Mission impossible achieved
Sir Henry Bessemer, who died 100 years ago next week, was one of the architects of the modern age. His invention of a process to convert iron into steel in large quantities economically gave engineers a new material of construction, whose effects were felt across industry and, indeed, society.
It made possible the construction of everything from bigger boilers to power larger steamships, structures like the Forth Bridge or the now ubiquitous tower block, and long-lasting rails for the rapidly expanding railway network. In this century, steel became the staple material for the car industry.
Back in the 1850s the engineer's choice was limited to brittle pig iron, while bridges and ships used malleable or wrought iron. Tools and cutlery were made from steel, but it was produced by methods which were not only costly but also incapable of being expanded to large-scale production.
Bessemer's interest was triggered by the need for stronger materials for ordnance. He was working on a rotating projectile intended to do away with the need for rifled grooves in the gun barrel. But he was unable to find cast iron strong enough for his needs. Rather than wait for metallurgy to catch up, he took matters into his own hands.
Though lacking scientific training, Bessemer was something of an inventor. At an early age he had devised a machine for tearing up gold foil into flakes for use in gold paint. In his obituary, The Engineer notes that the secret of his machine had probably gone with Bessemer to his grave: 'The machine described in his patent,' it said, 'will not work'.
He had also showed a model of a new design for a railway axle at the second meeting of the Institution of Mechanical Engineers in April 1847, but the meeting was unimpressed with the merits of the invention and he did not become a member.
In fact, said The Engineer's obituary in March 1898: 'his total lack of systematic training at once made him and marred him'.
A great deal of luck was inherent in Bessemer's discovery in the first place, and when difficulties arose in making the process repeatable, Bessemer adopted a rather hit-and-miss approach to fixing the problem.
At the same time, conventional scientific wisdom would have inhibited a trained scientist from pursuing the direction Bessemer took. 'He was carefully assured by those who were supposed to know all about steelmaking that the process was totally impossible,' said The Engineer.
The route he took was described, after 18 months of experiments, to a meeting of the British Association for the Advancement of Science on 11 August 1856 at Cheltenham, with the startling title of The Manufacture of Iron without Fuel. Bessemer discovered that blowing a charge of cold air through molten iron caused the material to heat up further and turn the hard brittle material into a ductile and malleable one.
Reporting on the announcement that week, The Engineer said that it 'promises completely to revolutionise the iron manufacture as at present conducted'.
In his Bessemer centenary lecture, reported in The Engineer in May 1956, James Mitchell says Bessemer's paper was also 'a very fine effort in salesmanship'. Within weeks of the meeting five firms paid a total of £27,000 for licences to make iron by the new process, and 'there was at least one other who attempted to operate the process without this formality'.
But success turned immediately to failure: by Bessemer's own admission, 'the results of the trials were most disastrous'. The brittle metal produced bore no resemblance to the sample bars Bessemer had produced in his own trials.
What had not been appreciated was the effect of impurities in the iron. Without modern analytical techniques, it took two years before it became clear that phosphorus was the key to the apparent failure of the process. By chance, the pig iron supplied to Bessemer for his original experiments happened to be low in this substance.
Bessemer embarked on a series of unsuccessful experiments aimed at removing the phosphorus. At one point, he imported some Swedish iron known to be low in sulphur and phosphorus, to reassure himself the process would work, and succeeded in turning it into pure malleable iron, and also steel essentially an alloy of iron and less than 2% carbon in various degrees of hardness.
This is thought to be the first time he foresaw the possibility of making a range of steels using the process: but this raised another problem. The effect of the blast of air in the Bessemer process is to burn the carbon off: the question now arose as to how to stop the blast of air when the right carbon content had been reached.
As The Engineer said in 1898, 'it was found, indeed, that steel could be made, but only with utter uncertainty as to the quality of the product'.
Here two other figures come into the picture. Goran Frederick Goransson had bought a licence to use the Bessemer process for Sweden in June 1857. He was able to determine the approximate carbon content by forging droplets of steel from the Bessemer converter. This method sufficed until the problem was solved in 1861 by the invention of the Eggertz carbon.
There was one further important ingredient, however, before steel could be made from British pig iron.
Within a few weeks of Bessemer's British Association lecture, Robert Mushet succeeded in making ductile iron by remelting it with spiegel, an ore of manganese. The addition of small quantities of manganese was the solution Bessemer had been searching for. (Goransson's steel had been naturally high in manganese and did not need additions.)
Thus, in 1859, Bessemer was ready to reintroduce his process. But though the unhappy licensees had been reimbursed, he had to overcome the credibility problem engendered by the earlier failure, and recruitment of licensees was slow. None of the Sheffield steelmakers would adopt the process except on exclusive terms, which Bessemer rejected. Instead, he set up his own successful steelworks in Sheffield, which was soon producing a million tons a year, accelerating the adoption of the process by others.
Nor was the engineering world always quick to take advantage of the new process. Boilermakers and shipbuilders took up the new material in place of wrought iron fairly quickly between 1859 and 1863; and the use of steel for railways rails was first recorded in 1862. But the War Office and the Admiralty were reluctant to take up the new material.
Bessemer was an innovator to the last, amassing scores of patents for devices from sugar cane crushing machinery to telescopes.
Not all his inventions were successful though. The Bessemer Channel steamer had an unusual arrangement of paddles, two fore and two aft. More remarkable was its saloon, mounted on gimbals and with a two-ton flywheel. The gyroscopic action of the rotating flywheel was supposed to stabilise the saloon and prevent seasickness. But it was no match for an English Channel gale. The ship itself was 'slow and unhandy' and collided with Calais Pier on her first trip.
Bessemer kept his health until the end, when he was taken ill, appeared to recover, then three weeks later collapsed and died suddenly, aged 85.
As The Engineer's obituary remarks, 'it is nearly certain that he never mastered the chemistry of his steelmaking process'. But while he lacked proportion and was incapable of evaluating the factors that led to the success or failure of an invention, Bessemer 'seemed to possess some special power of making things succeed which ought to have failed. Of course he committed a multitude of mistakes, but they were all swallowed up in his successes.'