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420
BOOK IX.
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are folded together. Since this kind of bellows does not give a vigorous blast,
because they are drawn apart and compressed slowly, the smelter is not able during a whole day to smelt much more than half a centumpondium of tin. Very good iron ore is smelted55 in a furnace almost like the cupellation
furnace. The hearth is three and a half feet high, and five feet long and
wide ; in the centre of it is a crucible a foot deep and one and a half feet
wide, but it may be deeper or shallower, wider or narrower, according to whether
more or less ore is to be made into iron. A certain quantity of iron ore is
given to the master, out of which he may smelt either much or little iron.
He being about to expend his skill and labour on this matter, first throws
charcoal into the crucible, and sprinkles over it an iron shovel-ful of crushed
iron ore mixed with unslaked lime. Then he repeatedly throws on charcoal
and sprinkles it with ore, and continues this until he has slowly built up a
heap ; it melts when the charcoal has been kindled and the fire violently
stimulated by the blast of the bellows, which are skilfully fixed in a pipe.
5SIn the following descriptions of iron-smelting, we have three processes described ;
the first being the direct reduction of malleable iron from ore, the second the transition stage then in progress from the direct to indirect method by way of cast-iron ; and the third a method of making steel by cementation. The first method is that of primitive iron-workers of all times and all races, and requires little comment. A pasty mass was produced, which was subsequently hammered to make it exude the slag, the hammered mass being the ancient " bloom." The second process is of considerable interest, for it marks one of the earliest descriptions of working iron in " a furnace similar to a blast furnace, but much wider " and higher." This original German Stückofen or high bloomery furnace was used for making " masses " of wrought-iron under essentially the same conditions as its progenitor the forge— only upon a larger scale. With high temperatures, however, such a furnace would, if desired, yield molten metal, and thus the step to cast-iron as a preliminary to wrought-iron became very easy and natural, in fact Agricola mentions above that if the iron is left to settle in the furnace it becomes hard. The making of malleable iron by subsequent treatment of the castiron—the indirect method—originated in about Agricola's time, and marks the beginning of one of those subtle economic currents destined to have the widest bearing upon civilization. It is to us uncertain whether he really understood the double treatment or not. In the above paragraph he says from ore " once or twice smelted they make iron," etc., and in De Natura Fossilium (p. 339) some reference is made to pouring melted iron, all of which would appear to be cast-iron. He does not, however, describe the 16th Century method of converting cast into wrought iron by way of in effect roasting the pig iron to eliminate carbon by oxidation, with subsequent melting into a " ball " or " mass." It must be borne in mind that puddling for this purpose did not come into use until the end of the 18th Century. A great deal of discussion has arisen as to where and at what time cast-iron was made systematically, but without satisfactory answer ; in any event, it seems to have been in about the end of the 14th Century, as cast cannon began to appeal about that time. It is our impression that the whole of this discussion on iron in De Re Metallica is an abstract from Biringuccio, who wrote 15 years earlier, as it is in so nearly identical terms. Those interested will find a translation of Biringuccio's statement with regard to steel in Percy's Metallurgy of Iron and Steel, London, 1864, p. 807. Historical Note on Iron Smelting. The archaeologists' division of the history
of racial development into the Stone, Bronze, and Iron Ages, based upon objects found in tumuli, burial places, etc., would on the face of it indicate the prior discovery of copper metallurgy over iron, and it is generally so maintained by those scientists. The metallurgists have not hesitated to protest that while this distinction of " Ages " may serve the archaeologists, and no doubt represents the sequence in which the metal objects are found, yet it by no means follows that this was the order of their discovery or use, but that iron by its rapidity of oxidation has simply not been preserved. The arguments which may be advanced from our side are in the main these. Iron ore is of more frequent occurrence than copper ores, and the necessary reduction of copper oxides (as most surface ores must have been) to fluid metal requires a temperature very much higher than does the reduction of iron oxides to wrought-iron blooms, which do not necessitate fusion. The comparatively greater simplicity of iron metallurgy under primitive conditions is well exemplified by the hill tribes of Northern Nigeria, where in village forges the negroes reduce iron |
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