At this time Sir Henry had no connection with the iron or steel trade, and knew little or nothing of metallurgy. But this fact he has always represented as being rather an advantage than a drawback. "I find," he says, "in my experience with regard to inventions, that the most intelligent manufacturers invent many small improvements in various departments of their manufactures,—but, generally speaking, these are only small ameliorations based on the nature of the operation they are daily pursuing; while, on the contrary, persons wholly unconnected with any particular business have their minds so free and untrammelled to new things as they are, and as they would present themselves to an independent observer, that they are the men who eventually produce the greatest changes." It was in this spirit that he began his investigations in metallurgy. His first business was to make himself acquainted with the information contained in the best works then published on the subject. He also endeavored to add some practical knowledge to what he learned from books. With this view he visited the iron-making districts in the north, and there obtained an insight into the working merits and defects of the processes then in use. On his return to London he arranged for the use of an old factory in St. Pancras, where he began his own series of experiments. He converted the factory into a small experimental "iron-works," in which his first object was to improve the quality of iron. For this purpose he made many costly experiments without the desired measure of success, but not without making some progress in the right direction. After twelve months spent in these experiments he produced an improved quality of cast iron, which was almost as white as steel, and was both tougher and stronger than the best cast iron then used for ordnance. Of this metal he cast a small model gun, which was turned and bored. This gun he took to Paris, and presented it personally to the Emperor,[23] as the result of his labors thus far. His Majesty encouraged him to continue his experiments, and desired to be further informed of the results.
As Sir Henry continued his labors, he extended their scope from the production of refined iron to that of steel; and in order to protect himself, he took out a patent for each successive improvement. One idea after another was put to the test of experiment; one furnace after another was pulled down, and numerous mechanical appliances were designed and tried in practice. During these experiments he specified a multitude of improvements in the crucible process of making steel; but he still felt that much remained to be done. At the end of eighteen months, he says, "the idea struck me" of rendering cast iron malleable by the introduction of atmospheric air into the fluid metal. His first experiment to test this idea was made in a crucible in the laboratory. He there found that by blowing air into the molten metal in the crucible, by means of a movable blow-pipe, he could convert ten pounds or twelve pounds of crude iron into the softest malleable iron. The samples thus produced were so satisfactory in all their mechanical tests that he brought them under the notice of Colonel Eardley Wilmot, then the Superintendent of the Royal Gun Factories, who expressed himself delighted and astonished at the result, and who offered him facilities for experimenting in Woolwich Arsenal. These facilities were extended to him in the laboratory by Professor Abel, who made numberless analyses of the material as he advanced with his experiments. The testing department was also put at his disposal, for testing the tensile strength and elasticity of different samples of soft malleable iron and steel. The first piece that was rolled at Woolwich was preserved by Sir Henry as a memento. It was a small bar of metal, about a foot long and an inch wide, and was converted from a state of pig iron in a crucible of only ten pounds. That small piece of bar, after being rolled, was tried, to see how far it was capable of welding; and he was surprised to see how easily it answered the severest tests. After this he commenced experiments on a larger scale. He had proved in the laboratory that the principle of purifying pig iron by atmospheric air was possible; but he feared, from what he knew of iron metallurgy, that as he approached the condition of pure soft malleable iron, he must of necessity require a temperature that he could not hope to attain under these conditions. In order to produce larger quantities of metal in this way, one of his first ideas was to apply the air to the molten iron in crucibles; and accordingly, in October, 1855, he took out a patent embodying this idea. He proposed to erect a large circular furnace, with openings for the reception of melting-pots containing fluid iron, and pipes were made to conduct air into the centre of each pot, and to force it among the particles of metal. Having thus tested the purifying effect of cold air introduced into the melting iron in pots, he labored for three months in trying to overcome the mechanical difficulties experienced in this complicated arrangement. He wondered whether it would not be possible to dispense with the pipes and pots, and perform the whole operation in one large circular or egg-shaped vessel. The difficult thing in doing so, was to force the air all through the mass of liquid metal. While this difficulty was revolving in his mind, the labor and anxiety entailed by previous experiments brought on a short but severe illness; and while he was lying in bed, pondering for hours upon the prospects of succeeding in another experiment with the pipes and pots, it occurred to him that the difficulty might be got over by introducing air into a large vessel from below into the molten mass within.
