Results from our investigation into the physical characteristics and remaining trace evidence of manufacture, treatment, corrosion and archaeology are presented below.
On mirrors 1,2 and 3, polishing marks were seen on both sides. On mirror 1 and 2, by using the polarized light microscope (PLM) with reflected light, parallel lines with sharp angles were observed. This type of line is likely caused by mechanical polishing of the surface. For mirror 1 the marks can be seen on-top of some protruding corrosion products rather than passing beneath them (Fig. 2a & 2d). This indicates that this polishing was done post formation of these corrosion products and that an aggressive cleaning of the surface was undertaken quite recently, perhaps to remove significant corrosion to get to the base metal surface in attempts to improve the mirror “aesthetics”. For mirror 2 it is clear that newer corrosion components have formed on top of some of the presumably older tool marks (see arrowed examples in the figure) while the surface has some inclusions that are many times (10X) larger than those small surface defects seen on mirror 1 (Fig. 2b&2e). Mirror 3 is similar to mirror 2 but has a factor of 3-5x more surface defects with a mixture of broader and finer tool marks. In some cases the cleaning tracks can be seen going through these surface defects. In another case (top left of Fig. 2c) these tracks are under the corrosion. These tracks also have a broader spread of angles across the surface indicating several different cleaning episodes. Due to the highly corroded surface, there is no polishing marks able to be observed on mirror 4.
Mirrors 1 and 2 have a black, shiny patina on their fronts (see Fig. 1b) common to mirrors from the Warring States period. The areas without corrosion still present a reasonably reflective surface. Mirror 3 has as a variable, very dark to lighter green patina. From a region towards the edge of mirror 3 where the overlaying shiny patina has become detached, the division of surface patina and further corroded metal underneath is distinctive. In this small area, where the lighter shiny olive green patina has come away, a lower level “crinkly” sub-surface is evident Fig. 3 (left). This is seen in many other bronzes from antiquity. An example is shown in Fig. 3 (right) of a 2nd century AD Roman brooch from Cirencester museum in the UK (photo taken by the 2nd author in-situ in December 2017). This is shown next to mirror 3 together with arrowed inserts showing the common patination features. Both mirror and brooch are from approximately the same time period, both are small in size and have similar light to dark olive green shiny patinas. However, in both examples the top level patina has flaked off in some places to reveal a crinkly, sub-surface condition. Such features are, to our knowledge, not present in modern replicas.
Remaining Archaeological evidence
Mirror 1 does not have any obvious archaeological evidence such as any encrusted earth deposits or any other phenomenon related to a burial history such as attached organic material, including textile pseudomorph marks from wrapping in cloth prior to burial. Mirror 2 does have some apparent encrustation remaining on the mirrors’ 3 mm wide edge that extends for about 2 cm. Images of this attached encrustation are shown in Fig. 4a&b at 40x and 1000x magnification respectively. A fine implement can remove this material quite easily. Both mirrors 1 and 2 appear to have been heavily cleaned (see Figs. 2a-b). For mirror 3, within the incisions of the inscribed pattern of the jade annular attachment, some traces of earthen encrustations were also observed. By touching by scalpel, these encrustations are very hard to remove (Fig. 4b). It looks natural. The formation of this type of crust on a jade surface is usually caused by a prolonged burial history. The jade annulus itself is 12 mm wide with an outer diameter of 52 mm and inner diameter of 28 mm with a typical late-Warring States simple, scroll-type, panchi pattern. From microscopic examination, it is clear this piece was not worked with modern machine tools but betrays the tell-tale marks and scratches typical for jade hand-worked from this period and evident in Fig. 4c. The jade annulus itself is possibly at least partially recessed into the bronze mirror being almost flush with the surface on one side with a protruding thickness of ~ 1.9 mm for the most part but closer to 3 mm for the inner edge closer to the central boss.
On mirror 4, the overall surfaces are quite heavily corroded and covered with deep greenish blue corrosion products raised from the base mirror surface together with some white crust along one edge. Textile pseudomorph marks were observed on this particular corrosion layer (see Fig. 5a,b). This area also fluoresces bright yellow under UV light indicating some organic material is present in this region (see Fig. 13) The impressed textile marks look natural. From the woven pattern (Fig. 5b), it is likely from a linin fabric. It is not rare to see such textile pseudomorph marks on bronze mirrors and other bronzes. As an important and elegant personal belonging, mirrors were usually buried with the owner, and sometimes wrapped with a textile [15, Fig. 10]. After a long time underground, the textile, which is an organic material, deteriorates away to nothing and only residual marks can be left on the surface corrosion, especially in the regions where underground water levels fluctuate actively. Several insect carcasses can also be seen embedded in the thick corrosion (see Fig. 6).
