Classical analysis of paper
The characterisation of SurveNIR reference collection was an enormous task, for which more than 18,000 working hours were needed, which translates to more than 2 years of non-stop analytical work. However, all this work was needed in order to obtain data based on which the chemometric calculations could be based. The higher the number of samples is, and the more variable they are, the more reliable is the chemometric model.
It was decided that the following parameters are of interest:
- Degree of polymerisation,
- Molecular weight,
- Lignin content,
- Gelatine content,
- Aluminium content,
- Ash content,
- Fibre composition,
- Presence of optical brightener,
- Rosin content,
- Reducing group content,
- Tensile strength,
- Tensile strength after folding.
In the following paragraphs, the methods are shortly described. For more information on paper characterisation, see ref. 1.
Is the average number of repeating D-glucose monomer units in cellulose, which is a linear polymer. The more degraded paper is, the shorter the cellulosic chains are, the more vulnerable paper is and the shorter is its remaining lifetime.
The standard viscometric method was used.2 Prior to the analyses, ash content had to be determined (see below). The method is based on dissolution of cellulose in bis(ethylenediamine)cupric hydroxide and measurement of the effluent time, i.e. the time that 1 millilitre of the solution needs to pass through a narrow capillary. Based on this, the degree of polymerisation is calculated.3
Is determined using size exclusion chromatography, which is an instrumental technique. In order to dissolve cellulose, it is first derivatized (i.e. chemically modified). In a constant flow of solvent, the clear solutions are injected onto a porous solid material. Due to migration of macromolecules into pores, larger molecules are retained less than smaller molecules, which migrate more easily. Based on this principle, the macromolecules are separated and their average size is calculated and so is their average molecular weight.4
The sample size needed for this analysis is indeed very small – samples can be extracted from paper using hollow needles of a diameter less than one millimetre.
is a highly unstable natural organic polymer and its limited photostability is important for works of art which are exhibited and thus exposed to light. During oxidation of lignin, coloured products are formed, which is why newspaper becomes discoloured if left in sunlight. Since discolouration is unacceptable for historical originals, knowledge of lignin content helps us to assess risks associated with discolouration.
Due to its complex structure, quantitative determination of lignin is problematic. We used the acetyl bromide colourimetric method. Using this methodology, lignin is derivatized, dissolved and its content in paper is determined based on the concentration of lignin in the prepared solutions.5
Gelatine was a common ingredient in papers from the 15th to 19th century, and while primarily used as a sizing agent it can enhance paper mechanical properties significantly. It was shown that there is a significant correlation between the condition of historic papers and the content of gelatine sizing.6
The procedure of gelatine determination consisted of extraction and hydrolysis of this protein to obtain free amino acids. The amino acid typical for gelatine, hydroxyproline, was quantified using liquid chromatography. Using this instrumental technique, different compounds are separated in a flow of solvent, which passes through a solid material, with which the compounds of interest interact chemically. Due to different types and intensities of interactions, separation is achieved.7
Acidity (pH) is one of the most important factors influencing the stability of paper. In acid media cellulose is degraded fairly quickly and due to the introduction of alum/rosin sizing of paper in ca. 1850, the lifetime of most papers produced between 1850 and 1990 is estimated at only a few hundred years. This is in a big contrast with paper produced before 1850, which is expected to survive several millennia if stored in good environmental conditions.
Acidity is commonly expressed as pH. The higher the acidity, the lower the pH. This parameter is determined using micro-samples of paper (see above), which are extracted in 5 microlitres of water, in which pH is determined using a micro-combined glass electrode.8
Aluminium (in the form of alum) was added to paper as a precipitating agent both during gelatine sizing and during rosin sizing. Alum is a source of acidity in paper, which is why its content is of interest.
After extraction in acid, aluminium was determined using atomic absorption spectroscopy. At higher temperatures (e.g. in a flame), atoms emit and absorb light of characteristic wavelength – the more intensive the absorption is, the higher is the content of these atoms in the flame. Based on this principle, the content of aluminium in paper extracts can be determined.
Ash content was determined by weighing (gravimetrically) according to the modified (sample mass 0.1 g) standard method. Samples are first dried, then weighed and after ashing at 900 °C, the remaining solid is cooled down and weighed.9
Determination of ash content is a necessary step prior to determination of the degree of polymerisation (see above). Inorganic substances in paper are added to improve colour, texture and other properties of paper.
The fibres used in papermaking mostly originate from either annual plants (cotton, linen, hemp, straw etc.) or from wood. As a rule, wood fibres are of inferior quality, it is therefore important to know the type of fibres contained in paper.
For determination of fibre composition, modified standard methods have been used.10 The type of fibres was determined using dyes (Graff C) which lead to different colouration of different fibre types. The fibres of the same colour are counted and the proportion of each fibre type is calculated.
Optical brighteners are added to paper to enhance brightness. The chemicals used as optical brighteners absorb light in the UV region and emit blue light – the human eye perceives such materials as whiter.
Optical brighteners are commonly contained in washing powders to enhance whiteness of white textiles.
Optical brighteners are instable compounds and degrade quickly, especially if exposed to light. It is thus important to know whether a paper-based object contains optical brighteners or not, in order to assess risks associated with exhibition and potential material yellowing due to exposure to light.
Abetic acid (AA) and dehydroabietic acid (DHAA) are the main components of colophony which was used as a sizing agent in paper production from ca. 1850 - 1990. In order to precipitate the acids onto fibres, aluminium sulphate was used. It is commonly assumed that the use of aluminium sulphate led to a decrease in the paper pH, which led to the well-known low stability of rosin-sized paper. Another potential source of rosin acids could be ground softwood, which can be used in papermaking directly.
For determination of rosin content, the samples were extracted in an acidified organic solvent (acetonitrile) and analysed using liquid chromatography (the same technique as used for determination of hydroxyproline in gelatine, see above).11
The presence of reducing functional groups in paper can be a consequence of production procedures (bleaching, as used in papermaking) or of degradation. An increased content of reducing groups is indicative of more extensive degradation and the parameter is thus of interest in paper conservation.12
A colourimetric method was used, where the paper sample reacts with a chemical compound leading to more intensive colouration in the case of high content of reducing groups and less intensive colouration in the opposite case.
Mechanical properties of paper are the parameters defining its usability. Loss of strength is a natural consequence of cellulose degradation during ageing of paper. Two mechanical properties of paper were determined: tensile strength13 and tensile strength after folding14, as they closely resemble the manual folding test sometimes used in collection surveys.
Tensile strength after folding was performed according to ref. 14. In order to fold paper strips in a standardised manner, its two ends are fixed together on a plane with 20° inclination and a cylinder of 460 g is triggered to roll down the plane. The tensile strength of the folded sample was measured in the same way as without folding. This test closely resembles manual fold determination.
Ageing and Stabilisation of Paper,
Comparison of Cellulose Molecular Weights Determined by High Performance Size Exclusion Chromatography and Viscometry,
Chromatographic techniques for evaluation of the condition of historical paper,
An improved acetyl bromide procedure for determining lignin in woods and wood pulps,
Gelatin sizing of paper and its impact on the degradation of cellulose during aging,
Determination of gelatine in historic rag papers based on NIR/Chemometrics, Durability of Paper and Writing 2,
A new electrode for micro-determination of paper pH,
Determination of Rosin Acids in Paper, Durability of Paper and Writing 2,
Determination of Reducing Carbonyl Groups in Cellulose in the Solvent System LiCl/N,N-Dimethylacetamide,
Die Beschreibung der Benutzbarkeitsqualität gealterter Papiere in Bibliotheken und Archiven,