Microstructural Analysis of A Steel Tool Rescued from Historical Site in Rio De Janeiro State
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MICROSTRUCTURAL ANALYSIS OF A STEEL TOOL RESCUED FROM HISTORICAL SITE IN RIO DE JANEIRO STATE Guadalupe do Nascimento Campos and Guillermo Solórzano Department of Materials Science and Metallurgy, PUC-Rio, Rio de Janeiro
ABSTRACT The present work has as its objective a metallographic analysis of a metallic archaeological artifact using methods of optical microscopy (OM) and scanning electron microscopy (SEM). The object of study is a scythe from the Brazilian colonial period, dated beginning of XVII century, removed from an archaeological site, Sítio Cruzeiro, in excavations conducted by researchers of the Brazilian Archaeological Institute. Sample preparation required a meticulous procedure in view of the fragility of the object. The scythe has been suffering the action of oxidation-corrosion for over 300 years, allowing the impregnation of mineral sediments. A detailed metallographic analysis coupled with spectroscopic SEM measurements concluded that of the object was made of steel with a carbon content of the order of 0.4%. The equi-axed microstructure together with the alignment of inclusions strongly suggest a metal working procedure followed by heat treatment and a slow cooling rate as the method used for fabricating the artifact.
INTRODUCTION Although archaeology is concerned with all material traces related with humankind, aiming to rescue their past, the analysis of metallic materials through metallographic procedures, with the objective of obtaining information regarding fabrication processes and the composition of these objects, has been rather limited. This aspect acquires particular relevance when such objects or artifacts are based on iron and has been exposed to oxidation and corrosion processes over the years. Due to the acquisition of a considerable heap of such objects, coming from different archaeological/historical sites1, an inter-disciplinary approach combining archaeological and metallurgical research has been undertaken. The site in question refers to “Sítio Cruzeiro", located in Santo Antônio da Jacutinga, Belford Roxo – state of Rio de Janeiro2, where the excavation was conducted in July 1985. The region is illustrated in Figure 1 which is a local cartographic map of the Brazilian colonial period. The site was dated to the beginning of XVII century by conventional archaeological procedures, and by stratigraphic observations of the ground, where the objects were found, and also through documents from the local church parish and from the National Library. The present study has as its goal to contribute to the knowledge about the metallurgical practices in Brazil in the colonial period3.
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Figure 1 – Cartographic localization of Sítio Cruzeiro. Original map of XVII century.
Due to the corrosion processes over the years, the integrity of the objects was severely modified. The object microstructure was expected to be seriously modified, mainly due to the transformation of the metal into iron oxide which requires in delicate handling during metallographic manipulation of the
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