Set of examination and testing protocols for the characterization of a patina layer both for sculpture that is not likely to have been buried and for archaeological bronzes. For the latter, these are very general notes; the question of burial might be more nuanced than presented here and can lead to much more complex situations (e.g., archaeological bronzes may be subsequently coated). See fig. 305 for more details on the relevant techniques of analysis.
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A. SCULPTURE THAT IS NOT LIKELY TO HAVE BEEN BURIED
If there is a suspected organic coating present such as wax or lacquer:
- Examine with strong raking and UV light, with and without magnification. Make note of any brush marks, brush fibers, drips, spray patterns, etc., and/or layering that may be revealed in areas of damage or loss.
- If multiple coating layers are found, note the order.
- Fluorescence color under UV may help with preliminary characterization of the coating(s) (Measday, Danielle, Charlotte Walker, and Briony Pemberton. 2017. “A Summary of Ultra-Violet Fluorescent Materials Relevant to Conservation.” AICCM National Newsletter, no. 137 (March): https://aiccm.org.au/network-news/summary-ultra-violet-fluorescent-materials-relevant-conservation/.). Discrete solvent testing by a trained conservator may also help with preliminary characterization.
- Consider whether the coating(s) may be part of original manufacture, from a historically significant restoration, or from a more recent conservation/restoration treatment.
If a coating appears to be original to the manufacture of the object:
- Do not remove (except for minute sample for analysis if possible).
- Microscopic examination is useful to identify the best-preserved areas of original coating.
- Look for the presence of pigment particles or transparent colorants. Note surface condition and texture (craquelure, etc.).
- Comparative surface elemental analysis of the coating with bare metal areas may reveal inorganic elements in the coating (fig. 305 A, II.6).
- If sampling is possible, attempt identification of organic and inorganic components with the appropriate analytical techniques (fig. 305 B and C).
- Consider whether an old coating may have changed over time to become darker or more opaque.
If a coating appears to be not original:
- Examine carefully, particularly in areas of damage or loss, to determine if (possibly fragmentary) coatings are present below. A minute cross section may be taken for microscopy to aid in this determination.
- Consider whether the non-original coating(s) may significantly alter or conceal the underlying appearance.
- Consider whether identification of the non-original material is warranted to better understand the history of the object.
Be aware that a chemical patina layer can sometimes be mistaken for an organic coating:
- Chemical layers are generally thinner than organic coating layers and may not show edges in losses. Use magnification to study edges of patina application or loss if possible.
- Consider whether the original color of a chemical patina may have changed over time. Compare to any areas protected from air and light if possible.
- Comparative surface elemental analysis of patina layer(s) with bare metal areas may reveal differences (fig. 305 A, II.6). The presence of certain elements may point to either a pigmented coating or to a chemical patina.
B. ARCHAEOLOGICAL PATINA LAYER
If the surface is metallic and free of corrosion:
- Examine porosities and surface texture under magnification; look for any residual traces of corrosion.
- Consider possible reasons for the lack of corrosion (burial environment, previous treatments).
If the surface shows corrosion products:
- Examine with strong raking and UV light, as well as under magnification/stereo microscope. Make note of any:
- inclusions
- accretions
- pseudomorphs
- embedded original surface elements (gilding, gesso, paint, inlays, patination)
- drip marks/patterns
- layers of different colors
- layer order
- degree of mineralization (X-radiography may be required)
- disfigurement of original decoration
- Consider whether corrosion is due to burial, post-excavation treatment, possible residual chemicals, or environmental causes.
- If sampling is possible, attempt identification of organic and inorganic components with the appropriate analytical techniques (fig. 305 B and C).
- Map the corrosion and any differences in color, drip marks, and any patterns that may indicate original use or orientation during burial.
- A minute cross section may be sampled using a micro scalpel in order to reveal layers and information about formation and corrosion mechanisms, as well as original manufacture.
- It is relatively common that materials used as inlays (metals, niello, etc., see I.9) may have become degraded by deposition and interfere with corrosion layers. Also, be aware of the possibility of staining from close contact with other materials such as iron or lead oxides.