Chinese ink sticks: Pyrolysing PAHs for differentiating soot sources

Sooty sources of Chinese ink sticks

Chinese ink has been the black ink of choice for painting and calligraphy for more than 2000 years in eastern Asia and retains its popularity today. It is produced in the form of blocks or sticks which have a long shelf life and are ground with a little water to produce the liquid form. Unlike most modern inks, Chinese ink does not fade when exposed to light, as evidenced by the quality of many ancient examples of calligraphy.

The main reason for its stability is its high content of soot, which is a stable substance. Traditionally, the soot was produced by controlled burning of pine wood or from lamp black formed by burning vegetable oils. It consisted mainly of carbon with organic compounds such as polycyclic aromatic hydrocarbons (PAHs).

The best soot was made by burning the pine wood in inverted pottery jars to trap it. After removal using feather brushes, it was mixed with glue made from animal hides or horn, to act as the binder. Sometimes, perfumes like camphor and cinnamon were also added to conceal the smell of the glue and make the inks more saleable.

The quality and size of the soot particles influences the quality of the aqueous dispersion prepared from the ink sticks and blocks. However, the PAHs might also have an important function for distinguishing between the different types of ink sticks.

A team of scientists based in Europe and China noted that there are no published studies on PAHs in Chinese ink sticks, an omission which they set about to correct.

Shuya. Wei and Manfred Schreiner from the Institute of Science and Technology in Art (ISTA), Vienna, and Xiaoyang Fang and Xuejun Cao from Department of Scientific History and Archaeometry at the College of Humanities Graduate University of Chinese Academy of Science, Beijing, initially examined several commercial samples of pine soot and lamp soot before examining two Chinese ink sticks from the ISTA and two commercial ink sticks produced in China.

PAHs for sourcing soot in ink sticks

The researchers compared the performance of pyrolysis-GC/MS with that of GC/MS alone. For the former, about 0.1 mg of the ink stick was placed in the pyrolyser and heated for 12 seconds at 600°C. This was sufficient to transfer the sample through the heated interface to column in a high-speed quadrupole GC/MS system. The same type of column, with a 5% diphenyl-95% dimethylsiloxane film, was employed for the Py-GC/MS and GC/MS experiments.

The eluting compounds were analysed by electron ionisation mass spectrometry and identified by matching with a commercial database.

In the first instance, the total ion chromatograms from Py-GC/MS were compared for the pine soot and lamp soot. The main PAHs detected were the same for each soot type: anthracene, fluoranthene, triphenylene, benzo[k]fluoranthene and their isomers. However, the PAH content was far lower in lamp soot than in pine soot, based on the intensities of the main peaks.

This looked promising, so the team examined the intensities of the individual PAHs to see if they could be used to differentiate the sources of soot in the ink sticks. For better accuracy, all of the soots were analysed by Py-GC/MS using selected ion monitoring rather than total ion scan mode, giving reproducibilities of less than 6%.

The levels of five PAHs differed between the types of soot but the clearest indicator was benzo[k]fluoranthene and its two isomers, with a relative concentration of more than 20% in pine soot and less than 17% in lamp soot.

This criterion was used to examine the two ISTA ink sticks, to show that one has been produced with lamp soot and the other with pine soot.

In addition to the PAHs, the chromatograms of the ink sticks also revealed the expected components from the animal glue used as a binder. They also identified camphor, borneol and isoborneol. Camphor was probably added to mask the glue smell and the borneols to prevent the ink sticks from deteriorating, said the researchers.

The same samples were analysed by GC/MS but this technique had the notable disadvantage of being unable to detect the PAHs, even though it found small amounts of fatty acids which Py-GC/MS failed to detect. This anomaly was explained in terms of relative sample size. Py-GC/MS analyses were carried out with 5 times less sample than GC/MS, so levels of the minor fatty acids were too low for detection in the total ion chromatograms.

So, pyrolysis-GC/MS can be used to differentiate between the types of soot used in modern and ancient Chinese ink sticks by comparing the signal intensities of the PAHs, notably benzo[k]fluoranthene and its isomers. The technique also detects the animal glue components and other additives such as camphor and borneol.

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