Close up to the Surface: Reflections on a Preliminary Forensic Study of Four Chinese Bronze Mirrors from a Hong Kong Private Collection

Mirror history in China

There are different opinions on where the first idea to make bronze mirrors by the ancient Chinese originated. Modern scholar Liang Shangchun [1] thought the ancient Chinese got the inspiration of creating a reflective surface to see the world from looking at still water in a lake or pond. Gradually, bronze basins and later polished bronze plates were developed for reflection. The earliest recognisable Chinese bronze mirror was unearthed in Gansu Province and has been dated to the Neolithic period’s Qijia culture. Over the subsequent 4000 years of history, Chinese bronze mirror design, technology and prominence experienced several important time periods including the three most important: the Warring States (475-221BC), the Han (202BC-220AD) and the Tang (618–907). By the time of the Song dynasty (960-1279AD) the Mirror industry was in decline though this period was the most varied in terms of mirror shape, even if design typically simple. This opinion is based on the distinctive characteristics, artistic design and material quality and the numbers of surviving examples from these periods.

The Warring States was a developing period of bronze mirror refinement, indicated by diverse artistic styles and delicate and intricate decoration. The Han dynasty was a period when the bronze mirror industry was flourishing as evidenced by the vast numbers of mirrors from this period that have come to light even if the execution of their designs were of lesser quality. In the Tang dynasty, in terms of shape, size, pattern and various decoration techniques such as with lacquer and mother of pearl, bronze mirrors were much more varied and sophisticated but again the surface precision and fineness does not match the excellence of the Warring States.

The bronze mirror industry started to decline during the Song dynasty as indicated by the significantly less numbers uncovered via archaeological excavations and the more simple designs compared to the preceeding Tang dynasty [2]. Some scholars believe that the decline of the mirror industry during the Song is related to wars and a reduction of copper mining. However, Zhang [3] has a different opinion, considering the reason for the lack of even basic decoration on Song mirrors being the result from the prevalance of adopting a ‘simple life’ philosophy of that time.

Mirror casting

In China, bronze mirrors were manufactured via the commonly adopted clay mould casting technique. The model, together with desired inscribed decoration, was made first and the mould was second [3, p. 32]. An archaeological excavation of a mirror workshop dated to the Western Han dynasty in Shandong province, P.R. China [4] proved this practice was employed. Before clay moulds, stone moulds were used [5]. There is no evidence found to indicate that the lost wax technique was used by the ancient Chinese to make bronze mirrors, perhaps due to the simple forms.

Mirror alloys

Like other ancient bronzes, a bronze mirror is an alloy that contains various fractions of copper, tin and lead. Although historic records did not specifically mention lead in their formulae, lead is commonly detected. According to the “six formulae: for making bronze for different uses given in K’ao Kung Chi 考工记 by Tai Chen, the ratio for making mirrors and specula is 50% Copper and 50% tin. However, this 50:50 ratio is not observed in extant studied mirrors based on modern scientific appraisal and metallurgical analysis. Qiu [2] pointed out that the scientific data collected from known bronzes is very far removed from the alluded 50:50 ratio. Chase et al. [6, p. 21] revealed that bronze mirrors typically contain approximately 70% copper, 25% tin and 4% lead. Barnard [7,table 9] gave the average percentage of each element, for a sample of both attested and unattested bronze mirrors as reproduced below:

Cu 68.05%, Sn 21.70%, Pb: 3.53% for mirrors dated to the Warring States;

Cu 67.14%, Sn 24.14%, Pb 5.43% for mirror dated to the early Han;

Cu 67.07%, Sn 25.42%, Pb4.47%, Zn up to 6.73% for mirrors dated to Han dynasty;

Cu 69.03%, Sn 24.62%, Pb:4.68% for mirrors dated to late Han;

Cu 66.68%, Sn 9.75%, Pb:18.99%, Zn 8.24% for mirrors dated to Song-Jin dynasty.

In all cases the copper fraction remains remarkably consistent while the tin and lead fractions also remain very similar until the late Han. No quantitative error estimation or sigma dispersion on these averages have been provided, despite the precise percentages reported. The varying zinc fractions found in mirrors from the Han and Song-Jin dynasties likely reflect contamination by this element in the base ores used and available at these times. Zinc has a much lower melting point than copper and even in ancient times was likely expelled as a gas during the bronze smelting process. It was not smelted in China as a metal until the middle of the 16th century [8]. High levels of zinc (> 10%) in a mirror would therefore indicate a modern reproduction or forgery.

