Study of the atypical formations in the corrosion bulks of an ancient bronze shield, by optical and electron microscopy

Abstract

This article presents the atypical formations in the structure of the corrosion crust and in the partially mineralized metallic core, which resulted during the underground stay of a bronze shield, dated between the 1st century B.C. and the 1st century A.D. For our study, we choose a representative fragment from the rim of the shield, which was analyzed by optical microscopy and by electron microscopy coupled with energy-dispersive X-ray spectroscopy, to study its morphology, its composition, and the location of chemical compounds on the surface and inside the bulk formed during the underground stay, by processes of chemical and physical alteration, assisted by contamination with structural elements from the site. Those processes, by monolithization and mineralization formed a series of structures consisting of congruent elements and phases with a complex composition. Those formations, defined as surface effects generated by exogenous factors and endogenous factors inside the bulk, are frequently found in ancient bronze objects (such as the exterior flat mole formations and the Liesegang effect in the stratigraphic structure of the bulk). Some of those structures have atypical characteristics as regards their structure, composition, and formation mechanism, which may be used in archeometry. Moreover, that includes the object in the category of special cases, in terms of artifact evolution during underground stay and of the atypical formations resulted from the action of pedological and environmental factors.

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“…Corrosion on ancient objects is formed during the time of manufacture (primary patina), of usage in the atmosphere (secondary patina) and during the time of burial in soil or water (tertiary or contamination patina). Even (Vink, 1986;Domenech-Carbo et al, 2008;Pracejus, 2008;Sandu et al, 2008Sandu et al, , 2012Sandu et al, , 2014Domenech-Carbo et al, 2011;Mircea et al, 2012). Figure 4 shows the corrosion products of all objects with crossed nicols (polariser and analyser) polarised light (left) with reflected light (right).…”

New Interdisciplinary Aspects on Science for Conservation of Cultural Heritage-Ii

“…Responsible for the Liesegang effect are the hydrogells of the amphotere metals with coating capacity and the behavior of membrane permeability specific to some cations and anions. At the solution/gel transition these structures allow by osmosis the stratified recrystallization of some compounds like: cuprite, nantokite, malachite and azurite [26] [27] [28] [29], [30]. …”

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“…• chiselling and encrustation of the surface of the object [3,6]; • tinning (initially by rubbing with tin ores reduced with charcoal at temperatures much lower than the bronze melting temperatures, as later by immersion in molten tin) [3,[7][8][9][10]; • silvering and gilding by polishing [11][12][13][14].…”