Ink marks, bronze crossbows and their implications for the Qin Terracotta Army
At the heart of bureaucratic practice during Warring States and early Imperial China were regular, small acts of accountancy in which objects and people were marked so that their movements could be kept track of, their quality checked and their numbers marshalled. In the mausoleum complex of the Qin Shihuang (259-210 bc, the First Emperor of the Qin Dynasty), the longer texts and shorter inscribed marks found on the bronze weapons of the Terracotta Army are reasonably well known, and such information helps us to understand aspects of Qin craft organisation and logistics at this crucial period of Chinese state formation. This paper’s modest starting point is a study of two further, less well-known ink inscriptions found on crossbow triggers from Terracotta Army Pit 1. Using multispectral photography, digital microscopy and Raman analysis, we uncover evidence of further marks on the same two triggers that suggest a similar pattern of ‘matching’ marks as suggested by the incised evidence. We also identify the black substance used to make the marks as a soot-based ink. Spatial analysis of both the inked and incised trigger marks then provides wider context for how such marking practices amongst Qin bronze-workers may have operated.
KeywordsEmpire Raman Multispectral Spatial analysis Craft production
This paper takes as its modest starting point, a re-investigation of two previously identified ink marks on bronze crossbow triggers from Terracotta Army Pit 1. In contrast to incised marks which are commonplace on a variety of weapons from the mausoleum [11, 12], ink marks have only very rarely been identified so far. In total, three black-painted marks have so far been identified on the over 200 crossbow triggers recovered from Pit 1 (with the characters Jia 甲 (1st of the ten Heavenly Stems notation), Wu 武 (martial) and Jiu 九 (the number nine), while a red-painted mark (Sigong, 寺工, referring to the Qin central workshop) has also been found on a lance scabbard [12, 13, 14]. By applying a range of analytical techniques—multispectral photography, digital microscopy and Raman spectroscopy—we are however able both to shed further light on known instances of ink marks and to identify previously unrecognised, matching marks on other parts of the same triggers and to suggest what substance was used to make the ink. Additional spatial analysis of all known marks on the crossbow triggers in Pit 1, and further attention to crossbow trigger micro-style and Qin documentary sources, extend our previous conclusions about Qin artefact marking practices, imperial workshop organisation and product quality control in this crucial period of early Chinese political consolidation. These insights also fit into a long-term programme of collaborative research on Qin imperial logistics [11, 12, 15, 16, 17, 18, 19, 20].
In what follows, we begin by discussing the use of multispectral photography and digital microscopy for better visualisation of existing marks and prospection of new ones. Then discussion moves on to consider the likely nature of the marking substance via Raman spectroscopy. Spatial analysis of the wider set of trigger marks from the easternmost trenches of Pit 1 then allows us to place these trigger marking practices in wider context, before a final section reflects on the broader implications for Qin manufacturing practices and the equipping of the first emperor’s mausoleum.
Multispectral imaging is increasingly important as an approach in archaeological and heritage materials science and for object-scale assessments in related subject areas such as conservation, digital humanities and art history [21, 22]. A full imaging survey of all the bronze weapons from the Terracotta Army pits—using one or more multispectral scanners spanning the visible, near- and short-wave infrared for example—is highly desirable in future. However, for the immediate and preliminary purposes of this paper, we have explored the potential of multispectral photography for characterising existing ink marks on the weapons and potentially identifying new ones, especially with a view to cheap flexible usage in a variety of laboratory and on-site settings. To achieve this, a specially-designed camera (Fujifilm IS Pro) was used with a sensor responsive to an unusually wide UV, visible and near infrared (VNIR) range of the electromagnetic spectrum (approximately 380–1000 nm). The following workflow was used for capture and processing: (1) an ordinary RGB photograph was taken by applying a UV/IR-cut ‘hot’ filter to the camera; (2) IR-pass filters were used to capture four infrared images of increasing spectral range. Then (3) slight adjustment of each image for tonal range and exposure, then creation of an image stack of all acquired images. (4) Because of the different focal lengths that pertain to different infrared filters, small spatial adjustments were made to each image in the stack to ensure accurate co-registration. (5) The background of all images was masked out to ensure it played no role in subsequent multivariate image processing. (6) An orthogonal transformation (ordinary PCA) of the image stack was conducted, and (7) a false colour composite produced using the two most insightful principal components. Given the preliminary nature of the sample, we have not formalised a predictive model discriminating ink versus non-ink surfaces, and have opted for PCA as a most widely-known ordination method, rather than cross-comparing many different approaches to multivariate discrimination. We also explored the potential of UV-excited florescence (via a UV torch and lamp) to study ink marks, corrosion and other variability on the weapon surfaces, but this proved to be of marginal added value. We studied six triggers in this manner, but focused on two that were already known to have one ink mark each. As a complement to the above results from multispectral photography, and alongside closer-range digital microscopy (with a Dino-Lite USB digital microscope), Raman spectroscopy was also performed with a Renishaw inVia Raman Microscope, using a 514 nm Nd:YAG laser for the full spectral range from 100 to 4500 cm−1. Cosmic noise was removed, and manual baseline correction and normalisation applied. Finally, we mapped all types of crossbow marks across the easternmost trenches of Pit 1 in order to place the rare ink inscriptions in a wider context provided by the spatial distribution of incised crossbow marks that are found in larger numbers.
