Technologies Used in the Study of Bamboo Slips

Bamboo slips or bamboo strips (Chinese:简牍) were the most important form of book in ancient China before the invention of paper. Ancient Chinese people chopped bamboo 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 ropes were used to tie the slips through the holes into rolls or pages (Chinese:册).

Bamboo slips had profound impact in the Chinese book history. The 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 were Tsinghua Bamboo slips, a collection of 2388 bamboo slips donated to Tsinghua University by an alumnus. The most astonishing finding 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 four different parts how new technologies enable archeologists to better study bamboo slips: conservation, authentication, image acquisition, and image post-processing.

Conservation

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 a collection 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.

With the assistance of water however, 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 a major problem. After the excavation of the bamboo slips, the temperature, pH, and the water content of the environment will dramatically change. If left unattended, water molecules will evaporate from the intervals between celluloses, 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 perpetually infiltrate 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, 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

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

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 induce damages on 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 and 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 increase the concentration of ethyl alcohol solution, sample can be completely dehydrated. Similar method is used in many biology lab 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 regarded as 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 to prove 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 how experts from the Tsinghua University proves that these bamboo slips are not counterfeit artifacts.

Accelerator Mass Spectrometry

The main function of Accelerator Mass Spectrometry (AMS) is to accelerate the different isotopes of carbon to a very high velocity so that mass 14 can be separated and distinguished from math 12 and other rare particles with masses similar to mass 14 carbon atoms. To prepare the sample, the carbon atoms from the sample has to be converted to carbon dioxide gas through a catalytic process. 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 a carbon ion beam. 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 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, calculations can inform us which position on the detector is bombarded by mass 14 carbon ions, and the particle detector can record the number of ions detected in the mass 14 stream. Since the volume of mass 12 is huge for individual ion detection, counts are determined by measuring the mass 12 and mass 13 electric current created in a Faraday cup which is position before the velocity selector. The number of mass 12 carbon ions can be calculated using Coulomb’s Law. Therefore, a fraction of mass 14 carbon/mass 12 carbon can be determined for a given sample.

The main logics behind carbon dating is that all living organisms contain a trace of radioactive carbon-14s. This is because carbon-14s are created from the collisions of nitrogen atoms with neutrons from cosmic rays in space. These carbon-14s will be incorporated into carbon dioxide in the atmosphere, carbon dioxide is absorbed by plants including bamboos, and plants are devoured by animals. 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 activity. 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-time 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 (480–221BC).