Technologies Used in the Study of Bamboo Slips

Inscribed bamboo-slips of The Art of War, unearthed in Yinque Mountain, Linyi, Shandong in 1972. The slips are dated back to the 2nd century BC.

Bamboo slips (Chinese:简牍) were the most important form of book in ancient China before the invention of paper. Ancient Chinese people chopped bamboos into small slips (Chinese: 简), used knife to smoothen the surface, and baked the slips over fire to kill the hidden insect eggs. Two or three holes were punched on each bamboo slip, and threads were used to tie the slips through the holes into rolls or pages (Chinese:册).

Bamboo slips had a profound impact in the Chinese book history. The book terminologies, the writing format, and the Chinese characters all inherited the traditions from the bamboo slips period. The first usage of bamboo slips was found during the Warring States Period (475-211 BC) and lasted until the Northern and Southern Dynasties (420-589 AD). Numerous famous ancient Chinese books were able to survive until today due to the inventions of bamboo slips, including The Analects of Confucius, Classic of Poetry, and Book of Documents. The most recent discovery was Tsinghua Bamboo Slips, a collection of 2388 bamboo slips donated to Tsinghua University by an alumnus. The most astonishing finding within this collection was a decimal multiplication table for multiplying numbers up to 99.5.

The advancement in technology enhanced our knowledge of these bamboo slips in many different ways. This wiki page will briefly discuss in three different parts how new technologies enable archeologists to better study bamboo slips: conservation, authentication, and image acquisition.

Conservation

Bamboo Stalks

Conservation is a crucial part of studying bamboo slips. Compared to other historical relics, bamboo slips are usually harder to preserve. Two obstacles archeologists have the encounter during conservation are the structural change and color change of the bamboo slips.

Bamboos, like all other plants, are composed of cells. Cellular components have carbohydrates, a perfect energy source for insects, bacteria, and fungi. The temperature (25-30 Celsius), pH (4.5-5.5), and water content (35-50%) in the tombs or wells where bamboo slips were found are ideal for the growth of microorganisms. These microorganisms are able to dissolve the cellulose in the cell walls into glucose, thereby destroying the structure of the bamboo cells.

However, with the assistance of water, water molecules are able to create hydrogen bonds with the hydroxyl groups in cellulose and support the structure of the cells. This explains why most bamboo slips that have survived until today are the ones soaked in water. Although water helps to preserve the bamboo slips, it also generates some major problems. After the excavation of the bamboo slips, the temperature, pH, and the water content of the environment will dramatically change. If the bamboo slips are left unattended, water molecules will evaporate from the intervals between cellulose, rendering the bamboo structures weak. Deformation, shrinkage, and ruptures are common problems.

Another risk is the exposure to the oxygen in the air. Bamboo slips have been buried for thousands of years underneath the earth without contact to oxygen. During this time, the iron (II) ions have perpetually infiltrated into the bamboo cells. The iron (II) ions are able to react with the oxygen to create iron (III) ions. The chemical equation is the following:

4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H20

The principal component of bamboo is tannin, which can go through demethylation and produce catechol. The two polar hydroxyl groups on catechol can react with iron (III) ions. This chemical change will result in the blackening of the bamboo slips. Moreover, the cellular components of bamboo contain chemicals such as vinyl groups, benzene ring, and quinone groups, while the cell walls have carboxyl, hydroxyl and alcohol groups. These groups, called auxochrome groups, can react with other chemicals and further the color changes. All these reactions will make it more difficult for the scholar to identify the characters on bamboo slips.

Decoloration

Oxalic Acid

Ball-and-stick model of the oxalic acid molecule, the simplest dicarboxylic acid. Atom positions based on a crystallographic study of oxalic acid dihydrate.

The most common method to decolor the bamboo slips uses oxalic acid. Under room temperature, soak the bamboo slips in 1% oxalic acid solution for twenty minutes. The color of the bamboo slips will change from a brown/black color to the original yellow color. The logic behind this method is that the oxalic acid can react with iron (III) ions to form iron (III) oxalate complexes. The color of these iron (III) oxalate complexes is light yellow. A major drawback of this method is that the ultraviolet light from the sun can decompose iron (III) oxalate complexes into the original iron (III) ions and oxalic acid. Therefore, after decoloration, the bamboo slips have to be kept in an environment that minimizes exposure to sunlight.

Ethylenediaminetetraacetic Acid

Stick model of the cobalt(II) EDTA complex anion.

