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Table 10: Scientific analytical techniques used in the study of bronze sculpture

Table 10
Table 10
Scientific analytical techniques used in the study of bronze sculpture
    Method of analysis Materials Sampling requirements and preparation Advantages Drawbacks and limitations See
Microchemical testing Minimally invasive techniques Microchemical/solvent tests
  • Core: elemental composition, testing for the presence of carbonates and gypsum
  • Alloy
  • Repairs
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
 Tests can often be performed under the microscope using very small samples
  • Economical, rapid, very precise way to qualitatively determine the presence of specific elements or substances
  • Quantitative analysis is possible in specialized laboratories
  • The range of compounds that can be identified is limited
  • In many cases, tests can only confirm the presence or absence of a given material or element
  • Only those elements are found that are specifically looked for
  • Can be misleading, as it may indicate the presence of a substance whose quantity may be relatively minor, or even an incidental or accidental inclusion
  • Detection limits are rarely known
  • Best used in conjunction with other methods
 II.6§3.4
Elemental anlaysis Non- or minimally invasive techniques X-ray fluorescence analysis (XRF)
  • Core
  • Alloy
  • Repairs
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
  • Useful information can be obtained without sampling
  • Quantitative analysis of alloys may require polishing a spot (2–10 mm diameter) to remove corrosion and expose base metal
  • Rapid, multi-element characterization
  • Inexpensive
  • Many analyses can be made over large areas
  • Quantitative alloy analysis is possible
  • Elemental mapping possible
  • Rigorous quantitative analysis is difficult
  • Geometry can affect readings (flat surfaces are ideal; curves can create issues)
  • Not effective for light elements such as carbon, oxygen, and sodium
  • Only a thin surface layer is analyzed for most materials
II.5§2.1, II.6§2.2, II.7§2.1.4
Particle induced X-ray emission (PIXE) and particle-induced gamma-ray emission (PIGE)
  • Core
  • Alloy
  • Repairs
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
  • Useful information can be obtained without sampling
  • Quantitative analysis of alloys may require polishing a spot (0.5–5 mm diameter) to remove corrosion and expose base metal
Relatively high spatial resolution (50 µm) enables mapping of complex patina or gilding layers as well as specific surface features (repairs, assemblies, inlays, overlays)
  • Expensive
  • Rarely available as a service analysis; limited to a few laboratories
  • Most laboratories cannot analyze objects larger than a few cm high
  • Only a very thin surface layer is analyzed for most materials
II.5§2.2, II.6§2.2, II.7§2.1.5
Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA)
  • Patina
  • Gilding and other metal plating
 Useful information can be obtained without sampling Depth profiling possible (analysis of different strata for patina and plating)
  • Expensive, not widely available (few laboratories can do such analysis)
  • Not suitable for complex layered stuctures
  • Needs to be combined with other techniques
Note: for NRA there is no issue of decontamination, as there is with neutron analysis 		II.6§2.3
Laser-induced breakdown spectroscopy (LIBS)
  • Core
  • Alloy
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
 Useful information can be obtained without sampling
  • Rapid, multi-element characterization
  • Inexpensive
  • Able to analyze light elements
  • Depth profiling is possible (analysis of different strata)
  • Minimally destructive; creates many small ablation craters (<1 mm diameter)
  • Small analysis spot may yield unrepresentative results
  • Not widely accepted as a method for rigorous quantitative analysis
II.5, II.6§2.3
Laser ablation inductively coupled plasma – mass spectrometry (LA-ICP-MS)
  • Core
  • Alloy
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
 Sampling may be necessary if object is not small (see “atomic spectroscopies” below)
  • Rapid, multi-element characterization
  • Able to analyze light elements
  • Depth profiling is possible (analysis of different strata)