Multispectral photography and digital microscopy
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  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).
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 .
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 . 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. , 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 ). 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 .
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.
Discussion and conclusions
“When quarterstaffs [shu 殳], halberds [ji 戟], and crossbows [nu 弩] (marked) in black or red (literally lacquer and cinnabar, xiutong 髹彤) have become confused (xiangyi 相易), this is not to be considered as a surplus or a shortage but is to be condemned according to Statute concerning marks not corresponding to the register.” [5: 74; 6: B21].
It remains unclear whether this edict is referring to crossbows that were disassembled into wooden stocks, trigger parts and other components, or is referring to whole crossbows. If the latter is the case, then the edit suggests yet an additional set of marks: those made to be seen on the outside of finished objects. In contrast the marks discussed in this paper would usually have been covered up by the wooden stock of the crossbow and usually invisible to the user, unless the crossbow was disassembled.
Also of interest are the insights provided here about marking materials, lacquer and soot-based ink. Lacquer was exploited much earlier in Chinese history than ink, for example for lacquer vessels, as an adhesive agent, for painting, and even for writing. Historical documents and archaeological evidence both indicate that ink was produced during the late Zhou Dynasty (1050-221 bc) to write on bamboo and wooden slips [33, 36, 37], such as the two fragmentary wooden slips found at Qingchuan . In addition, a broken piece of soot for ink-making, 2.1 cm in diameter and 1.2 cm in height, was found in the Qin tomb of Shuihudi in 1976 [4, 38], while the tomb of Marquis Yi (ca. 475-433 bc) of the Zeng State  yielded earlier bamboo slips with ink characters. Given the preliminary evidence presented here for soot-based ink marking evidence on the Qin bronze triggers, an important question remains how widespread such painted/inked marking is on the wider assemblage of crossbows from Pit 1 or in other parts of the rest of the mausoleum complex. There is a much greater likelihood that such inked marks will have faded or disappeared altogether from the trigger surface, and/or further vestiges can probably only be identified with close analytical prospection. We suggest a more comprehensive programme of study involving multispectral imaging and verification by SEM and Raman spectroscopy is worthwhile in future.
AB, XL and MMT designed the research described in this paper. AB, XL, MM-T, ZZ, JH, SL, NX, YX, SM collected the data and shared technical expertise at several stages. AB, XL and MMT drafted the majority of the manuscript. All authors read and approved the final manuscript.
Xiuzhen Li’s post-doctoral research is supported by Rio Tinto via the Institute for Archaeo-Metallurgical Studies (IAMS). The authors are grateful to the Emperor Qin Shihuang’s Mausoleum Site Museum and the UCL Institute of Archaeology for their consistence support, as well as to the late Peter Ucko, who made the institutional collaboration possible. The Imperial Logistics project has been adopted as a flagship British Academy Research Project, and we are very grateful for this endorsement. Many thanks also to due to Thilo Rehren for his practical advice and valuable suggestions. Further thanks are due to colleagues in the Conservation and Collection departments of the Museum for their help with data collection.
The authors declare that they have no competing interests.
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Beyond university and museum support, three external funding bodies kindly supported this research (the British Academy, the Institute for Archaeo-Metallurgical Studies and Rio Tinto). None of them had a role in the study design, data collection, interpretation of results or writing-up.
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