Technologies in 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

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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.

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