A Ghost Story: Remnant Structures in Corroded Ancient Iron Objects
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A Ghost Story: Remnant Structures in Corroded Ancient Iron Objects Michael R.Notis Archaeometallurgy Laboratory Department of Materials Science & Engineering Lehigh University Bethlehem, PA 18015, USA ABSTRACT The applicability of a broad spectrum of laboratory analytical methods to the study of remnant structures still observable in the corrosion products of ancient iron objects is discussed. These methods range from examination by light optical microscopy and SEM to direct microanalysis using x-ray mapping in EPMA. Samples ranging from low carbon iron to steels to cast iron have been examined and it has been found possible to observe remnant structures and infer fabrication information from the large majority of objects studied. INTRODUCTION A number of years ago, at the first MRS Symposium on Materials Issues in Art and Archaeology, I reported on the general aspects of the use of electron optical and x-ray analytical methods as applied to archaeometallurgy, and how this suite of experimental methods could be used together with an understanding of materials science in order to characterize metal artifacts and to provide insight into ancient fabrication methods [1]. In this present paper I would like to demonstrate the use of these methods, in a more focused way, in order to examine the structural and micro-chemical details of the prior metal structure remaining in the corrosion products of ferrous materials. BACKGROUND Plain carbon ferrous alloys may be classified according to their carbon content as steels, with less than about 2 weight percent carbon, or as cast irons, with carbon greater than about 2 weight percent. When steels are slow cooled from the higher temperature face-centered cubic solid phase (austenite) they undergo a reaction and transform to body-centered low carbon iron (ferrite) and iron carbide (cementite, Fe3C). For a steel with composition at about 0.8 weight percent carbon ("1080 steel"), ferrite and cementite form cooperatively and simultaneously in the form of a lamellar structure called pearlite. For steels with lower carbon content ferrite forms first and then the balance transforms to pearlite; and for carbon contents higher than this, cementite forms first and then the balance forms pearlite. With prolonged heating the lamellar pearlite and the cementite spheroidize and thus give an indication of this type of heat treatment. If these same steels are rapidly quenched from austenite they form a new high hardness single phase structure, martensite. Cast irons, depending on the presence of impurities such as Si, P, Mn and S, when cooled from liquid iron, may form an eutectic between austenite and graphite ( producing "grey cast iron") or between austenite and cementite (producing "white cast iron"); the austenite then transforms as described above. Thus, in both cases, the microstructure of
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steels and cast irons are strongly dependent upon carbon content, the presence of other impurities, and the cooling conditions. An excellent review of the corrosion behavior of ferrou
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