Since the major component causing the darkening is the iron (III) ions, latest studies have shown that ethylenediaminetetraacetic acid (EDTA), a chemical that binds and holds on to metals, can increase the efficiency of decoloring the bamboo slips. Under 40 degrees Celsius, soak the bamboo slips in 1% EDTA solution with minimal amount of inorganic salt for a week with frequent mixing. The color of the bamboo slips will turn yellow, and the bamboo slips decolored from this method resist color change even under exposure to sunlight. Although EDTA can make the decoloration almost permeant, it is relatively costly compared to oxalic acid (the unit price of oxalic acid is 5 times less than that of EDTA), and the whole procedure takes a relative long time.

Sodium borohydride or sodium dithionite

Using sodium borohydride or sodium dithionite, scholars can bleach the bamboo slips to eliminate the black color. At fifty degrees Celsius, submerge bamboo slips under 1% sodium borohydride solution or 3% sodium dithionite solution. After bleaching, the bamboo slips look brand new with a shining yellow color. This method is different from the previous two methods discussed in the sense that instead of targeting the iron (III) ions, it focuses on the auxochrome groups. The protons generated in this reaction will make chemical changes in the auxochrome groups, rendering them colorless. However, this method is usually not used in preserving bamboo slips because both sodium borohydride or sodium dithionite can create highly basic conditions that can damage the valuable bamboo slips.

Dehydration

Natural dehydration

Natural dehydration is a method that uses air pressure difference to dehydrate the sample under 95% humidity and 16-27 degrees Celsius condition. The slips will be wrapped with wet cotton paper and stored in the basement. This method is relatively easy but can only be used on slips made of wood with hard texture. For example, the wooden slips from Zoumalou in Changsha, Hunan were dehydrated using this method because these slips were made of Chinese fir.

Alcohol—Ether Dehydration

The most common dehydration method is submerging bamboo slips in alcohol first then in ether. The water molecules between the bamboo celluloses can dissolve in ethyl alcohol through osmosis. By gradually increasing the concentration of ethyl alcohol solution, sample can be completely dehydrated. Afterwards, bamboo slips are soaked in diethyl ether solutions. By gradually increasing the concentration of diethyl ether solution, the remaining ethyl alcohol inside the bamboo slips will be replaced by diethyl ether. The bamboo slips soaked in diethyl ether can be easily dried out when exposed to air due to the high volatility of ether. This alcohol—ether dehydration method has multiple advantages. It successfully prevents the possible ruptures caused by water evaporating from the bamboo slips because both alcohol and ether have lower surface tensions than water. The evaporation of alcohol and ether is faster than water. Moreover, alcohol and ether are relatively inert chemicals that will not induce damages to the bamboo slips. Similar method is used in many biology or chemistry labs to dehydrate animal samples.

Vacuum Freeze Drying

The technology of vacuum freeze drying was invented first in 1811 for the conservation of bacteria, viruses, and blood serum. The biggest problem for natural dehydration is that the high surface tension of water will destroy the structure of the bamboo slips during evaporation. Under low temperature in a vacuum environment however, water molecules will sublimate out of the bamboo slips instead of evaporating. This method is generally considered the optimal dehydration method because it preserves the quality of the bamboo slips to the utmost extent without changing their characteristics. The problem with this method is that the volume of water will increase upon solidifying. Therefore, it is recommended to soak the bamboo slips in tert-butanol and then perform vacuum freeze drying. Compared to water, tert-butanol will not have an increase in volume after freezing.

Authentication

Due to the high historical value of bamboo slips, it is inevitable that many technically skilled, educated, and well-funded forgers tried to make counterfeit bamboo slips and sell them through illicit market. Therefore, establishing the true antiquity of these historical relics is very important. The most famous recent case proving the significance of authentication is the discovery of Tsinghua Bamboo Slips in 2008. This collection of bamboo slips was not archaeologically excavated but purchased from a foreign auction. The mysterious origin caused many scholars to doubt the authenticity of the Tsinghua Bamboo Slips. We will illustrate in the following section how experts from the Tsinghua University proves that these bamboo slips are not counterfeit artifacts.

