Multispectral photography and digital microscopy
Figure 2 provides an enhanced RGB image and two IR images of a trigger with a clearly visible, previously known mark (a Chinese character, Jia 甲). However, these contrast-stretched images already suggest a possible second mark on part A (the trigger itself or handle), and the false colour composite of PCA bands from multispectral photography in Fig. 3a makes this clearer still. Figure 3b shows a false colour composite of another trigger with a previously known ink mark (the numeral 9, Jiu 九, next to a chiseled character Xu 戌 7th character of the twelve Earthly Branches notation), and again we were able to use the newly acquired images to identify a second 九 mark on part C (the rocking lever of the trigger). A chiseled non-character symbol was found on part B (the tumbler) of the same trigger.
As a complement to the above results from multispectral photography, closer-range digital microscopy allowed us to confirm the presence of carbonaceous, black material in the locations identified as definite marks (Fig. 4a, b). The very fine-grained texture of these marks, as well as their well-defined contours, suggests that they were painted using liquid pigment with a brush, as opposed to, for example, drawing with a harder crayon. Further assessment via scanning electron microscope provided a clear image of brush painting with high carbon identified for the black lines of Chinese character against its bronze background.
At the same time we took the opportunity to compare examples of chiselled marks on the triggers and confirmed evidence from a previous study using SEM [11] that the incised marks were made after the trigger parts had been finished used an abrasive tool of some kind. However, the portability of digital microscopes now allows examination of a much larger number of objects quickly, for patterns in the use of instruments which might be indicative of different artisans, tools or techniques in the different stages of the production, including for example styles of chiselling (Fig. 4c, d).
Raman spectroscopy
The Raman spectroscopy results demonstrate a clear difference between ink and non-ink surfaces on the triggers (Figs. 5, 6), allowing for check of candidate ink marks observed through multispectral images. Those areas that appear red in the multispectral images shown in Fig. 3b could be confirmed as dominated by malachite [Cu2CO3(OH)2], a common corrosion product in archaeological copper alloy objects, with the strongest peaks at 152, 181, 272 and 432 cm−1. Raman spectra of better preserved metal surfaces (green in multispectral image) did not show any strong bands.
Of more interest, Raman spectra obtained on the black lines and patches show two bands at ~ 1350 cm−1 and ~ 1600 cm−1 that correspond, respectively, to the so-called ‘Deffect’ or D1 band, and the ‘Graphite’ or G band (which also includes the D2 band) of carbon black. The high signal intensity or ‘plateau’ between both peaks is explained by an additional Deffect or D3 band at ~ 1500 cm−1. The relatively high D/G intensity ratio, together with the broadening of the D and G peaks and the D3 plateau are all diagnostic of amorphous or low order carbon, while the lack of a shoulder at 1200 cm−1 (D4) indicates a relatively low organic content. No peaks were recorded at ~ 2700 cm−1, which if present would point to the presence of charcoal or graphite [23,24,25,26,27]. Similarly, no bands were identified in the 2700–3000 cm−1 region, which if present would have denoted the presence of proteinaceous materials such as animal glues that could have been mixed with the pigment as a binder [28].
Altogether, these features allow the identification of the material as carbon black and, more specifically, the group of ‘flame carbons/soots’ that are produced in the gas phase, from incomplete combustion of hydrocarbons at relatively low temperatures [25]. Raman spectra of carbonaceous materials are highly susceptible to variation depending on Raman wavelength, orientation effects and other parameters [23,24,25], and thus comparisons between studies should be made with caution. Even so, it is worth noting that of the many carbon-based black pigments analysed by Coccato et al. [25], our spectra best match that of ‘furnace black’ (47,250) which is a form of amorphous carbon obtained by condensing the smoke of a luminous flame (oil, tar, pitch, resin; see also [26]). The possibility that oil-derived soot (‘lampblack ink’) may have been used, as opposed to wood-derived soot (‘pine ink’), should be verified. However, the use of pine, wood-derived ink was much earlier in China from at least the Qin Dynasty (221-206 bc), while so far oil-derived, lampblack ink has only been shown to appear much later (the Southern and Northern Dynasty, ad 420-589 [26,27,28,29,30, 37, 38] and references therein). Moreover, it remains difficult to distinguish the two types of Chinese ink through Raman spectra, so further analyses by gas chromatography-mass spectrometry (GC-MS) would be useful to resolve this question, as well as to verify if any possible organic binder was involved [30,31,32,33,34,35].
