Chromatic alterations induced by preservation treatments on paper: The case of Ag-functionalized nanocrystalline cellulose

Abstract

Artifacts made of paper (books, manuscripts, and other documents) represent a valuable part of cultural heritage. They are particularly fragile compared to other objects due to the multiple physical, chemical, and biological factors able to degrade the paper basic constituents, i.e., cellulose, hemicellulose, and lignin. Moreover, hemicellulose and lignin represent an ideal growth substrate for proliferation of microorganisms and fungi. Regarding this latter aspect, researchers have experimented several methods to prevent and control fungal development on paper documents, but, sometimes, these treatments present themselves collateral effects, being responsible for the variation of paper chromatic appearance. Recently, a suspension made of crystalline nanocellulose with the adding of silver nanoparticles was tested as antifungal method. Actually, the crystalline nanocellulose has been already experimented as starting material for preservation and consolidation treatments on paper, but the adding of silver nanoparticles also provided the suspension with a broad-spectrum antimicrobial action. The aim of the research is to investigate the potential effects of the so-prepared suspension in modifying the optical characteristics of the treated paper. To do that, two different types of paper were analyzed: Whatman and Amalfi. Squared paper samples (3 cm side) were prepared, subjected to artificial ageing, and then inoculated with nanocrystalline cellulose (CNC) and with Ag-functionalized nanocrystalline cellulose (CNC/Ag). Spectral reflectance measurements were performed on the samples, and the CIE L*a*b* chromatic coordinates under D65 were obtained. Comparing the measurements referred to samples treated with CNC and those treated with CNC/Ag, it has been possible to describe the effects due to the presence of silver in the suspension. Obtained results demonstrate that the chromatic variations of the samples treated with CNC/Ag are higher than those referred to the samples treated with CNC. Specifically, the chromatic variations ΔE*D65 range from 0.2 to 10.5 for the Whatman paper and from 0.3 to 5.1 for the Amalfi one. Moreover, it has been shown that the two types of paper react in a different way to the treatments and that the alterations are not homogenous on the entire paper sample, but more evident in the inoculation region in which pinkish spots arise. Finally, it has been proved that the alterations are not stable over time, but, especially for Whatman paper, they become more evident after one month and half from the preparation of the samples.