Though he entertained grave doubts as to the practicability of carrying out this idea, chiefly owing to the high temperature required to maintain the iron in a state of fluidity while the impurities were being burned out, he determined to put it to a working test; and on recovering health he immediately began to design apparatus for this purpose. He constructed a circular vessel, measuring three feet in diameter and five feet in height, and capable of holding seven hundred-weight of iron. He next ordered a small, powerful air-engine and a quantity of crude iron to be put down on the premises in St. Pancras, that he had hired for carrying on his experiments. The name of these premises was Baxter House, formerly the residence of old Richard Baxter; and the simple experiment we are now going to describe has made that house more famous than ever. The primitive apparatus being ready, the engine was made to force streams of air, under high pressure, through the bottom of the vessel, which was lined with fire-clay; and the stoker was told to pour the metal, when it was sufficiently melted, in at the top of it. A cast-iron plate—one of those lids which commonly cover the coal-holes in the pavement—was hung over the converter; and all being got ready, the stoker in some bewilderment poured in the metal. Instantly out came a volcanic eruption of such dazzling coruscations as had never been seen before. The dangling pot-lid dissolved in the gleaming volume of flame, and the chain by which it hung grew red and then white, as the various stages of the process were unfolded to the gaze of the wondering spectators. The air-cock to regulate the blast was beside the converting-vessel; but no one dared to go near it, much less deliberately to shut it. In this dilemma, however, they were soon relieved by finding that the process of decarburization or combustion had expended all its fury; and, most wonderful of all, the result was steel! The new metal was tried. Its quality was good. The problem was solved. The new process appeared successful. The inventor was elated, as well he might be!
The new process was received with astonishment by all the iron-working world. It was approved by many, but scoffed at by others. As trials went on, however, the feeling against it increased. The iron so made was often "rotten," and no one could tell exactly why.
Bessemer, however, continued to investigate everything for himself, regardless of all suggestions. Some ideas of permanent value were offered to him, but were set at nought. It was not till another series of independent experiments were made that he himself discovered the secret of failure. It then appeared that, by mere chance, the iron used in his first experiments was Blaenavon pig, which is exceptionally free from phosphorus; and consequently, when other sorts of iron were thrown at random into the converter, the phosphorus manifested its refractory nature in the unworkable character of the metal produced. Analyses made by Professor Abel for Sir Henry showed that this was the real cause of failure. Once convinced of this fact, Sir Henry set to work for the purpose of removing this hostile element. He saw how phosphorus was removed in the puddling-furnace, and he now tried to do the same thing in his converter. Another series of costly and laborious experiments was conducted; and first one patent and then another was taken out, tried, and abandoned. His last idea was to make a vessel in which the converting process did not take place, but into which he could put the pig iron as soon as it was melted, along with the same kind of materials that were used in the puddling-furnace. He was then of opinion that he must come as near to puddling as possible, in order to get the phosphorus out of the iron. Just as he was preparing to put this plan into operation, there arrived in England some pig iron which he had ordered from Sweden some months previously. When this iron, which was free from phosphorus, was put into the converter, it yielded, in the very first experiment, a metal of so high a quality that he at once abandoned his efforts to dephosphorize ordinary iron. The Sheffield manufacturers were then selling steel at £60 a ton; and he thought that as he could buy pig iron at £7 a ton, and by blowing it a few minutes in the converter could make it into what was being sold at such a high price, the problem was solved.