Some individual spectra of the black pigment on the triggers include other peaks that could be identified as cuprite (Fig. 6b, strongest peak at 640 cm−1), quartz (Fig. 6c, strongest peaks at 464 and 209 cm−1) and possibly calcite (Fig. 6d, strongest peaks at 1079, 710 and 269 cm−1). Supporting this identification is the fact that minuscule white or translucent minerals could be seen embedded in the black material during microscopic investigation (Fig. 4a and b). While the cuprite signal is no doubt derived from the surface patina of the underlying bronze substrate, the other minerals may result from the incorporation of soil particles on the object’s surface during burial. However, an alternative possibility is that these minerals were mixed with the pigment during manufacture, or perhaps as contamination while grinding pigment pellets on an inkstone prior to use. This latter hypothesis should be investigated further through more detailed microscopic examination of both painted and control surfaces on a wider range of objects, possibly aided by experimentation. Note that quartz was also found in one of the two samples of pigment from a Han-period inkstone analysed recently [30].
Spatial analysis
Overall, the bronze weapons from Pit 1 exhibit a large number of different kinds of incised marks. These include longer, dated texts on lances, dagger-axes and halberds, shorter inscriptions on swords and spears, and simple characters on crossbow triggers and long weapon ferrules. The crossbow was a particularly complex and important piece of Warring States and Qin period military equipment. It is therefore perhaps unsurprising that the marks on crossbow triggers are especially complex. They often appear on multiple component parts of the same trigger, with a combination of ordinary Chinese characters, numerals, ‘the Heavenly-Stem and Earthly Branch notation’ and further unknown symbols. Figure 7 offers four viewpoints on these marking practices and how they are distributed across the pit. Despite the complexity of the spatial patterns at first glance, they do exhibit a certain amount of insightful structure that helps to tease out some of the patterning suggested by the matching ink marks. Figure 7a reproduces an analysis first conducted elsewhere [18] in which we made detailed measurements of different trigger parts and then used these to make groups of those examples that exhibit very similar shapes. Put simply, while it is well-known that Qin triggers from Pit 1 are highly standardised in their overall shape, the above measurements and certain typological observations nevertheless demonstrate that certain trigger shapes are more similar to each other than others. These micro-stylistic sub-groups for individual trigger parts are likely to exist because these parts were made in same or similar moulds by particular workshop cells. Similar triggers parts then stayed together when they were assembled into a whole weapon, suggesting that part manufacture was followed fairly quickly by weapon assembly, and/or that it was important to find parts that matched closely and would have operated better as an assembled mechanism. The resulting similarities continue to be visible in the pit: for example, the smaller activity areas in Fig. 7a may reflect batches of weapons from related moulds (perhaps the same workshop cell), while a preliminary interpretation of the larger identified activity areas is that they may have been parts of the pit that were equipped with weapons at the same time. Figure 7a also shows the location of the two analysed triggers with matching ink marks. These are both from what Li et al. [18, Table 2] referred to as Assembly Group 6. This group is particularly interesting because (a) it is found mainly on the flanking corridors of Pit 1; (b) it exhibits lower overall shape variation than other groups; (c) it is frequently marked with a Gong (工, which may be an abbreviation of Sigong 寺工, and hence also referring to a central official workshop for producing metal and ceramic objects) inscription that appears to refer to a more central royal workshop (although the inked examples do not happen to have a Gong (工) reference); (d) it exhibits an unusually high propensity for multiple marks that match across parts, and (e) it demonstrates unusual consistency, for example, in exactly where on a given trigger part, the artisan made a mark (Fig. 7b–d). In other words, Assembly Group 6 exhibits more artisan care, more evidence for quality control, better final morphology and a probable link to the most important of the Qin central workshops. Given this background, it makes sense that an added bureaucratic level of ink-based marks were implemented for this group in particular.
Stepping back, it is clear that other crossbow triggers from other parts of the easternmost trench of Pit 1 (such as the front and central corridors), are more variable in shape (when looked at very closely). They also have fewer incised marks overall (and no ink marks yet) and exhibit different locations where any marks are typically placed on the trigger part (e.g. on the side, in the middle, between the two prongs part B, etc. Fig. 7c). Such greater variability may be due to a different supply chain behind how these other crossbows (rather than the more standardised Gong-related group) came to the pit. For example, perhaps they were stored for a longer time in a military arsenal (rather than coming straight from a manufacturing workshop) or perhaps they were made by different and/or less centralised workshop cells.
