Conservation and Restoration of Cultural Heritage Master’s Book and Paper Specialisation
A cross-section is worth a thousand words: a study of methods
for sampling and preparation of paper cross-sections.
Master’s Thesis
Student: Julia Owczarska Student number: 11354011 Supervisor: Bas van Velzen Second Assessor: Femke Prinsen 19 June 2018
Table of contents
Abstract 4
English 4
Nederlands 6
1. Introduction 8
2. Current state of knowledge 10
2.1 Cross-sections in conservation 10
2.1.1 Cross-sections in paper conservation 11
2.2 Cross-sections in the paper industry 12
3. Case study 14 4. Methodology 16 4.1 Research questions 16 4.2 Hypothesis 17 4.3 Experiment outline 17 4.4 Ion milling 18 4.5 Sampling 21 4.6 Embedding procedure 22 4.6.1 Embedding moulds 24 4.6.2 Poly-Pol PS230 25 4.6.3 Technovit 2000LC 26
4.6.4 Methyl methacrylate/N-Butyl methacrylate 27
4.6.5 LR White 28
4.6.6 Unicryl 28
4.6.7 Dr. Spurr’s Low Viscosity 29
4.6.8 Barrier media 29
4.6.9 Finishing steps 30
4.7 Analysis of cross-sections 33
4.7.1 Optical microscopy protocols 34
4.7.2 SEM protocols 35
5. Experimental 38
5.1 Ion milling results 38
5.1.1 Discussion 40 5.2 Sampling 44 5.2.1 Sample holder 45 5.2.2 Cutting methods 47 5.2.3 Results 48 5.2.4 Discussion 50 5.3 Poly-Pol PS230 results 54 5.3.1 Discussion 55 5.4 Technovit 2000 LC results 58
5.4.1 Discussion 59 5.5 Methyl methacrylate/N-Butyl methacrylate results 62
5.5.1 Discussion 63
5.6 LR White results 64
5.6.1 Discussion 66
5.7 Unicryl results 68
5.7.1 Discussion 69
5.8 Dr. Spurr’s Low Viscosity results 71
5.8.1 Discussion 72
5.9 Barrier methods 74
5.9.1 Discussion 75
6. Comparative analysis of cross-sectioning methods 76
7. Conclusion 79
Acknowledgments 81
Bibliography 82
Appendix I: Making of silicone moulds 88
Appendix II: SEM-EDX report 89
Appendix III: Cork embedding 92
Abstract
This thesis project was carried out in fulfilment of requirements for the Master of Arts program of Conservation and Restoration of Cultural Heritage at the University of Amsterdam.
English
Keywords: paper cross-section, cross-section topography, sample embedding. This thesis presents a comparative study of methods for the sampling and preparation of cross-sections of paper. The analytical technique will be examined through a combination of a literary review and experimental study, with the aim of establishing its potential use in the field of paper conservation. The research hopes to introduce preliminary guidelines as well as provide a reference library of images to aid the analysis of cross-sections. A large aspect of the discussion will centre on the accessibility of the discussed cross-sectioning methods.
In conservation, cross-sectioning of samples is an investigative technique for the study of layer structures found in historical artefacts. This analytical technique has a long history of use with materials such as paint, metal or ceramics. In many cases, the cross-sectioning of samples is an established step in object observation and an invaluable aid in formulating treatment proposals. However, this practice has not taken as firm of a hold in the field of paper conservation. Cross-sections of paper sparsely appear in publications and there are no paper-specific guidelines to aid the sampling and preparation process. Many are often reluctant to proceed with this technique in fear of destroying valuable samples. Because of a lack of references, it is somewhat unclear what benefits could be obtained through a paper cross-section or how to analyse them. This thesis aims to address the outlined issues and establish paper as a three-dimensional material benefitting of cross-section analysis. The goal of this research is to improve our understanding of layer structures found in paper artefacts as well as to lay the necessary foundations for further use and study of cross-sections in the field of paper conservation.
