Studies On The Carbonization Mechanism Of Ancient Silk Textiles And Corresponding Countermeasures

China is the first country to cultivate silkworms,reel silk and weave silk fabrics.Silk textile has witnessed the development and evolution of the ancient civilization and is one of the symbols of the splendid Ancient Chinese culture.Therefore,it is one of the most important historical materials in studying the ancient Chinese civilization.However,only quite small part of ancient silk textiles has survived during the long-term burial underground.Furthermore,most of the unearthed silks are suffering from discoloration,strength weakening,carbonization,etc.due to various factors such as temperature and humidity.Up to date,the carbonized silk textiles have not drawn enough attention and the in-depth basic theory on the conservation of ancient silks has not been established.The present conservation theory failed to provide effective guidance on the conservation work.Therefore,the study on the carbonization mechanism of ancient silk textiles becomes quite meaningful on the present conservation process.In this paper,in-depth and systemic investigations on the carbonization mechanism and corresponding conservation strategies have been performed aiming at solving the problems and meeting the requirements in the conservation of carbonized silks based on the survey of large amount of literatures.Both macro-and micro-characters of carbonized silk textiles have been studied using various techniques,including video microscope,scanning electron microscope(SEM),Fourier Transform Infrared Spectroscopy(FTIR),Raman spectroscopy,X-ray diffraction(XRD),X-ray photoelectron spectroscopy and electron paramagnetic resonance(EPR),to summarize their common typical features and provide evidences for the simulation of silk carbonization experiments.Results reveal that carbon free radicals were produced during the carbonization,accompanied by serious discoloration and appearance of amorphous carbon.In the perspective of molecular level,the characteristic infrared absorption peak of amide ?,? and ? get weakened by different levels indicating the rupture of amide bonds.Modern blank silks were heated under helium atmosphere using TG-FTIR-MS combination equipment,which is able to conduct the in-situ detection of the decomposed small gas molecules.Characterization results of the residuum via FTIR,Raman,XRD,XPS and EPR indicate that it possesses all of the essential features of carbonization,which proves the reasonability of the aforementioned method.As a result,the gas that generated during the carbonization process of silks is mainly composed of CO2,NH3 and H2O,as well as small amount ofSO2 and carboxylic acid.Thereby,the carbonization of silk can be divided into the following three procedures:?The volatilization of free water and bound water in silk protein,and the disulfide bonds ruptured in the S-containing amino acids such as methionine and cystine,and thus SO2 were generated.? Decarboxylic reaction occurred in the acidic amino acid residue such as glutamic acid and asparaginic acid to produce CO2.Due to the instability of amino acid residues,the amido bond in the main chain of silk fibroins broke which generated NH3,meanwhile,decomposition at different levels occurred on tyrosine,phenylalanine,lysine,tryptophan and serine in the side chain.Mass spectra results indicate that the degraded products of tyrosine contains phenol,4-methyl phenol,hyacinthin and 4-hydroxy phenylethylamine;the products of decomposed phenylalanine contains methylbenzene,ethylbenzene,phenethylamine and phenylethylene;the decomposition of lysine produces the hydrogenated pyrrole,pyrrole and 3-amino-nitrogen heterocyclic heptane;the product of decomposed tryptophan is consisted of benzpyrole,3-methyl indole and 3-ethyl indole;the H2O generated by the decomposition of serine leads to the hydrolyzation of silk fibroins and carboxylic acids are produced,which is decomposed into CO2 due to the instability.? Stable amino acid started to degrade under heatingor longtime burial.Finally,elements such as H,N and O were sloughed and elemental carbons with a certain degree of crystallinity were generated.The EPR spectrometer was applied to the mechanism of carbonization of silk in the perspective of residue,and carbon free radicals were firstly discovered in carbonized ancient silk textiles.The signal is an asymmetric single peak without hyperfine structure,the width from peak height to peak valley is approximately 1 mT and the g-factor is 2.0037.These characters are quite in accordance with that of carbon free radicals found in coal.Besides,the formation mechanism of carbon free radicals was studied based on the theory of free radical chemistry.The results showed that the carbonization of ancient silk fibroins is mainly resulted from the rupture of the amido bond,which is consistent with the previous TG-FTIR-MS results.The unveiled carbonization mechanism of ancient silks is more precise and convincible through the mutual corroboration of the analyses on both gas products and residues of thermal aged samples.In order to evaluate and demarcate the carbonization degree of ancient silk textiles,the concept of Degree of Carbonization is firstly proposed via measuring the concentration of carbon free radicals.The feasibility of this method in evaluating the carbonization degree of silk has been proved by comparing with the residual tensile strength.Satisfactory results have been achieved in the conservation ofancient silks.In the last part,tea polyphenol(TP)and superoxide dismutase(SOD)were selected as alternative inhibitors aiming at suppressing the aforementioned carbon free radicals in ancient silk textiles.Based on a series of non-repeated two-factor cross grouping tests and corresponding variance analyses,we found that the two free radical inhibitors performed quite similarly in obliterating carbon free radicals in simulated carbonized silks in circumstances that proper concentration of inhibitors and sweeping time have been chosen.However,since TP is one of polyphenol type materials which can be easily reacted to be acidic,it is harmful to the conservation of silk textiles.Therefore,the SOD is designated to be the proper inhibitor of carbon free radicals.The SOD at an appropriate concentration was utilized in the obliteration of newly generated carbon free radicals in ancient samples and satisfactory results have been achieved.Furthermore,the appearance and tensile strength of as-treated ancient silks were not significantly changed,confirming to the basic principles of cultural relics preservation.The study of the carbonization mechanism ofancient silk textiles makes the aging mechanism of silk textiles supplemented.Meanwhile,the research also provides theoretical basis for the conservation,the application of which is expected to rescue precious silk relics,it will be of great significance for the inheritance of Chinese ancient civilization.