authentication studies of metal artifacts: analysis and...
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Authentication Studies of Metal Artifacts:Analysis and CharacterizationJoseph G. Barabe
Senior Research Microscopist McCrone Associates
Jude M. Runge, Ph.D.President
CompCote International, Inc.
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Authentication studies of metal artifacts
• No standard dating methods• Alloy chemistry and metallography• Some sampling can be intrusive• Ideal non-destructive method is X-ray
fluorescence• Thermoluminescence sometimes
possible on sand core material• Outside expertise sometimes required
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Benin “bronze” head
• Provenance fairly good – major collector from major gallery
• Ancient ?– (16th – 19th century)
• Or modern forgery?
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Benin “bronze” head
• Visual Examination results– Lost wax casting
• Uneven wall thickness
• Multiple seams
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Benin “bronze” head
• Metallographic Examination– Many sample sites
internal to figure– Samples taken near
one ear and inside the bottom lip
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Analysis of Corrosion Layer
Sampling flashing
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Cuprite Corrosion Thickness
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Brass matrix
Lead inclusion
Corrosion layer
Brass matrix
Lead inclusions
Corrosion layer
Analysis of cross-section
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Metallographic Analysis Results• Flash samples always reflect alloy
segregation – rich in second and third phase constituents– Areas also effects of high temperature
rework• Examination disclosed thick continuous
oxide scale (14 – 62 microns)– Comprised of three (3) layers: dark gray;
mixed light gray, red; green-brown and orange
– Dezincification beneath the oxide layer with particulate oxidation in the interdendritic spaces
• EDS analysis determined the alloy composition to be brass rather than bronze (greater than 15% zinc)
• Oxide was comprised of copper oxide as tenorite; black, CuO, and cuprite; red, Cu2O.
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What the Results Mean• Lost wax process, cast in multiple
pours and slow-cooled.• Spherical oxides throughout
microstructure indicates pour was not degassed.
• Alloy Composition: – 65% Copper, 35% Zinc
• Oxide composition and stratification typical for slow cooled components: cuprite, red and yellow; tenorite, dark gray; zinc oxide, white.
• Component was held at or near the casting temperature for some time – supports multiple pour theory.
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Significance• Cuprite: Large crystals of cuprite that reflect
a good pleochroic red indicate corrosion crust has grown slowly, over longer periods of time.
• Depth and penetration of the crust is thick and uniform, requires time to form and grow.
• Uniform dezincification with transition to sound cast metal is consistent with age both due to environmental exposure and process quality.
• “Bronze” is brass! No evidence of recent manufacture, Alloy chemistry and cast technique establish that 17th to 19th century AD is a reasonable estimate historically and metallurgically.
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Conclusions
• Identify the materials• Characterize the process
• Collaboration with experts in other fields when necessary
• Importance of identifying and working with talented consultants