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Table 13: Technical examination techniques used in the study of bronze sculpture

Table 13
Table 13
Technical examination techniques used in the study of bronze sculpture
Method of examination Surfaces Structure Materials Importance and advantages Drawbacks and limitations* See
Naked-eye examination examination, description, and measurement of technical features on the inner and outer surfaces, including:
  • tool marks
  • color
  • coating
  • casting artifacts
  • ondition issues, etc.
 identification, description, and measurement of visible joints, repairs, internal supports, inlays, etc. examination and description of materials, identification of good sampling locations
  • important first step of any examination/analysis
  • essential for comparison with other images (X-ray, UV, etc.); universally accessible, allows good overview of an artifact
limited to visually accessible areas II.1§2
II.1§3
II.2§1
II.4
Stereo microscopy and digital microscopy provides information on the surface of bronzes:
  • tool marks
  • color
  • coatings
  • restorations
 provides information on the structure of the metal, joints, repairs, surface coatings, core material useful for the removal, preparation, and examination of samples of core material, surface coatings, corrosion products, and metal, and for observation of microchemical tests
  • useful supplement to naked-eye examination and generally essential for sampling
  • can reveal many significant details not visible to the naked eye
  • if not already available, acquisition of a stereo microscope can be expensive
  • many microscopes are set up for viewing samples on a horizontal stage that cannot be adapted to sculpture
  • binocular microscopes on stands that allow free movement of the head can be very expensive
II.2§2.1
Reflectance transformation imaging (RTI)
  • can greatly enhance subtle surface details on bronzes such as tool marks, defects, repairs, and inscriptions
  • . augments naked-eye examination and photography
can greatly enhance subtle surface characteristics associated with joints in the sculpture or the model may show up microstructure of metal and/or pseudomorphs can be done with a minimum investment in apparatus and software beyond standard photography equipment investment required in time and training II.2§2.3
Photogrammetry and 3D laser or structured light scans
  • both methods can provide a detailed 3D model of the surface of a sculpture
  • photogrammetry and most 3D scanners will capture surface color and texture
3D scanning allows detailed and sophisticated measurement and comparison between related pieces; with proper setup photogrammetry will also allow precise measurements; useful in deciphering fabrication methods -
  • creation of 3D models allows direct and sophisticated comparison of dimensions between related sculptures (often in disparate locations) without having to bring them together physically
  • photogrammetry can be done with a minimum investment in apparatus beyond standard photography equipment
  • for both methods, considerable investment is required in time and training
  • 3D scanning apparatus can be expensive to acquire
  • obviously, accurate comparison between related pieces requires that data be acquired with sufficient accuracy
II.2§5
Ultraviolet examination helps to discern certain types of original coatings, adhesives, fillers, and overpainting may aid in the detection of structural repairs and restorations
  • fluorescence under UV is used to detect the presence of organic materials such as resins, oils, proteins, and aliphatic glues
  • certain pigments and minerals may also be detectable
low-cost and accessible method that allows rapid examination of objects with minimal equipment and training requires a darkened examination space and eye protection; documentation with UV photography requires greater investment in equipment and training; does not allow identification of exact composition of materials II.2§3.1
Endoscopy examination and description of inner surfaces identification and description of internal evidence of casting and assembly processes (joints, repairs, internal supports, armature, core pins, core material remains) can help identify the type of material used for core material or armature useful aid for visual examination of areas that are difficult to access

 may be hard to maneuver, and to gauge the exact location and proportions of features observed; access may be limited II.2§2.2
X- and gamma radiography examination, description, and rough measurement of technical features on the inner surfaces, including casting artifacts, inlays, condition issues, etc.
  • reveals internal structure and technical features (e.g., joints, repairs, internal supports)
  • reveals internal structure and technical features (e.g., joints, repairs, internal supports)
records differences in physical and chemical density; as a result it may:
  • discriminate between different inlays
  • reveal metal plating such as gilding
  • reveal the presence or absence of core
 radiography is a key tool in technical studies of bronzes:
  • can provide a great deal of detailed information related to fabrication method (especially of hollow casts) that other methods cannot provide, particularly if no access to inside of hollow cast and endoscopy is not possible
  • radiographs are often very aesthetic and didactic
  • real-time radiography is helpful in selecting angles
  • equipment required is expensive and not standard in museums, including dedicated space
  • requires skilled technicians experienced with industrial methods and bronze sculpture
  • serious health and safety issues, especially with portable equipment and gamma radiography (both higher kV than medical)
  • radiographs can be difficult to interpret, requiring training and experience; multiple views are helpful; multiple exposures of each view may also be helpful, particularly for sculptures with large variations in metal thickness
  • requires comparison with object
  • will render future luminescence dating of the core material difficult
II.3§1
Neutron radiography although some surface features might be revealed, X-ray is much preferable reveals artifacts made of a lower-density material than their surrounding: typically organic fibers, paper, etc. trapped in the bronze, or glass, copper, silver, and gold implements embedded in a lead sealing, core material records differences in atomic number; as a result it may discriminate between different materials (inlays, metal plating, core materials, other materials trapped in the internal cavity) very informative, particularly for visualization of low-density (organic or core) material in the interior of a sculpture
  • requires a specialized facility, typically a nuclear reactor, with highly trained staff
  • very few examples of this technique for bronze sculpture for comparison
  • insurance and transportation costs can be significant
  • spatial resolution and contrast usually inferior to X-radiography
  • interpretation of radiographs requires specialized training
  • will make future luminescence dating of the core material difficult
  • some objects may need to be quarantined for decontamination (de-activation), that is, kept at the neutron facility for anywhere between several minutes to several days before returning to their owner. The quarantine depends on a number of parameters, including neutron flux and energy, alloy composition, etc., which can create temporary radioactivity of the material. Copper alloys are actually less impacted (unless they contain large amounts of gold or arsenic). For example, no quarantine was needed for the Javanese statuettes in Case Study 4.
II.3§3
Computed tomography (CT scanning using X-ray or neutrons) tomography can provide, if properly calibrated, a dimensionally accurate model of the inner and outer surfaces of a sculpture can provide detailed and dimensionally accurate models of internal structural elements can discriminate between different inlays
  • tomographic 3D models and associated renderings may be easier to interpret than conventional radiographs
  • accurate measurement of dimensions, including wall thickness, is easier than with conventional radiographs
  • requires expensive equipment rarely found in museums
  • lengthy exposure to radiation
  • postprocessing requires significant investment in time by highly skilled technicians
  • resolution is generally lower than with conventional radiography
  • will make future luminescence dating of the core material difficult
II.3§2
Other nondestructive testing techniques (ultrasonic [US] testing, thermography, eddy currents)   reveal defects, repairs, assemblies; US enables to measure the wall thickness of hollow bronzes reveal metal plating such as gilding
  • access to most of the sculpture thus good representativity of thickness variation
  • low to middle cost of device (standard camera for termographyn, circa 2,000€ in 2018 for US testing)
  • private companies may carry out US testing for relatively low cost
  • the measurement of wall thickness by US may be altered by any kind of perturbation in the metal wall (porosity, crack, large inclusion, etc.). The latter may echo the signal and may be confused with the internal surface. Calibration is mandatory. A skilled operator is needed
  • US is slightly invasive: need for a binding medium between the bronze surface and the detector of the instrument, typically a gel (water)
II.2§3.2
II.4§2.3.3
II.5§2.3.2
*
Time and training are mentioned below. This may involve hiring an expert if the necessary skills are not available within the institution performing the examination.