The surfaces of mirror 1 have been well preserved but extensive abrasive cleaning seems also to have been undertaken perhaps in the recent past. On the front surface, towards the mirror edges severe pitted corrosion can be seen in several places that has eaten into the smooth, light green surface patina. By looking at these areas where the surface patina has been broken-up, 3 distinct layers can be distinguished (see Fig. 7). The top layer is a light green shiny patina, underneath is a black layer and under this is a layer of greenish powdery, pitted depressions. It is hard to tell if the corrosion starts from inside the metal or has ingressed from outside. We believe the black middle layer is the original mirror surface akin to what is seen for mirror 2 and resulting from application of the “xuan xi” technique referred to earlier.
However, given the absence of metal surface where the pits are exposed and the trend of the increasing density of number of pits around the powdery area, it is very likely that the corrosion starts from each single pit where moisture has gotten into the metal to stimulate the process of decuprification to form green corrosion and then turn into powdered corrosion. In certain regions the original metal surface is gradually replaced by this green corrosion. Along with forming increasing green powder corrosion, the green surface has been eaten away. Therefore, the green surface is the middle phase of the whole deterioration process. On the back surface, towards the mirror edges, the green layer is absent where a black rough metal underneath is exposed (Fig. 8).
For mirror 2 both front and back surfaces are very well preserved with the surfaces very dark, almost black in colour. However, a similar corrosion phenomenon (Fig. 9) to mirror 1 can also be seen on parts of the front surface of mirror 2 although in this case there is no additional green surface patina on top of the dark layer. It is possible this has been removed by aggressive cleaning. Both mirrors one and two were acquired at the same time from the same vendor. On the back surface towards the edge in one area in particular, a rougher surface is observed. It was likely formed during the manufacturing process since part of the relief is disturbed (Fig. 10).
For mirror 3 the front surface is mainly smooth and dark. However, there are regions of bright green malachite corrosion along with small regions of red corrosion which is assumed to be cuprite, as shown in Fig. 11. The corrosion looks natural. On the front surface there is also a meandering hairline crack within the body of the mirror (refer Fig. 14) that does not extend to the edges. It has an overall extent of ~ 5 cm.
Mirror 4 is covered with thick green corrosion products. In some regions, small, black “charcoal” like inclusions can be seen within the corrosion (Fig. 12 right). The overall corrosion looks natural.
UV fluorescence Imaging
UV fluorescence imaging has been a valuable diagnostic tool in the art and archaeology fields since the 1920s . Four mirrors were subjected to UV illumination to detect if any modern restorations, e.g. using adhesive/paint to reattach surface patina, have been done. For Mirror 3 and 4, due to the potential surface organic archaeological evidence remaining, observation was also carried out on these specific targeted areas. The results show that no restorations have been found on all mirrors. Mirror 3 had evidence for a brown “glue” under the jade annulus that is fixing it onto the back surface of the mirror. UV imagery did not reveal any fluorescence indicating that whatever was used to fix the jade annulus in place was not any modern resin or adhesive. Furthermore, the strongly attached encrustations here and there within the jade carvings did not fluoresce either so are not modern glued-on material sometimes used to create an impression of authenticity by the unscrupulous. However, UV imagery of Mirror 4 did reveal in stark relief the grey-white areas of corrosion associated with the textile pseudomorph regions. This indicates that organic components may remain in the fluorescent region.
Surface Metallurgy from pXRF measurements
For each mirror, 3–4 test spots were chosen for analysis using pXRF. Here it is crucial to appreciate that the pXRF measurements only penetrate a small distance into the surface. Hence, where the corrosion is thick, only these products can be assessed. All data was therefore from the surface but the cleanest regions where chosen wherever possible. For mirror 4, the data was collected from the area where with less green corrosion. The “vacuum” attachment and lower kV and high current was used for the corroded area in order to detect chloride and lighter elements. Bruker® Artax analytical software was used to analyse the data. By analysing the net count rates, which are the number of photons recorded for each element after removing other elemental interference and background, a percentage by abundance for each recognized element was calculated. Given the data was taken from the surface layers, where any corrosion or residues would affect the absolute value, the results should be regarded as semi-quantitative only. Table 2 indicates average percentage of major elements on the surfaces of each mirror from the combined 4 spot measurements. The blanks mean that the listed element was not detected at the low abundance limit of the instrument.
Copper alloy of four mirrors from pXRF analysis
Recorded percentages of the rare earth metals Pd, Rh and Ru are from the pXRF instrument itself and so are not included in the Table 1 but help explain why the totals do not add up to 100%.
Data was at 40 kV, 20 µmA, yellow filter for metal analysis, 120 second exposure for each spot. The results are the averages from the 3–4 spot measurements. The K12, L1 and M1 designations refer to electron shells where K denotes the first shell (or energy level), L the second shell and M, the third shell.
The four mirrors are all bronze, i.e. a copper-tin-lead ternary alloy. However, the ratios vary markedly. Zinc is not over 10% in any of these mirrors which indicates that none are poorly made and easily acquired modern fakes where high levels of zinc are common [17, 18].
