Precursors of the Blast
Excerpt from The Epic of Steel by Douglas Alan Fisher, 1963, Harper & Row, NY, pg. 26-29
While the iron industry languished over most of Europe in the early centuries of the Christian era, it kept on uninterruptedly in northern Spain where it had existed for hundreds of years. The metallurgical skill of the Catalonians was encouraged by the Visigoths who held sway over most of the peninsula from the fifth to the eighth centuries A.D., when their kingdom was destroyed by the Moors. The latter provided still greater incentive to the iron industry of Spain. During their dominion over that country the Catalan forge was developed.
The Catalan forge represented the first important metallurgical advance in iron smelting since classical times. The hearth was usually a slightly cup-shaped stone about thirty inches square, built up with stones at the front and on two sides to a height of three feet. Since the furnace was generally placed against a hillside the hill itself formed the back wall of the structure. A short distance above the hearth near the base of the front wall, was an opening for the admission of the nozzle, or tuyere, of the leather bellows. As furnaces grew a little taller and a stronger draft was needed, a flue was extended from the top of the furnace along the surface of the ground a short distance up the hill. The hearth was filled with charcoal to the level of the tuyere. On top of this layer, charcoal and iron ore were piled in two separate columns, the charcoal at the front of the furnace and the ore toward the back. A blast of air from the bellows caused the burning charcoal to give off hot carbon monoxide gas which combined with oxygen in the ore, reducing it to a pasty mass of iron, essentially free of slag. The lump of iron was removed from the hearth and hammered to compact the metal and to drive out any remaining cinders and slag. Afterwards it was beaten into bars which were marketed to the smiths. The furnaces preceding the Catalan forge were capable of producing only fifty pounds of iron at one time. The Catalan forge could yield 350 pounds of metal in a five-hour beat and for this reason assumed importance as a commercial producer.
In the eighth and ninth centuries iron smelters in Austria, Saxony, and along the Rhine, in an effort to increase the capacity of the Catalan forge, raised the height of the furnace shaft to ten feet and later to sixteen feet. This became known as the stückofen, or wolf furnace, because the large metallic mass extracted from it was called a stücke or wolf. Later, in France the lump of iron was called a loup and in England a bloom, the latter term being derived from the Anglo-Saxon bloma. The primitive English furnace took the name of bloomery. It never did evolve into the high bloomery or stückofen. Britain copied the blast furnace after Continental models, without going through the intermediate stages. All these taller furnaces in Europe, whatever their names, were essentially a Catalan forge extended upwards in a quadrangular or circular shaft.
The stückofen was the final development of furnaces in which malleable iron was produced directly from the ore. The stückofen yielded an iron mass weighing from 400 to 700 pounds-compared to 350 pounds in the Catalan forge-which was cut into two equal parts called stücke. Each half was divided into smaller sections which were worked on the forge into bars and other forms for the trade. The annual production of a stückofen was from 100 to 150 tons but it did not work the year round, generally being shut down during dry summer months.
In the earliest stückofen the air blast, was supplied by a pair of leather bellows operated by the feet or hands. Charcoal and ore were charged into the top of the shaft and were replenished from time to time as the smelting proceeded. By degrees the furnaces grew taller until the stack reached a height which a blast from humanly operated bellows was unable fully to penetrate. The solution to this problem was provided by the application of water power. It had been driving simple rotary mechanisms such as grain mills for centuries, but when it was applied to operate shafts and cams is not known. As early as the first decade of the thirteenth century, water power was driving bellows and hammers in the silver mines of the South Tyrol and soon spread to the other European ironmaking regions. The Cistercians, who played a prominent role in the erection of ironworks operated by water power on the Continent, probably introduced this productive device into England when they settled there and there is evidence that such a mill was built at a Cistercian abbey in Yorkshire about 1200. The water-driven bellows were heart-shaped and consisted of two wooden boards at the top and bottom, with collapsible sides and back made of ox or horsehide. The bellows at first were quite small-about five feet long and two and a half feet wide at the back end, the widest part. As the furnaces were built taller, the bellows grew in proportion in order to provide a blast powerful enough to reach the upper portions of the furnace stack. In England the largest bellows were twenty feet in length and four feet in width at the back. There were usually two bellows attached to a furnace, working alternately and thus supplying a steady blast of air. Water-driven bellows caused a relocation of the iron industry. Formerly the site of an iron-smelting furnace was determined primarily by the accessibility of wood for charcoal and frequently the furnace was built deep in the forest, often on the mountain slope. Water was needed for quenching and other purposes but not in sufficient volume to turn a water wheel. Now the iron smelters came down into the valleys and built their furnaces near the banks of swiftly running streams and rivers.
