Morphological and Chemical Analyses of Manganese Dioxide Accretions on Mexican Ceramics
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Morphological and Chemical Analyses of Manganese Dioxide Accretions on Mexican Ceramics Caitlin R. O’Grady Department of Materials Science and Engineering, University of Arizona, Tucson, AZ, U.S.A. ABSTRACT Microscopic analysis of West Mexican ceramics (dating between in the collection of the Department of the Arts of Africa, Oceania and the Americas at the Metropolitan Museum of Art has established five different morphological types of manganese dioxide (MnO2) accretions. The accretions result mostly from a combination of bacterial and weathering activity in postdepositional contexts. Optical microscopy, scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDS) and x-ray fluorescence (XRF) were successful; whereas microchemical spot tests, x-ray diffraction (XRD) and Raman spectroscopy proved inconclusive in the identification and differentiation of the five morphologies. Accretion morphology and substrate characteristics, together, suggest scenarios for the mechanism of formation and offer a qualitative tool for authentication. Accretions were documented using photomicrography, categorized based on empirical observations; sampled and analyzed using SEM-EDS and NDXRF. INTRODUCTION During the past thirty years, articles have reported the presence of manganese dioxide accretions on ceramic and stone artifacts. Due to its distinctive physical appearance and assumed millennial long rate of formation, the presence of this surface phenomenon has been interpreted as an indication of authenticity on archaeological objects including West Mexican ceramics [1] and Egyptian stone artifacts [2]. The most controversial use of this surface phenomenon as a means of authentication is related to the Mansoor collection of Amarna period limestone sculptures. Despite being originally proclaimed genuine by the presence of rock varnish/manganese dendrites, the Mansoor collection is considered suspect by an overwhelming majority of art historians, conservators and archaeologists due to inconsistencies in style and the collection’s poor quality of manufacture [2]. The use of rock varnish and manganese dendrites as indicators of authenticity can be aided by study of a wide variety of samples. The presence of the accretions is not irrefutable evidence of authenticity. This paper aims at identifying the morphology and chemistry of manganese dioxide accretions; and testing these hypotheses through analysis of excavated samples. BACKGROUND Our knowledge of manganese dioxide accretions is derived primarily from the investigation of natural terrestrial weathering processes, referred to as rock varnish and manganese dendrites. Structural and chemical comparisons indicate that they all refer to the same surface phenomenon. However, the accretion’s genesis and rate of formation are not understood.
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Rock varnish has been recognized in the scientific literature since the 1800’s and has been known by a variety of terms including desert varnish [3], Wüstanlack, Schutzrinden and patina [4]. It is commonly described a
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