The aim is not to provide a rigid cross-sectioning procedure, but rather to outline several flexible guidelines based on the results of the experimental research part of this thesis. In the experimentation, several sampling and cross-sectioning methods have been carried out on wallpaper fragments belonging to the Dutch Foundation for Historical Wall-hangings and Decorations (Stichting Historische Behangsels en Wanddecoraties). Wallpaper was selected as an appropriate case study for this research because it was thought to present a more complex layer
structure than most paper objects. Customarily, wallpapers were pasted one-over-another during redecorations, which often resulted in a complex matrix of paper, adhesive, paint, dirt etc. This complexity allows for presenting a wide-range of different layer structures that can be found in paper objects and which are not limited to wallpaper.
Optical microscopy and Scanning Electron Microscopy (SEM) photomicrograph analyses of paper cross-sections will form the basis for the comparative analysis of cross-sectioning methods. Ion-milled cross-sections will act as a control variable for the experiment and will illustrate how sampling, embedding media and different preparation steps affect the morphology of paper’s layer structures. In conclusion, this thesis explores the reasons why this diagnostic technique needs to be approached differently when it comes to paper and the large variety of photomicrographs is meant to provide reference for the analysis of paper cross-sections.
Nederands
Sleutelwoorden: papier-doorsnede, doorsnede topografie, voorbeeld inbedding. Dit proefschrift presenteert een vergelijkend onderzoek naar methoden voor bemonstering en bereiding van doorsneden van papier. De analytische techniek wordt onderzocht middels een combinatie van literair en experimenteel onderzoek, met als doel het vaststellen van het potentieel van de analytische methode in het onderzoeksveld van papier-conservatie. Het onderzoek hoopt elementaire richtlijnen te introduceren, alsmede een referentiebibliotheek met afbeeldingen te bieden voor de analyse van doorsneden. Een groot deel van de discussie zal gaan over de toegankelijkheid van de besproken methoden.
Bij conservering is het doorslijpen van monsters een onderzoekstechniek voor de studie van laag-structuren die te vinden zijn in historische artefacten. Deze analytische techniek heeft een lange geschiedenis van materiaalgebruik met verf, hout of keramiek. In veel gevallen is het doorslijpen van monsters een vastgestelde stap in object-observatie en is van onschatbare waarde bij het formuleren van behandelvoorstellen. Deze methode heeft echter geen vaste voet aan de grond gekregen in de papierconservering. Dwarsdoorsneden van papier verschijnen zelden in publicaties en er zijn geen papier-specifieke richtlijnen om het bemonstering- en bereidingsproces te ondersteunen. Velen zijn vaak terughoudend met het gebruik van deze techniek uit angst om waardevolle monsters te vernietigen. Vanwege een gebrek aan referenties is het enigszins onduidelijk welke voordelen kunnen worden behaald met een papieren doorsnede of hoe deze te analyseren. Dit proefschrift heeft tot doel de geschetste kwesties aan te pakken en papier te definiëren als een driedimensionaal materiaal dat profijt heeft van een dwarsdoorsnede-analyse. Het doel van dit onderzoek is om ons begrip van laag-structuren in papieren artefacten te verbeteren en tevens de noodzakelijke basis te leggen voor verder gebruik en studie van doorsneden op het gebied van papierconservering.
Het doel is niet om een strikte procedure voor ‘cross-sectioning’ te bieden, maar eerder om enkele flexibele richtlijnen te schetsen op basis van de resultaten van het experimenteel-onderzoekende gedeelte van dit proefschrift. In de experimenten zijn verschillende methoden voor bemonstering en dwarsdoorsnede uitgevoerd op behangfragmenten van de Stichting Historische Behangsels en Wanddecoraties. Behang werd geselecteerd als een geschikte casestudy voor dit onderzoek, omdat het een meer complexe lagenstructuur vertoont dan de meeste papieren objecten. Gewoonlijk werden behangsels naadloos over elkaar heen geplakt tijdens herinrichtingen, wat vaak resulteerde in een complexe matrix van papier, lijm, verf, vuil enz. Deze complexiteit maakt het
mogelijk om een breed scala van verschillende laag-structuren te presenteren die te vinden zijn in papieren objecten en die niet beperkt zijn tot behang.
