Spectroscopic analysis of historical documents

Abstract

The aim of this was work to assess the degradation risk and suggest conservation methods of certain historical documents. All the documents studied were obtained from the National Library of South Africa except one that was obtained from a private library at the city of Timbuktu, Mali. Five methods were used to carry out the assessment, namely Abbey pH pen test, X-Ray Flourescence (XRF), Mössbauer spectroscopy, Fourier Transform Infrared Spectrometer (FTIR) and Polarised Light Microscopy (PLM). The Abbey pH pen method was used to check the pH level of all the samples. It was found that all the samples except two were acidic. These are the Het Leven and South Africa (Barrow) samples. The sample from the Timbuktu manuscripts was found to be extremely acidic with a pH level of below 5.0. It is recommended that all the acidic samples be de-acidified using the Bookkeeper process in order to retard the process of degradation due to hydrolysis. Energy dispersive X-ray fluorescence technique was used to study the elemental composition of the above mentioned samples. In all the samples six elements namely Fe, Cu, Mn, Ca, K and S were detected. It was found that older documents had higher concentrations of Ca and hence have a considerable alkaline buffer than recent documents. It was also observed that the levels of Ca dropped significantly in the samples dating between 1800 and 1890 coinciding with the period during which paper making technology changed. The concentrations of K and S also decreased around 1890. Iron remained considerably high and was detected in all the samples. Copper and manganese were found to be at very low concentrations compared to Fe. This research confirmed that Fe has the potential to impact negatively on paper permanency unless de-acidification is undertaken because of relative abundance of Fe compared to Cu. The valence state of iron in five of the samples studied was determined using the Mössbauer spectroscopy. This is the first time paper is studied in this way. This technique was time consuming because of the trace quantities of Fe in the samples. Spectrum collection lasted for several days making the method unsuitable for analysing many of samples normally found in libraries and archives. It was found that all the samples had Fe³⁺. Only one sample namely, the Wildsport of Africa sample showed the presence of both Fe²⁺(21%) and Fe³⁺(79%). The detection Fe²⁺ in the Wildsport of Africa sample showed that oxidative degradation is also occurring in this document. It was recommended that the use of radical scavengers or chelation of transition metals in paper be used to slow down oxidative degradation of the historical documents. The nature of the fibres that make up the samples studied was determined using FTIR. The results showed that all the samples were made of cellulose. The Courier sample was also found to have both lignin and hemicellulose. The Total Crystallinity Index (TCI) of each of the samples was also calculated in order to determine the susceptibility of cellulose to degradation agents. This index is the ratio of the integrals of the FTIR band at 1372 cm­­-¹ to that at 2900 cm-¹. The integrals were taken over the ranges 1390 – 1339 cm-¹ and 2959 – 2830 cm-¹ for the bands at 1372 cm-¹ and 2900 cm-¹ respectively. The Lateral Order Index (LOI) was also calculated using the ratio of the integrals of the absorption bands at 1420 cm-¹ to that at 898 cm-¹. These bands are known to be sensitive to the relative amounts of crystalline versus amorphous structure in the cellulose. It was also observed that all the samples had clay but none of the samples had gypsum as a filler material. Traces of calcium carbonate were found in four of the samples studied. Kaolin was also found in all the samples except two. Only one sample showed the presence of gelatine. The optical properties of the sample fibres were investigated using PLM. Birefringence measurements showed that only one sample was made from linters fibers, seven from hemp and four from rammie fibres. It was also observed that the morphology and birefringence of the fibres were not affected by ageing. This research work showed for the first ever that Mössbauer spectroscopy can be used to determine the valence state of trace amounts of iron in paper. A direct implication of the result of this study is that MS analysis of any sample on paper substrate will also contain information about the valence state of the iron in the substrate. This is particularly important in the MS analysis of inks on historical documents. If the analysis of the ink is done in situ, that is without removing the ink from the paper, it is necessary to separate the MS spectrum of the paper form that of the ink.