Granulating Iron in Filarete's Smelter

Abstract

Historians of metallurgy will be greatly indebted to Professor Spencer for his excellent translation of Filarete's interesting treatise. There are a number of points which seem to differ from later European practice, and these particularly should be searched for hints as to the conditions surrounding the beginning of cast iron production in Europe. Professor Spencer and others have assumed that Filarete's description of the water-pit is a garbled description of some tempering or slag disposal process. It seems to me, however, that it should be taken literally. If one makes the assumption that the earliest blast furnaces were simply precursors of later ones casting heavy sows to be carried to the finery, there is indeed a difficulty in explaining the need for water. Its presence near the pig-bed would even be somewhat dangerous. However, an alternative interpretation would be that the metal was being poured into a water-pit in order to produce granules. After all, the main purpose of the operation at that time was to obtain material for subsequent processing into wrought iron. Would it not be more natural to give the product of the blast furnace a form similar to the metal which the iron smelter had regularly been obtaining by the direct reduction of iron ore in a hearth-in other words, granular lumps of iron weighing a few grams or at most a few hundred grams? Compared to a heavy cast pig, this product would be more easily transported to the hearth and more readily handled when in it. Still more important, it would come to nature more quickly because of the larger surface area that would be exposed to the blast before melting. Filarete may, therefore, be simply describing a granulation process which was in use as a logical intermediate stage between the old and new refining schemes. It need not have lasted long, for the preference for granules over sows would largely disappear when the scale of operation had increased and mechanical handling became essential.