But there was yet one thing wanting. He had now succeeded in producing the purest malleable iron ever made, and that, too, by a quicker and less expensive process than was ever known before. But what he wanted was to make steel. The former is iron in its greatest possible purity; the latter is pure iron containing a small percentage of carbon to harden it. There has been an almost endless controversy in trying to make a definition that will fix the dividing line that separates the one metal from the other.[24]
For our present purpose, suffice it to quote the account given in a popular treatise on metallurgy, published at the time when Bessemer was in the midst of his experiments. "Wrought iron," it says, "or soft iron, may contain no carbon; and if perfectly pure, would contain none, nor indeed any other impurity. This is a state to be desired and aimed at, but it has never yet been perfectly attained in practice. The best as well as the commonest foreign irons always contain more or less carbon.... Carbon may exist in iron in the ratio of 65 parts to 10,000 without assuming the properties of steel. If the proportion be greater than that, and anywhere between the limits of 65 parts of carbon to 10,000 parts of iron and 2 parts of carbon to 100 of iron, the alloy assumes the properties of steel. In cast iron the carbon exceeds 2 per cent, but in appearance and properties it differs widely from the hardest steel. These properties, although we quote them, are somewhat doubtful; and the chemical constitution of these three substances may, perhaps, be regarded as still undetermined." Now, in the Bessemer converter the carbon was almost entirely consumed. In the small gun just described,[25] there were only 14 parts of carbon for 1,000,000 parts of iron. Bessemer's next difficulty was to carburize his pure iron, and thus to make it into steel. "The wrought iron," says Mr. I. L. Bell, "as well as the steel made according to Sir Henry Bessemer's original plan, though a purer specimen of metal was never heard of except in the laboratory, was simply worthless. In this difficulty, a ray of scientific truth, brought to light one hundred years before, came to the rescue. Bergmann was one of the earliest philosophers who discarded all theory, and introduced into chemistry that process of analysis which is the indispensable antecedent of scientific system. This Swedish experimenter had ascertained the existence of manganese in the iron of that country, and connected its presence with suitability for steel purposes." Manganese is a kind of iron exceptionally rich in carbon, and also exceptionally free from other impurities. Berzelius, Rinman, Karsten, Berthier, and other metallurgists had before now discussed its effect when combined with ordinary iron; and the French were so well aware that ferro-manganese ores were superior for steel-making purposes that they gave them the name of mines d'acier. So Bessemer, after many experiments, discovered a method whereby, with the use of ferro-manganese, he could make what is known as mild steel. The process of manufacture, when described by Sir Henry Bessemer at Cheltenham in 1856,[26] was so nearly complete, that only two important additions were made afterwards. One was the introduction of the ferro-manganese for the purpose of imparting to his pure liquid iron the properties of "mild steel." The other was an improvement in the mechanical apparatus. He found that when the air had been blown into the iron till all the carbon was expelled, the continuance of "the blow" afterward consumed the iron at a very rapid rate, and a great loss of iron thus took place. It was therefore necessary to cease blowing at a particular moment. At first he saw no practical way by which he could prevent the metal going into the air-holes in the bottom of the vessel below the level of the liquid mass, so as to stop them up immediately on ceasing to force the air through them; for if he withdrew the pressure of air, the whole apparatus would be destroyed for a time. Here, again, his inventive genius found a remedy. He had the converter holding the molten iron mounted on an axis, which enabled him at any moment he liked to turn it round and to bring the holes above the level of the metal; whenever this was done the process of conversion or combustion ceased of itself, and the apparatus had only to be turned back again in order to resume the operation. This turning on an axis of a furnace weighing eleven tons, and containing five tons of liquid metal, at a temperature scarcely approachable, was a system entirely different from anything that had preceded it; for it he took out what he considered one of his most important patents, "and," he says, "I am vain enough to believe that so long as my process lasts, the motion of the vessel containing the fluid on its axis will be retained as an absolute necessity for any form which the process may take at any future time." The patent for this invention was taken out about four years after his original patent for the converter.
Uncle Fritz showed them a picture of this gigantic kettle, which holds this mass of molten metal and yet turns so easily.
"But," said Helen, "you have a model of it here, Uncle Fritz." And she pointed to her Uncle Fritz's inkstand, which is something the shape of a fat beet-root, with the point turned up to receive the ink. Uncle Fritz nodded his approval. These inkstands, which turn over on a little brazen axis, were probably first made by some one who had seen the great eleven-ton converters.
Uncle Fritz showed the children the picture in the "Practical Magazine," and they spent some time together in looking over the pages of the volume for 1876.