Metallurgical Investigation of a Steel Miner’s Chisel From Ponte Val Gabbia III Site

Abstract

Recent research on ancient ferrous artefacts belonging to different historical periods has indicated significant differences in various parameters characterizing the slags entrapped in these artefacts, in cases where they were obtained by using different production methods. Such differences have been observed by comparing ‘‘ancient period’’ artefacts with ‘‘subsequent period’’ artefacts. ‘‘Ancient period’’ products were obtained by direct reduction of iron ore, without carburizing and at temperatures below the melting point of the reduced sponge iron. In the ‘‘subsequent period’’, the indirect process started to be introduced, with the production, in a first reduction stage, of a liquid cast iron that had to be converted to wrought iron during a second fining operation. The understanding of the characterizing parameters of the slags has in fact progressed to the point where they represent a useful tool not only for inferring the origins of the starting ore but also for distinguishing between direct or indirect production. In the present research work, an accurate study of the entrapped slags has been carried out on an artefact from the Val Gabbia III site, identified in previous studies as a miner’s chisel. This study aims to carry out further metallurgical investigation into the miner’s chisel microstructure and the entrapped slags in order to help ascertain which production method was in use at the Val Gabbia III site; in fact, based on the intrinsic characteristics of the chisel, and the fact that the site where it was found, i.e. layers of the V–VI cent AD in Val Gabbia III site, was characterized by the presence of an almost 3.5 kg cast iron block, previous investigators were led to suppose that it may be a very early site of indirect iron smelting. While the slag characterizing parameters obtained in the present investigation appear to be consistent with published results related to the direct method, the discussion on the relationship between the indirect method production effects on entrapped slag and the experimental findings substantiate, although not definitively, the hypothesis that the production method of the miner’s chisel is indirect.