Accelerator Mass Spectrometry

Different regions in an ion mobility spectrometry mass spectrometer

The main function of Accelerator Mass Spectrometry (AMS) is to accelerate the different isotopes of carbon to a very high velocity so that carbon-14 can be separated and distinguished from carbon-12 and other rare particles with masses similar to carbon-14. To prepare the sample, a small amount of sample powder is obtained from bamboo slips. The carbon atoms from this sample will be converted to carbon dioxide gas. The carbon dioxide gas is then converted to filamentous carbons (small carbon tubes) through a catalytic process. These filamentous carbons are then placed in the accelerator’s ion source. The ion source will convert carbon into a single negatively charged ion through the bombardment of carbon ion beams. In the first stage of acceleration, the negatively charged carbon atoms will encounter a stripping canal. This stripping canal will eliminate some electrons on the carbon atom, producing carbon ion with different positive charges. Then, the beam of positively charged carbon ions will accelerate to an even faster speed and pass through a velocity selector. This velocity selector has a known electrostatic and magnetic field and only transmits particles with the proper speed and charge. The speed of the particle has to equal E/B, the strength of the electrostatic field over the strength of the magnetic field. This is because the electrostatic force and the magnetic force applied on the charged carbon ion are in opposite directions. In order to pass through the selector without hitting the walls, these two forces have to cancel out each other. Finally, the charged electrons will pass another magnetic field, which will cause the ions to move in circular arcs and eventually hit a particle detector. Ions with different masses will move in circular arcs with different radius. After measuring the radius, researchers can use calculations to deduce which position on the detector is bombarded by carbon-14 ions, and the particle detector have already recorded the number of ions detected in different locations. Since the volume of carbon-12 is huge for individual ion detection, counts are determined by measuring the carbon-12 electric current created in a Faraday cup which is position before the velocity selector. The number of carbon-12 ions can be calculated using Coulomb’s Law. Therefore, a fraction of carbon-14/carbon-12 can be determined for a given sample.

Radioactive decay of Carbon-14.

The main logics behind carbon dating is that all living organisms contain a trace of radioactive carbon-14. This is because carbon-14 is created from the collisions of nitrogen atoms with neutrons from cosmic rays in space. Carbon-14 will be incorporated into carbon dioxide in the atmosphere, carbon dioxide is absorbed by plants including bamboos, and plants are devoured by animals. This is a simple description of the carbon cycle. While an organism is alive, since it constantly brings in materials and expels wastes, it maintains the same carbon-14/carbon-12 ratio as found in the atmosphere. However, after the organism dies, this ratio is not maintained through biological activities. The amount of carbon-12 will remain the same because carbon-12 is not radioactive. However, carbon-14 is radioactive and will turn into nitrogen-14 through beta emission. Mathematically, this decay can be modelled exponentially. The half-life of carbon-14, or the time required for a given amount of carbon-14 to reduce to 50% of its original value, is 5568 years according to Willard Libby. Therefore, to determine the correct date of the bamboo slips, we calculate the fraction of modern carbon (Fm): the isotopic ratio of the sample to the standard (a given constant)—the activity of mass 14 carbon in 1950 AD before the perturbation of the signal caused by fossil-fuel burning. The equation for the age of the bamboo slips is calculated through the following, where 8033 is called the Libby decay constant (the mean-life derived from Libby's half-life of 5,568 years).

Age = -8033*ln(Fm)

Using AMS, the researchers were able to determine that the Tsinghua Bamboo Slips were created in 305±30 BC. Carbon dating not only proves the authenticity of the bamboo slips but also establishes the fact that these bamboo slips are created during the mid-to-late Warring States Period.

Light Microscopy

A compound microscope in a Biology lab.

Even simple light microscope can give us a lot of useful information about the bamboo slips. After dying the sample, scholars can view the structure of the bamboo slips clearly under the microscope. The hollow bamboo is made of three components: the outer skin, the inner skin, and the culm wall. The outer skin, by its definition, is the outer surface of the bamboos. It has a hard texture, smooth surface, and tissues that are densely populated. The inner skin covers the inner cavity of the bamboos. It also has a hard and brittle texture. Compared to the outer skin which has a light green color, the inner skin has a light-yellow color. The culm wall lies between the inner and the outer skins. The culm wall is made of parenchyma and vascular bundles. The vascular bundles are dispersed inside the culm wall with certain patterns. From outside to inside, the vascular bundles change from a collection of many dense and small bundles into a collection of a few big and sparse bundles. The growth pattern of these vascular bundles can also offer a lot of information. There are three different types of growth patterns: not differentiated, half differentiated, and fully differentiated. Analyzing the growth pattern of the vascular bundles, the scholars can determine the kind of bamboo used in making the slips. For example, the bamboo slips donated to Peking University were sampled and imaged under Zeiss Axioskop 40 biology microscope. The researchers found that all samples showed fully differentiated growth pattern, which is a special characteristic of phyllostachys. The genus of bamboos used to make bamboo slips depends on the dynasty and location. By cross analyzing the species of sample bamboo slips and other well-known historical relics, scholars can obtain a good estimate of the date and location.