Kong et al. [9] also showed that a copper/tin ratio found for 8 mirrors dated to the Warring States was 3:1, consistent with the table from [7] that is partially reproduced above. Modern research also reveals the copper/tin ratios found in Chinese mirrors is typically close to 3:1 [10–12]. The high tin content compared to other typical bronze objects (such as sacrificial cooking vessels and ornaments) yields an alloy that is hard and brittle. However, it can provide a nice silvery-white colour and is suited to enabling a very good polish and hence an excellent reflective surface for a mirror.

Mirror finishes

Bronze mirrors were polished after casting to make the surface as smooth as possible to render the best reflective property. Unlike other bronze objects the mirror has a specific and special function so the reflective surface has to be treated to increase the reflective effectiveness. Before the Song dynasty, Xiuwu xun in Huainanzi, 淮南子-修務訓[13] mentions that quicksilver was applied to the surface of the mirror and polished in by rubbing with white felt. Quicksilver is a mixture of tin and mercury paste. This process is also called xuan xi 玄錫. Later historic records by Tiangong Kaiwu 天工開物 (Ming dynasty by Song Yingxing) [14] described a similar polishing technique. It was in the Ming dynasty that the combination of mirror manufacturing and associated cultural flourishing led to an accompanying commercial business for maintaining the reflective surfaces of mirrors in a good state.

High tin bronze patina

Just like other archaeological bronzes, excavated Chinese bronze mirrors present diverse patinas and corrosion products. These depend on the drainage and condition of the burial region and patination process as the soil mineralogical constituents and water content have a very significant effect on the bronze. The exact constituents of the copper alloy and manufacturing finishes also affect the corrosion process. However, in many cases, bronze mirrors have a tin-rich surface due to the final treatment by the so-called “xuan xi” technique previously described. In these circumstances the mirror surface will be oxidized to varying degrees and react with the underground burial environment and this can form an anti-corrosion layer. This is commonly called qi gu 漆古 and results in a dark black or deep green surface colour and a typically shiny patina. In more modern times there has been much analytical interest in such Chinese mirrors whose surface patina appears very dark or even black as is the case for Mirror 2 in this study in particular. It is generally agreed that the surfaces of these “black” mirrors are no longer metallic but are composed of tin oxides. These result in a ‘tin’ enriched surface together with significant copper depletion compared to the copper fraction in the main alloy (see later for mirror 3).

Introduction to the four mirrors

The four mirrors presented together in Fig. 1a&b. are all round in shape and each has a central ring-knob that allows the mirror to be suspended on a cord or for a rod to be inserted. Based on style and design, mirror 1, with the feather hook pattern (see later), and mirror 2 with a coiling dragons, panchi pattern are nominally dated to the Warring States period (475-221BC). Mirror 3 is significantly smaller than the others and has a very rare jade, patterned annular ring attached to the back surface and is dated to the Han dynasty (202BC-220AD). Mirror 4 is the most corroded but is also unadorned and simple without any cast pattern. It is likely a shamanistic mirror and dated to the Song dynasty (960–1279). The iconography and design of these four mirrors are fully compatible with the estimated time periods of their potential manufacture.

The basic physical characteristics of these 4 mirrors are given in the Table 1 below. The observed variation in thickness is due to the variable corrosion across the surfaces.

Microscopic examination

A standard binocular microscope and an Olympus S2 × 10 binocular microscope with U-TV1X-2 adapter and attached digital camera and a “Nikon Eclipse LV100N Pol” polarized light microscope (PLM) with NIS-Elements and with” D5.00.00” 64-bit software were used to observe and record the microscopic morphology of manufacturing marks, corrosion and archaeological evidence on all four mirrors. An 1000X endoscope was also used for high magnification imagery.

UV examination

Ultraviolet light, as provided by a Inova X5MT-WUVT lamp with 365–400 nm wavelength emission, was used to illuminate the mirrors. Any resultant UV fluorescence can help detect any organic residues, resins or adhesives or modern restoration evident on any of the mirrors.

X-ray fluorescence spectroscopy

A Bruker® Tracer IV portable X-ray fluorescence spectroscope (pXRF) was used to provide surface elemental composition via spot testing. On each side of each mirror, no less than 3 spots were tested. Bruker Artex software was used to analyse the resultant data. It is important to appreciate that due to the thickness of the surface corrosion products and the low surface penetration of the pXRF, the base alloy composition for each mirror is essentially unknown.

Tool Marks

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.

Patina

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).

Corrosion morphology

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 [16]. 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.

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 [19], 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 [20] 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 [20] and given in a scientific report [21] 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 [20], 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)[24]. 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 [20]. 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 [23]. 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%).