. It was only in 1740 after he moved to Handsworth near Sheffield, and after years of experimenting in secret he perfected his process. Huntsman's system used a coke-fired furnace capable of reaching 1600 °C, into which ten or twelve clay crucibles, each holding about 15 kg of iron, were placed. When the pots are at a white heat they are charged with blister steel broken into lumps of about ½ kg, and a flux to help remove impurities. The pots are removed after about 3 hours in the furnace, impurities skimmed off, and the molten steel poured into ingots. Sheffield's Abbeydale Industrial Hamlet has preserved a water-wheel powered, scythe-making works dating from Huntsman's times, which is still operated for the public, several times per year using crucible steel made on the Abbeydale site.

Before the introduction of Huntsman's technique, Sheffield produced about 200 tonnes of steel per year based on Swedish ore. The introduction of Huntsman's technique changed this radically; one hundred years later the amount had risen to over 80,000 tonnes per year - almost half of Europe's total production. This discovery enabled Sheffield to develop from a small township into one of Europe's leading industrial cities.

Another form of crucible steel was developed in 1837 by the Russian engineer, Pavel Anosov. His technique relied less on the heating and cooling, and more on the quenching process of rapidly cooling the molten steel when the right crystal structure had formed within. He called his steel bulat; its secret died with him.

Crucible steels remained the world's best, although very expensive, for some time. The introduction of the Bessemer process replaced it outright however, able to produce steel of similar (or better) quality for a fraction of the time and cost.