authentication studies of metal artifacts: analysis and...

Authentication Studies of Metal Artifacts:Analysis and CharacterizationJoseph G. Barabe

Senior Research Microscopist McCrone Associates

Jude M. Runge, Ph.D.President

CompCote International, Inc.

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

Benin “bronze” head

• Provenance fairly good – major collector from major gallery

• Ancient ?– (16th – 19th century)

• Or modern forgery?

Benin “bronze” head

• Visual Examination results– Lost wax casting

• Uneven wall thickness

• Multiple seams

Benin “bronze” head

• Metallographic Examination– Many sample sites

internal to figure– Samples taken near

one ear and inside the bottom lip

Analysis of Corrosion Layer

Sampling flashing

Cuprite Corrosion Thickness

Brass matrix

Lead inclusion

Corrosion layer

Brass matrix

Lead inclusions

Corrosion layer

Analysis of cross-section

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.

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.

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.

Conclusions

• Identify the materials• Characterize the process

• Collaboration with experts in other fields when necessary

• Importance of identifying and working with talented consultants