Near-Infrared Spectroscopy and Machine Learning for Accurate Dating of Historical Books

Non-destructive, fast, and accurate methods of dating are highly desirable for many heritage objects. Here, we present and critically evaluate the use of near-infrared (NIR) spectroscopic data combined with three supervised machine learning methods to predict the publication year of paper books dated between 1851 and 2000. These methods provide different accuracies; however, we demonstrate that the underlying processes refer to common spectral features. Regardless of the machine learning method used, the most informative wavelength ranges can be associated with C-H and O-H stretching first overtone, typical of the cellulose structure, and N-H stretching first overtone from amide/protein structures. We find that the expected influence of degradation on the accuracy of prediction is not meaningful. The variance-bias decomposition of the reducible error reveals some differences among the three machine learning methods. Our results show that two out of the three methods allow predictions of publication dates in the period 1851-2000 from NIR spectroscopic data with an unprecedented accuracy of up to 2 years, better than any other non-destructive method applied to a real heritage collection.

Determination of Cellulose Degree of Polymerization in Historical Papers with High Lignin Content

Determination of cellulose degree of polymerization (DP) is one of the most commonly used methods in paper degradation studies, performed either by a standardized method using viscometry (as average degree of polymerization (DP_v)) or size-exclusion chromatography (SEC) (as weight average molecular mass (M_w)). Due to the insolubility of papers with high lignin content in cupriethylenediamine (CED), such as groundwood papers, viscometric determination is not possible; therefore, pretreatment is required to allow subsequent dissolution of the papers. In this study, the pretreatment of historical papers containing groundwood with sodium chlorite in acetic acid was investigated, which enables dissolution of the paper samples in CED and determination of the cellulose average degree of polymerization by viscometry (DP_v). Kappa number was determined to estimate the lignin content in the papers. The suitability of SEC UV-VIS analysis for determination of M_w in papers with high lignin content had been verified before it was used as a comparative method for viscometry. Using SEC, changes in the weight average molecular mass (M_w) of cellulose tricarbanilate (CTC) derivative during delignification were evaluated. The results indicate that no significant depolymerization occurred in the selected samples under the studied delignification conditions, which was additionally confirmed with determination of monosaccharides by ion chromatography. The results of the M_w determinations by SEC and DP_v by viscometry are in good correlation, justifying the use of viscometry after chlorite/acetic acid pretreatment to determine the cellulose average degree of polymerization in historical papers with high lignin content.

Stabilisation Treatments for Paper with Green Copper Pigment Verdigris

The focus of this research is the stabilisation of paper with Verdigris, a green copper pigment. Due to its corrosive effects on paper, many important documents, paintings and maps are in danger. The efficiency of several commercially available deacidification agents on paper samples with Verdigris was tested, including magnesium-based Bookkeeper® dispersion, nano calcium hydroxide containing Nanorestore® and CaLoSiL® dispersions as well as a recently developed dispersion of nano calcium carbonate. The antioxidant tetrabutylammonium bromide was tested either alone or in combination with nano calcium based deacidification agents. The effect of the treatments was evaluated using colour, tensile strength, degree of polymerization and pH measurements. The results indicate that acidic degradation does not play a major role during accelerated degradation of paper containing Verdigris with moderately acidic pH value and that oxidative decay could be the main culprit of the decay. The method described, which involves the use of antioxidant tetrabutylammonium bromide in combination with calcium carbonate based deacidification dispersion, was proved to have a superior effect against degradation of paper with Verdigris in comparison to the treatments which involve deacidification agents only. Nano calcium hydroxide based deacidification agents result in high pH values of the paper samples and therefore cannot be advised for use on paper documents.

Comparison of Trimethylsilyl Cellulose-Stabilized Carbonate and Hydroxide Nanoparticles for Deacidification and Strengthening of Cellulose-Based Cultural Heritage

Herein, colloidal dispersions of alkaline nanoparticles (NPs: CaCO₃ and Mg(OH)₂) are stabilized by trimethylsilyl cellulose (TMSC) in hexamethyldisiloxane and employed to treat historical wood pulp paper by an effortless dip-coating technique. Both alkaline NPs exhibit high stability and no size and shape changes upon stabilization with the polymer, as shown by UV-vis spectroscopy and transmission electron microscopy. The long-term effect of NP/TMSC coatings is investigated in detail using accelerated aging. The results from the pH-test and back-titration of coated papers show a complete acid neutralization (pH ∼ 7.4) and introduction of adequate alkaline reserve even after prolonged accelerated aging. Scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and infrared and water contact angle measurements showed the introduction of a thin and smooth hydrophobic NP/TMSC coating on the paper fibers. Acid-catalyzed desilylation of TMSC was observed by declining C-Si infrared absorbance peaks upon aging. The CaCO₃ coatings are superior to Mg(OH)₂ with respect to a reduced yellowing and lower cellulose degradation upon aging as shown by colorimetric measurements and degree of polymerization analysis. The tensile strength and folding endurance of coated and aged papers are improved to 200-300 and 50-70% as illustrated by tensile strength and double folding endurance measurements.

Evaluation of Treatments for Stabilization of Verdigris and Malachite Containing Paper Documents

Copper ions and acids lead to enhanced degradation of the paper carrier. Stabilization treatment of documents with pigments such as verdigris and malachite should therefore include addition of alkalis to combat acid hydrolysis of cellulose and antioxidants to retard oxidative degradation catalysed by copper ions. The paper reviews present approaches for stabilisation of green copper pigments. Furthermore, two recently proposed treatments containing tetraalkylammonium bromide antioxidants and alkali are evaluated. The first treatment is based on interleaving of paper samples containing malachite or verdigris pigments with papers, impregnated with alkaline buffer and antioxidant at elevated relative humidity and applied pressure. The second one is based on the use of non-aqueous suspensions of nano calcium carbonate and/or magnesium oxide particles and tetrabutylammonium or tetrapropylammonium bromide. Size exclusion chromatography is used to monitor the extent of degradation of samples during accelerated ageing, while colour changes of pigments are determined using colourimetry. Results demonstrate that effective stabilization of paper samples can be achieved with tetrabutylammonium bromide and calcium carbonate containing interleaves at 90% RH. When non-aqueous immersion treatment is applied, tetrabutylammonium bromide at a concentration 0.09 M in combination with calcium carbonate nano particles had the most beneficial effect on the paper as well as on the colour of the copper based pigments.