True Blast Furnace Is Born
The transition from the stückofen to the blast furnace was gradual. In the taller furnaces the iron ore remained exposed to the reducing action of charcoal for a longer period, and this, combined with higher temperatures from the water-driven blast, generally, but not always; caused some of the iron to melt and trickle from the bottom of the furnace, where it solidified. This iron, having absorbed enough carbon to transform it into cast iron, which is brittle and unworkable in the forge, was an annoyance to the smelter whose object was to produce low carbon wrought iron. As yet he had no use for cast iron and returned it to the furnace to be remelted. In the early part of the fourteenth century, a new term began to appear among iron smelters-flüssofen, that is, a flow oven, clearly indicating that it was capable of producing molten iron. It was also known in German as a hochofen and in French as a haut fourneau. The increasing appearance of molten iron running from the furnace presented the smelter with a problem. We are left to conjecture what may have passed through his mind. In the proportion that iron flowed from his furnace, the quantity of wrought iron which he obtained was lessened. At the same time, the return of the solidified iron to the furnace for remelting interfered with his operations as a producer of wrought iron. Bronze was then being cast in many forms. Among the chief-if not the chief-cast bronze products were church bells. The iron smelter was certainly familiar with the bronze foundry industry. What could have been more natural than for the producer of cast iron and the bronze foundryman to have been brought together? The circumstances under which this may have occurred are obscure, but it appears most likely that church bells were the first cast iron products extensively produced, followed by a much greater demand for cast iron cannon and cannon balls.
Alert to these new outlets for cast iron, more smelters adjusted their furnaces to produce the metal in a molten state and the true blast furnace came into being. During this transition period, some furnaces, notably the blauofen, could be worked to produce either wrought iron or cast iron, depending on the demand. Since the evolution of the blast furnace was gradual and was under way for some time, it is impossible to set a year for its first appearance. From the tenth century onward we hear of stückofen and flüssofen in various parts of Germany, and it is generally recognized that the blast furnace was brought to completion in the Rhine provinces with the French, Belgians, and Germans probably sharing honors in this great technological triumph. There is a record of a flüssofen in operation at Marche-les-Dames, Belgium, in 1340 and of hauts fourneaux existing near Liége in 1400. These furnaces were capable of producing molten iron. In 1409, there was a blast furnace in the valley of Massevaux, France, and it is claimed that there were many such furnaces in that country by 1450.
Water power was responsible for a second important technological advance in the iron industry: the introduction of a mechanical hammer.
Ever since man worked malleable metals, the smith wielded a hammer to refine and shape the hot plastic mass on the anvil. Hand forging required that the hot piece of metal be small enough for one man to manipulate Now the tilt hammer came to the aid of the smith. It was a long wooden arm with a head of iron weighing several hundred pounds. The axle shaft of the water wheel caused the arm to rise and fall rhythmically, beating the hot iron. The tilt hammer could perform the labor of twenty men and greatly increased productivity. It was used to do the first rough kneading of the large mass, while the smiths with their ringing hammers still did the finishing work on small sections. Tilt hammers were known to have existed in the iron-producing regions of Europe during the Middle Ages, but they did not appear in England until the late fifteenth and early sixteenth centuries.