Fig 1.
Drawing of the Japanese artefacts from the British Museum investigated in this study, with the analytical spots, reported in Table 2.
The link to the online Catalogue for the three objects are the following: https://www.britishmuseum.org/collection/object/A_TS-244, https://www.britishmuseum.org/collection/object/A_1952-0211-31, https://www.britishmuseum.org/collection/object/A_1958-0730-56-d.
Fig 2.
Visual appearance of the patinated alloys.
Each rectangle is 2.6 x 1.8 cm.
Fig 3.
OM pictures showing patinas developed on the alloys without tin (top row) and with tin (bottom row) through chemical patination. Pictures at 50X (each rectangle is 1.2 x 0.9 mm), non-polarised light.
Fig 4.
The a*/b* ratio of the patinas made with chemical patination.
Each data point represents the mean of five measurements and the error bars represent the standard deviation. Full results are presented in S4 Table in S1 File.
Fig 5.
SEM images of the surfaces of the alloys after patination at 4000x (area: 31.5 x 20.0 µm), secondary electron.
First row: alloys without tin; second row: alloys with tin. The image shows a substantial difference between the microstructure of the patina on alloys without tin, fine and smoot, and the patinas on alloys with tin, with larger particles.
Table 1.
Surface compositions of the alloys before (columns on left) and after (columns on right) patination.
EDS analysis was carried out at 100X magnification (area of analysis: 1.3 x 0.8 mm). Each value is the mean of 3 measurements. Highlighted rows indicate the most common alloy compositions described in historical sources. The presence of oxygen before patination is likely due to the tarnish of the patinas in air occurred from the moment of the creation of the alloy to the day of the analysis.
Fig 6.
SEM images of the cross section of the alloys after patination at 4000X (area: 31.5 x 20.0 μm), secondary electron.
CuAuAg (L), CuSnAuAg (R). On the CuAuAg alloy (L) there is the presence of a thin layer of cuprite only continuous to the alloy. On the CuSnAuAg alloy (R) there is the presence of a further layer of cuprite grains unevenly distributed on the surface of the continuous layer of cuprite alloy.
Fig 7.
Raman spectra of the patinas obtained by chemical patination on alloys without tin (top) and with tin (bottom). The phases identified are stoichiometric (red lines) and defective (green lines) cuprite, tenorite (black lines) and unidentified peaks (blue lines). Raman signatures from [21, 22].
Table 2.
Composition of different points of analysis performed on the Japanese artefacts.
pXRF analysis carried out using a 5 mm diameter spot, with one measurement in each case. Elements <0.1 wt.%: Ni, Sn, Sb. The full results are reported in S9 Table in S1 File. Highlighted rows indicate the shakudo alloys. Note that in the analysis of small inlays the spot size of the analysis (around 5mm) can include the copper substrate.
Fig 8.
The a* and b* values of the analysed patinas on the Japanese artefacts (black, square symbols) and of the experimental patinas (red, dot symbols).
The ellipses represent groups of similar composition. Error bars indicate standard deviation of 5 measurements.
Fig 9.
Chemical patination mechanism for samples not containing tin.
The presence of Au and Ag particles is proposed in the cuprite patina immediately over the solid-solution alloy substrate, not visible in the experiments performed. (a) before patination, (b) short patination, c) long patination.
Fig 10.
Chemical patination mechanism for samples containing tin.
Large tin oxide (SnO2), Au, and Ag particles in cuprite form the patina over the solid-solution alloy surface. (a) before patination, (b-c) short patination, d) long patination.
Table 3.
Data from [30].