Under the microscope, the thread used to tie the bamboo slips together can also offer a good amount of information. The common types of fiber used in Ancient China includes ramie, hemp, flax, and velvetleaf. The magnifying power of microscope can be used to measure the length of these fibers precisely to determine the species. The fiber length of ramie ranges from 60-250 millimeters. Hemp and flax are typically shorter compared to ramie, with the fiber length of 5-55 millimeters and 9-70 millimeters respectively. Velvetleaves, however, only have a fiber length of 2 to 4 millimeters. When comparing hemp and flax which have similar fiber length, researchers also need to look at the cross section of the fiber. Flax usually has an oval-shaped cross section, while the cross section of hemp is a irregular polygon. Another distinction between flax and hemp is the width of the fiber. Flax fibers are wider with an average of 50 micrometers, while hemp fibers are thinner with an average of 25 micrometers. For the case of bamboo slips donated to Peking University, although the threads have been seriously rotten, scholars were still able to observe the wide and oval shape of the fiber near the bend position under the microscope, which is typical for flax. This finding matches previous discovered bamboos slips made during the West Han dynasty, which also contained flax threads.

Laser-induced Raman Spectroscopy

Schematic of one possible Raman spectroscopy setup using CCD detection (adapted from Thomas Schmid; Petra Dariz (2019). "Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions". Heritage. 2 (2): 1662–1683. doi:10.3390/heritage2020102. ISSN 2571-9408.).

Raman spectroscopy, named in the honor of its inventor C.V.Raman, is a versatile tool of analyzing a wide range of chemical samples, both qualitatively by measuring the frequency of the scattered radiations (type of chemical bonds) and quantitatively by measuring the intensity of scattered radiations (number of chemical bonds). This technique is based on Raman effect, the inelastic scattering of radiation when photons interact with vibrating molecules of the medium. The rainbow-colored light obtained by passing natural light through a dispersive prism is an example of Raman effect. In Raman spectroscopy, sample is illuminated with a monochromatic laser beam which interacts with the molecules of sample and originates a scattered light. A Raman Spectra caused by inelastic collision between incident monochromatic radiation and molecules of the sample can be constructed. There are two different kinds of scattering. Rayleigh scattering will make the scattered radiation and the incident radiation have the same frequency, while Raman scattering will result in different frequencies between the incident light and the scattered light. If the end state has higher frequency than the beginning state, this shift in frequency is called the Stokes shift. If the end state has lower frequency than the beginning state, this shift in frequency is called the Anti-Stokes shift. The Raman spectrum is presented as an intensity vs wavelength shift. Every chemical contains chemical bonds between different atoms. The wavelength frequency can indicate which types of chemical bond is present in the chemical. This is because the change in frequency depends on the polarizability of the bond. If a bond is strongly polarized (C-O, N-H, O-H), vibration will cause a small change in the bond length and weak Raman scattering. Relatively neutral bonds (C-C, C-H , C=C), however, suffer large changes in polarizability during a vibration. The bond length changes a lot and causes strong Raman scattering. The intensity of the peak on a Raman spectrum can also indicate the relative amount of the corresponding chemical bonds present in the chemical.

The red color was used in many places in the Peking University bamboo slips, mostly for separation lines and pictures. Laser-induced Raman Spectroscopy can be used to analyze the chemical components of the paint without damaging the sample. Using the Thermo Nicolet Scientific Laser-induced Raman spectrometer, researchers from the Chinese National Museum confirmed that the red pigment was made of cinnabar. Cinnabar has many colors including dark red, bright red, orange, and etc. The darkness of the red color depends on the diameter of the particle. Higher diameter will result in a darker red color. Cinnabar is a powerful dye but is very light sensitive. Under the exposure to the sunlight, the hexagonal crystal system of cinnabar will change into tetragonal crystal system, darkening the red color. The red pigment from the Peking University bamboo slips have a bright yellowish orange color. This indicates that the cinnabar used was ground thoroughly into small particles, and the slips had limited exposure to the sunlight. Both conditions are very rare cases for bamboo slips, which increase the historical values of these relics.

Imaging Techniques

Digitalizing bamboo slips can be very challenging. After thousands of years buried underneath the earth, most of the bamboo slips have been rotten, and the ink on the slips has been washed away by the water, leaving the scholars with minimal trace of the Chinese characters on it. Another issue is that most of the bamboo slips have been contaminated with soil. Dusting away the soil from the scrolls can simultaneous damage the scroll or erase the ink writings. Moreover, many bamboo slips were rolled up and stored in scrolls. Due to the delicate structure of the bamboo slips, unrolling the bamboo slips may incur damages on them. To solve these problems, researchers use modern imaging technologies such as infrared thermography or 3-D X-ray CT scanning as alternative ways to obtain images of bamboo slips non-invasively.