  • Minimally destructive; creates many small ablation craters (<<1 mm diameter)
  • Small analysis spot may yield unrepresentative results
  • Quantitative analysis of copper alloys is difficult due to matrix effects
  • Only for small sculptures if sampling is not possible (although specific extensions are under development that allow analysis of large sculptures)
II.6§2.3
Neutron diffraction
  • Core
  • Alloy
 No sampling required
  • Able to measure the composition of the metal throughout the thickness of the metal wall
  • Possible to detect evidence of cold working
  • Highly specialized technique requiring large-scale research facilities, typically nuclear reactors
  • Only certain elements can be detected and quantified using neutron diffraction
  • The error of measurement is relatively high compared to other techniques
  • Some objects may need to be quarantined for decontamination (de-activation) at the neutron facility for anywhere from several minutes to several days before returning to the owner depending on a number of parameters, including neutron flux and energy, alloy composition, etc., which can create temporary radioactivity of the material
II.5§2.3.3
Invasive techniques Scanning electron microscopy (SEM-EDS or SEM-WDS)
  • Core
  • Alloy
  • Repairs
  • Patina
  • Inlays and overlays
  • Gilding and other metal plating
 Small samples, <1 mm2, usually embedded in cross section and carefully polished
  • Best spatial resolution of all techniques (beam spot size of a few nm)
  • Enables precise mapping of complex patina or gilding layers
Expensive II.5§3.4, II.6§3.3, II.7§2.1.3
Atomic spectroscopies: atomic absorption spectrometry (AAS) and inductively coupled plasma – atomic emission spectroscopy (ICP-AES)
  • Core
  • Alloy
  • 50–100mg of drillings (or pieces) for cores
  • approx. 20 mg of clean metal drillings (or pieces) for alloys
  • High sensitivity for trace element analysis
  • Much more sensitive than XRF or SEM-EDS
 More laborious, more expensive, and less easily accessible than EDXRF II.5§3.1
Inductively coupled plasma – mass spectrometry (ICP-MS)
  • Core
  • Alloy
  • 50–100mg of drillings (or pieces) for cores
  • approx. 20 mg of clean metal drillings (or pieces) for alloys
The most sensitive technique for elemental analysis (approx. 100 times more sensitive than ICP-AES)
  • The most laborious of the invasive spectroscopic techniques
  • More expensive and less accessible than atomic spectroscopies (including ICP-AES), although in the 2010s it has greatly evolved and is slowly replacing atomic spectroscopies
II.5§3.2, II.7§2.1.1
Neutron activation analysis (NAA)
  • Core
  • Alloy
 50–100 mg of sample material (alloy or core) Relatively good sensitivity and accuracy on a large range of elements
  • Expensive, not available as a service analysis for bronzes
  • Some objects may need to be quarantined for decontamination (see "neutron diffraction" above)
II.5§3.3, II.7§2.1.2
Isotope analysis (mainly Pb isotopes) Invasive technique Multi-collector – inductively coupled plasma – mass spectrometry (MC-ICP-MS)
  • Alloy
  • Metallic repairs
  • Metallic inlays and overlays
 Small sample of material (approx. 20 mg for alloys) Useful for provenancing of metals
  • Relatively expensive
  • Requires a database and a good knowledge of the prehistoric/historic context to be useful
  • Lead isotopes have limited utility for intentionally leaded alloys
II.5§5
Structural analysis* Invasive techniques Optical microscopy (metallography) Metal Polished samples of a few mm2 to 1 cm2 are usually required for the study of microstructure
  • Microstructure can be used to distinguish as-cast metal, cold-worked metal, and annealed metal
  • Can help to identify joinery techniques such as welding and brazing
  • Can be used to characterize corrosion and patina
Large sample size II.5§6
Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy
  • Organic and inorganic fills for repair
  • Organic and inorganic patina
  • Organic and inorganic inlays and overlays
  • Samples <<1 mm2 required
  • Microscopic samples suffice if FTIR microscopy is available
Rapid and easily accessible; most laboratories have this technique (at least FTIR)
  • Spectra require interpretation by a scientist experienced in artworks
  • Reference spectra are required for comparison; some materials may not have adequate reference spectra