The Cu-Sn ratios on surface in Mirror 1 fall within recorded common ranges as around 3:1, Mirror 2 is about 2:1. As for lead fraction, according to Wang , lead on the surface of mirrors dated to the Warring States ranges from 9.35–12.83%. The 20.20% lead value measured for Mirror 2 is much higher than the reference record. Furthermore, the range in the amounts of tin and lead found in mirrors 1 and 2 does not match the opinion of He [20,p. 76] who states that mirrors dated to the Warring States in most cases have a tin fraction that is much higher than lead. Here it is only 17–24% tin c.f. 8–20% lead. He  also indicated that the mirror dated to the Warring States with low fraction of iron (Fe) were probably unearthed from central china Shaanxi, Shanxi province or from inner Mongolia.
For mirror 3 the surface copper content is extremely low while the tin content is 50%. The likely applied “xuan xi” surface treatment would contribute to this outcome. Although this data was achieved from surface measurements, the present of a crack indicates the brittleness of the alloy on this mirror(Fig. 14). Similar data on Cu-Sn ratios was achieved on mirror surfaces by He  and given in a scientific report  by the lab in University of Science and Technology of China in 1988. The drawback of high tin is that it can lead to a brittle copper alloy. Indeed on mirror 3, a crack on the reflective front has already been noticed (see Fig. 14). Most published data [11, 12, 20–22] on the lead content on the surface of mirrors dated to the Han dynasty are less than 10%. Wang (1995) gave a wider range of 7.27–20.4%. Therefore, the lead in mirror 3 is actually in the reasonable range for provenanced Han bronze mirrors.
For mirror 4, the Cu-Sn ratios are far from 3:1. Other archaeologically provenanced mirrors dated to the Song dynasty with high lead content have also been reported. According to He & Song , a Song mirror unearthed from Linxi, Shandong province (northern China) present a high lead content (35.11–35.75%) on its surfaces. Conversely, mirrors unearthed from Echeng City in Hubei province (southern China) show a relatively low lead component (average 20.220% for 12 mirrors). An interesting phenomenon for mirror 4 is the much more highly corroded surface compared to the other three mirrors and that the measured lead fraction is high, averaging ~ 38% on the front surface. There are two possible explanations and sources for the lead. 1). It is migration to the surface from within the alloy itself; 2). It arises from the lead mercury finishing paste used to keep the reflective surface shiny . This could at least also partially explain why the front surfaces for mirrors 2, 3 and 4 also have a higher lead content. No matter what the source is, lead is an active factor in causing more corrosion. This is also true for other types of bona-fide archaeologically excavated bronzes with high lead content . The known process of the migration of lead within the alloy also affects the composition of the distinct but localised white crust that bears the textile marks (pseuodmorphs) on mirror 4. The pXRF result shows that this crust on mirror 4 has a very high lead value (Pb L1 50.28%, Pb M1 0.41%).
Presence of mercury (Hg)
Trace but reliable levels of mercury (Hg) were detected on both mirrors 3&4, indicating that these two mirrors at least were likely treated with the quicksilver polishing technique described in the historical records. The nominal low level detection of Hg on the decorated reverse side of mirror 1, though not considered reliable, is interesting and could also indicate past such treatment where aggressive cleaning of the reflective surface has removed almost all traces of mercury on that side. No mercury was detected on mirror 2. It is hard to conclude that no mercury was used to treat the mirrors since if heat was involved during the process, the mercury may evaporate .
pXRF of the corroded areas
The pXRF with settings of 15Kev, 36 µmA, vacuum implemented and with 120 second exposure testing times were used to analyse chosen corroded areas on pitted areas of mirror 1 and 2 to detect chloride (Cl) which commonly causes ‘bronze disease’, seen as greenish white powder on bronzes . However, no chloride was detected. For mirror 3, data was collected on the green corrosion on the reflective surface as in Fig. 11. Green corrosion and earthen crust on mirror 4 were also analysed. Chart 1 shows the elemental distribution on the targeted areas on four mirrors.
Presence of chromium (Cr)
There is chromium (Cr) present on both mirrors 1 and 2 as a trace element. A few ancient bronzes have been discovered bearing chromium that have been studied in the past 30 years. Scholars are interested in whether such chromium was added deliberately by the ancients for anti-rust purposes. However, most investigations [27–29] pointed out that the chromium was not an intentional addition but a contamination from the natural environment, except perhaps for one bronze Pan dated to the Warring States Chu Culture. The content of chromium in the sample surface-film by Luo  for this bronze Pan is 35.61%, much higher than the average content in any other publication and much higher than found here for mirrors 1 and 2 (less than1% chromium). As chromium(III) oxide (Cr2O3) is a modern polishing product used on metal we conclude that the detected chromium is more likely a result of cleaning with chromium oxide more recently, especially considering the sharp angled modern polishing tool marks observed on the reflective surfaces on mirrors 1 and 2. Interestingly, these two mirrors came from the same dealer. This may also explain the absence or very low levels of mercury on those two mirrors, where a heavy, abrasive polish could have removed any remaining trace mercury.