Infrared Thermography

Thermography of my cats. Wavelength: 8 to 14 micrometer, Sensor: Microbolometer 320x240, Lens: Germanium triplet For those who are not browsing the Thermography set

Infrared thermography (IRT), or thermal imaging, takes advantages of the infrared radiation emitted by all objects. According to Planck’s Law, or commonly known as the black-body radiation law, every physical body spontaneously and continuously emits electromagnetic radiation, and the emissive power called the spectral radiance of the body denoted by the letter B has a positive correlation with temperature. These radiations come in relatively low frequency (9000-14000 nanometers), difficult to be detected by human eyes. However, thermographic cameras can capture these images of radiation, enabling researcher to observe the object with or without visible illumination.

When the bamboo slips are exposed to infrared light, the area covered by ink and the area not covered by ink will have a distinguishable difference in the absorption of the infrared light. This difference can be detected by the thermographic cameras and processed into images. The science basis behind this is that different materials have different heat absorption capacities when exposed to infrared light. This technique is very useful in the case of studying bamboo slips because after soaked in water for thousands of years, the characters on the bamboo slips can be very difficult to detect. However, even for the case where the ink is completely washed off by water, the remnant ink chemicals can still make a difference on a thermograph.

3-D X-ray CT Scan

An elderly man appeared in the ER incoherent and cachectic. Brain CT scan was interpreted as showing extensive edema in the left frontal lobe that appears to be related to an underlying 23 mm mass (at arrow). There is a second large area of edema in the left parietal-occipital region.

Computer tomography (CT) was the first non-invasive radiological method allowing the generation of tomographic images of human body. It is typically used in the medical field but has many other applications including archaeology. In 1917, Johan Radon first mathematically demonstrated the possibility of reconstruction of a 3D object from multiple projections. However, the math is too complicated for this technology to be used in reality. It was not until 1963 when Allan Cormack introduced the Fourier Transform in the calculations and made this invention possible. The engineer that designed the first CT machine was Godfrey Hounsfield who won the Nobel Award in Physiology or Medicine in 1979 because of this breakthrough. To perform a CT scan on an object, X rays are emitted to penetrate the object at various angles. For the emission at each angle, the detector behind the object is able to detect the intensity of the X-rays. If the X-rays pass through dense material, the X-rays' intensity will be low because diffraction can take place. Otherwise, if the X-rays pass through light material, the X-rays' intensity will be high due to lower amount of diffraction. The detector will automatically change the intensity levels into a gray scale where light color means low density of the object and the dark color means high density of the object. This image is called back-projection. For a single angle, the image will look just like a projection of a 3D object onto a 2D page, which offers limited information about the shape and structure of the object. However, when obtaining this gray-scaled images through numerous angles, mathematical equations (Fourier transform) can be applied to superimpose these images to generate the 3D structure of the object. However, the image acquired from this back-projection scheme is inherently blurry. The solution to this is to amplify the difference in each gray-scaled image before superimposition through a filter. This technique is called the reconstruction algorithm or kernel. The attenuation (or the decrease in intensity) of an X-ray beam through a material can be modeled by the Beer-Lambert Law, where I denotes the measured beam intensity at the detector, I0 the incident beam energy of the X-ray source, d the thickness of a material, and u the linear attenuation coefficient that will be different for each different type of material.

I = I0 * e^(-u*d)

CT offers a great way to acquire the images of the bamboo slips without unrolling the bamboo scrolls or dusting off the soil. Daniel Stromer and his team have invented an innovative way of digitizing soiled bamboo scrolls in a non-invasive manner. The scans of the bamboo scrolls were obtained with a beam geometry micro-CT system consisting of an X-ray source, a flat-panel detector, and a turntable. The turntable rotates 360 degrees to acquire a set of projection images from which a 3-D volume is reconstructed. The distance between the source and the detector was set to be 1377.28mm, and the distance between the source and the object was set to be 507.44mm. When scanning objects with 3-D X-ray CT, the reconstructed volume can suffer from nonexistent artifacts when the X-ray beam length through an object is of high variation. However, because the bamboo scroll was held in an upright position, and the scrolls had a constant diameter throughout the z direction, this artifact was reduced to a minimal amount. After retrieving the CT images, virtual cleaning was done through a Random Sample Consensus (RANSAC) to smooth the output. After virtual cleaning, the bamboo scrolls can be virtually wrapped into a two-dimensional representation using the Enhanced Correlation Coefficient (ECC) image alignment technique. Comparing the photograph of the manually unwrapped scroll versus the digitally unwrapped scroll, Stromer found that his streamlined digitalization method achieved similar quality.