Nanoscale degradation indexes of ancient Chinese and Italian papers for conservation & restoration applications

Joint research project

Project leaders: Mauro Missori, Shan Wang
Agreement: CINA - CACH - Chinese Academy of Cultural Heritage
Call: CNR/CACH triennio 2019-2021 2019-2021
Department: Physical sciences and technologies of matter
Thematic area: Physical sciences and technologies of matter
Status of the project: New

Research proposal

For centuries, a growing number of ancient documents have been accumulating libraries and museums all over the world [1]. Conservators and restorers have as own goal to save these relics. However, at the same time, they want to know what happen in the material during aging and the extent of paper deterioration. This knowledge is critical in order to decide the conservation intervention and apply the best restoration method. In particular, the Chinese government wants to establish general rules and procedures in order to regulate the conservation activity. There is a strong demand of technical specifications and scientific advices to create guidelines and standard procedures based on degradation indexes to choose the best restoration procedures.
Paper is a complex material whose composition depends on the production period and technology employed. Chinese papermakers developed materials and technologies different from the West. From very ancient times, selected cellulose fibers from bast plants, tree bark, stalks of grasses, hemp, mulberry, rattan and bamboo were employed [2]. In order to change the writing performance of the paper, wood ash water, calcium and magnesium carbonates, talcum, and vegetable starch as an adhesive were used in the papermaking process. The ancient rags based Italian papers contain mainly cellulose from hemp, cotton and linen and animal glue as sizing compound. It shows high permanence due to beneficial action of the lime added to animal glue. Later on, modern soft or hardwood paper may include lignin and additives such as alum, resin or chalk [3].
In the paper relic system, every element is of great unity and harmony with the outer environment through energy flow, material cycle and physicochemical reactions (surface environmental balance). The degradation starts when the environmental conditions changes [2,3]. The degradation always begins at the nanoscale level by hydrolysis of the ²-(1,4)-glycosidic bonds and oxidation of the ²-D-glucopyranose units of cellulose polymers. These processes gives rise to development of acid byproducts, carbonyls acting as chromophores and recrystallization of cellulose fibers; macroscopically, to the weakening of the mechanical properties of the sheets and their yellowing [3].
Although hydrolysis and oxidation have clear physical and chemical consequences, conservation scientists have no proposed so far specific indexes for paper degradation, apart pH, as a reference for restorers and conservators. In Italy, in the Technical specifications for paper restoration of the Italian Minister of Cultural Heritage (MiBACT) the value of paper pH is recommended as the unique parameter in order to decide the conservation intervention (de-acidification when pH is less than 5) [4].
The aim of this joint research project is to develop degradation indexes of ancient Chinese and Italian papers for restoration applications. These activities require the use of specific physical and chemical diagnostics technique at the nanoscale suitable for the cellulose based materials. In this contest, the Italian and Chinese units have great experience.
Reflectance spectroscopy will be used for non-destructive measurement of the concentration of chromophores due to the oxidation of cellulosic hydroxyl groups to either carbonyl or carboxyl [3]. Consequently, degraded cellulose shows a broad absorption spectrum peaked at about 4.8 eV (260 nm), with an absorption tail towards the visible range. Only recently, experimental and theoretical studies have clarified the spectral response of the carbonyls responsible for the yellowing of ancient paper, allowing the measurement of their concentration [5, 6]. The practical diagnostic method can be performed in-situ by using a simple portable set-up. It was applied to the famous Leonardo da Vincis's self-portrait [6]. These results will be confirmed by FTIR-ATR non-invasive technique [3], able to identify the degradation products (in terms of oxidation index) of paper material.
Chromatographic analysis will be used in order to evaluate the amount of acid compounds, produced during the aging and present on the paper artwork, confirming the data obtained by the reflectance measurements and improving the pH index. The Italian Unit optimised an easy chromatographic procedure, based on high-pressure liquid chromatography (HPLC) with UV-vis detection, able to identify the acid degradation production of cellulose, using a sponge for sampling or analysing the water after the cleaning treatment. In particular, the attention will be focused on ascorbic, malic, lactic, oxalic, citric, and succinic acids (main cellulose degradation products).
In order to study the degradation at the nanoscale artificial aging in different environmental conditions will be performed by the Chinese research group. Scanning electron microscope, fiber analyzer, FTIR and other paper physical property test instruments will be employed. Results will be correlated with those obtained at the nanoscale for a full characterization of paper samples, which will be prepared in the paper restoration laboratory of CACH. Based on the results, by the identification of degradation indexes, the traditional calligraphy and painting mounting technique and procedures will be improved. The traditional technique will meet higher standard for cultural heritage conservation thanks to the developed degradation indexes.
References
[1] D Hunter, Papermaking: The History and Technique of an Ancient Craft. Dover Publications (1978)
[2] D Gong, et al. (2014), J. Cultural Relics 9,85
[3] T Lojewski, et al (2010), Carbohydr Polym 82,2:370
[4] Capitolato Speciale Tecnico, MiBACT, 2009, http://www.icpal.beniculturali.it/allegati/2009/Capitolato_Speciale_Tecnico_Tipo.pdf
[5] M Missori (2016), Il Nuovo Cimento C 39,2,293
[6] A Mosca Conte, et al. (2014), Appl Phys Letters 104,22:224101
[7] L Micheli, et al (2018), Microchem J 138,369
[8] L Micheli, et al. (2016), Microchem J 126,32

Research goals

1) Comparison of the ancient and traditional hand-made Chinese and Italian papers in terms of production techniques, composition, preservation environment and resulting mechanical, morphological and optical properties.
2) Review of the degradation mechanisms of cellulose in terms of physical and chemical modifications at the nanoscale and correlation with the effects observable at macroscopic scale in Chinese and Italian paper relics, such as the variation of mechanical and optical properties.
3) Develop systemic non-destructive diagnostic tools able to provide information of the physical and chemical degradation mechanisms of Chinese paper at the nanoscale. Methods will be based on optical and vibrational spectroscopies as well as on chromatographic methods.
4) Building of a map of degradation indexes of paper from nanoscale phenomena as a reference for the selection of restoration procedures and materials for Chinese and Italian relics in comparison and improving the Technical specifications for paper restoration of the Italian Minister of Cultural Heritage (MiBACT).
5) Supply of the degradation indexes map for Chinese ancient paper in comparison to Italian one's.
6) Apply diagnostics methods to unaged and aged materials to identify their durability, also by using artificial aging.
7) Evaluation of the use of some new suitable materials for restoration based on the results of the diagnostic characterization of Chinese ancient paper.

Last update: 22/11/2024