  • Complex mixtures generate complex spectra that are not easy to interpret; best for pure materials or simple mixtures
  • Often complemented by other techniques
 II.6§2.4, II.6§3.4
Gas chromatography with mass spectrometry (GC/MS or pyrolysis (Py)-GC/MS)
  • Organic fills for repair
  • Organic patina
  • Organic inlays and overlays
 Microscopic samples required
  • Qualitative and quantitative analysis (sensitivity typically in ppb) of organic materials
  • Gas chromatography has the advantage over infrared spectrometry that it permits the identification of complex mixtures
  • Chromatograms and spectra require interpretation by an experienced scientist
  • Reference spectra are required for comparison; some materials may not have adequate reference spectra
II.6§3.4
X-ray diffraction (XRD)
  • Core
  • Inorganic fills for repairs
  • Inorganic patina
  • Inorganic inlays and overlays (other than metals)
Microscopic samples required, although portable instruments are becoming available that may enable in situ measurements
  • Quite rapid
  • Very informative, often a necessary step in the identification of compounds
  • Widely available
  • Some mineral compounds are difficult to detect (SnO2, sulfur compounds, etc.)
  • Complex mixtures may be hard to decipher, though software to aid interpretation is improving
II.6§2.4, II.6§3.4
Petrography** Invasive techniques Optical microscopy (petrography) Core Samples of a few mm2 to 1 cm2 are required for thin section preparation
  • Rapid
  • Widely available
  • Used in conjunction with digital image analysis, many distinctive characteristics of a core can be quantified (such as porosity and particle size, shape and distribution)
May not be sufficient to fully identify the material II.7§2.2.1
Cathodoluminescence (CL) microscopy Core Polished samples of a few mm2 to 1 cm2 are usually required
  • Most scanning electron microscopes can be fitted with a CL detector
  • Rapid imaging
  • Can be useful for classifying mineral species and core types, usually in conjunction with other techniques such as SEM-EDS
  • Rarely applied to the study of core material in bronzes to date
  • Relies on comparative analysis; currently few documented samples are available for comparison
  • Not widely available in cultural heritage laboratories
II.7§2.2.2
Dating Invasive techniques Trapped charge dating; thermoluminescence dating (TL) and optically stimulated luminescence (OSL) Core: mineral constituents 100–200 mg of core material uncontaminated by radiation (including UV radiation); samples must be taken in the dark
  • An absolute dating method (if the annual dose can be measured)
  • Applies to all cores containing silicates (quartz, feldspars, etc.)
  • Although not routinely used for bronzes, it is becoming more widespread
  • New, single-crystal OSL techniques offer the potential for more precise and robust dating
  • Calculations require estimation of many parameters, leading to imprecision (results of analyses of the same material by different laboratories do not always coincide)
  • Analysis of core that was not sufficiently heated during casting can yield an overestimate of age
  • Exposure to subsequent sources of radiation (such as X-rays during radiographic exams) can yield an overestimate of age
  • Few labs currently have the equipment and experience necessary
II.8§1
Radiocarbon dating (carbon-14) Core: organic constituents 2–5 mg
  • Simple collection procedure if the core is accessible and has substantial organic content
  • Readily available as a service analysis in many radiocarbon laboratories
Errors are possible if old organic matter was naturally present in the core's raw materials, particularly in clays II.8§2
Radiocarbon dating (carbon-14) Armature (carbon in iron) 100–1500 mg depending on the carbon content (0.8 %wt of C => 125 mg, 0.3 %wt of C=> 333 mg , etc.) Currently the only accepted method to date iron armatures
  • Large sample required
  • Very recent technique, not available as a service analysis, few laboratories are able to undertake such analysis
II.8§2
*
See II.6§2.1 for a definition of “structural analysis.”
**
Although petrography enables structural analysis, it is presented separately, since for bronze sculpture it is exclusively devoted to core analysis.