Weathering of a Metal Artifact in a Saharan Environment: Evidence for a Novel Form of Desert Varnish

摘要

Desert varnish is a term commonly used in reference to a natural coating which often forms on rock surfaces in arid regions worldwide. Varnish coatings consist largely of clay minerals, oxides and hydroxides of manganese and/or iron, plus minor amounts of organic matter and other materials derived from air-borne dust and other sources external to substrate. A dark-brown coating on the exterior of a World War II metal fuel can recovered in southwestern Egypt may be a variation of the desert varnish that develops on rock surfaces in arid regions. This suggestion is supported by results of infrared spectroscopic work, x-ray diffraction, scanning electron microscopic studies, and x-ray fluorescence. Exterior portions of the can resemble heavily varnished rock, differing completely in appearance and texture from rusty areas which had developed on portions of the interior surface. The coating consists principally of microscopic mineral deposits, some of which appear to be largely Fe-oxides and Fe-oxy-hydroxides. However many deposits are assemblages of Fe-oxides, clay minerals, and other elements typically occurring in forms of desert varnish often observed on the undersides of desert pavement stones in contact with the soil. Associated with the mineral matter is an organic component apparently deriving from residues of an original paint layer which had been removed by wind/sand scouring. Index headings: Desert varnish; Weathering of metal; World War II artifact; Infrared; X-ray diffraction; X-ray fluorescence; Reprints. (aw)