Ancient Marble Sculpture: Geochemical Characterization of Surficial Weathering Products

Quantitative scientific determination of the authenticity and age of marble sculpture is an important goal of geo-archaeologists and conservation scientists. Geochemical and petrographic techniques are used here to investigate rock weathering and mineral alterations responsible for the “patina” and alteration layers on marble sculpture. We present oxygen and carbon isotopic, scanning electron microscopic and electron microprobe analyses of both fresh marble and weathering crusts materials from cores taken from Cycladic and Archaic Greek sculptures and from ancient quarries, to evaluate these techniques as indicators of antiquity. Calcitic marbles exhibit an altered weathering crust of variable thickness, where calcite has been recrystallized and interpenetrated with inclusions of iron oxide, clay minerals, gypsum and other authigenic minerals. The thickness and composition of these crusts varies with soil and water chemistry as well as marble density, texture and age. Microprobe analyses indicate trace element gradients from fresh to weathered calcite. Carbon and oxygen isotopes can differentiate between insitu alteration and precipitated carbonate. Dolomitic marbles can exhibit calcitic surficial layers formed by dedolomitization, which can be confirmed by isotopic and microprobe analyses. Analyses of known forgeries, ancient quarry samples and artificially weathered marbles have further documented our criteria and show that the majority of diagnostic geochemical and mineralogical features seen on ancient Greek sculptures cannot be accurately duplicated by artificial means.