Optische microscopie en Scanning Electron Microscopy (SEM) microfoto-analyses van papier-dwarsdoorsneden zullen de basis vormen voor de vergelijkende analyse van methoden voor dwarsdoorsneden.
Ion-gefreesde doorsneden zullen fungeren als een controlevariabele voor het experiment en illustreren hoe bemonstering, inbedding-media en verschillende bereidingsstappen de morfologie van de laag-structuren van papier beïnvloeden. Als laatste onderzoekt dit proefschrift de redenen waarom deze veel gebruikte analysetechniek anders moet worden benaderd als het gaat om papier en de grote verscheidenheid aan fotomicrografieën is bedoeld als referentie voor de analyse van papier-dwarsdoorsneden.
1. Introduction
This thesis project was carried out in fulfilment of requirements for the MA degree in Conservation and Restoration of Cultural Heritage from the University of Amsterdam. It was imperative that the research carries an investigative element and therefore this study focuses on the development of a diagnostic tool, the cross-section. Cross-sections are an established and frequently used analytical methods in many fields of conservation, such as paintings, metal or ceramics, however, in paper conservation this technique rarely makes an appearance and there are no paper-specific guidelines for the practice. This research aims to examine the potential benefits of this technique to the field of paper conservation and in the process aspires to optimise the diagnostic method for use with historical paper artefacts.
It has been long established that the use of cross-sections can be of great assistance to the conservator and that the benefits of information gained through analysis of layer structures greatly outweigh the destructive nature of sampling (Gettens 1932, p.20). Unlike many other historical artefacts, paper may initially appear flat and two-dimensional, however, a view from the side can in fact provide the conservator with valuable information. A single cross-section can reveal information about the manufacturing process, artists’ techniques, as well as previous conservation treatments (Graphics Atlas.org, Identification tab). This kind of knowledge undoubtedly would influence conservation choices and could be imperative to selecting an appropriate treatment plan.
Wallpapers are an example of paper artefacts which are composed of many layers, and which are among the most complicated of stratigraphies a paper conservator might encounter in their practice. These complex stratigraphies can consist of paper, adhesive, paint, dirt, varnish, size and/or coating; they are the result of the practice of pasting wallpapers one-over-another during redecorations. This complexity allows for presenting a wide-range of different layer structures that can be found in historical paper objects and which are not limited to wallpaper. It is therefore anticipated that the findings of this research could be applicable to a wide range of paper artefacts. Two wallpaper fragments, belonging to the Dutch Foundation for Historical Wall-hangings and Decorations (Stichting Historische Behangsels en Wanddecoraties) were selected as a case study.
This research project was undertaken based on the supposition that cross-sections of paper would likely provide conservators with valuable information, however, that the preparation methods would have a large effect on the level of clarity and representation of the sampled artefacts. In order to assess the full
potential of this diagnostic tool it is therefore pertinent to optimise the technique for use with paper. It is crucial for the cross-sectioning techniques to be of minimal disturbance to the samples, in order to observe the stratigraphies in their true relationships, and to see the undisturbed interfaces between the layers. It is likely that the negative side effects of cross-sectioning methods cannot be totally avoided; hence this thesis aims to document the ways in which paper is disturbed. It is hoped that this reference information will aid conservators in their analysis of paper cross-sections and will help separate degradation damages inherent to the paper from the damages caused by the cross-sectioning processes, which could interfere with accurate interpretation.
In order to evaluate the tested cross-sectioning methods a clear set of criteria was established and visual observation was chosen for the assessment of results. Visual observation was structured through protocols in order to allow for comparative analysis and the finished cross-sections were observed using an optical microscope (OM) and a scanning electron microscope (SEM). The results will be illustrated in this thesis with the use of photomicrographs.
The thesis is structured in seven chapters and starts with a literature review, which presents the current state of knowledge on this subject. Publications, both from the field of heritage conservation, as well as other sciences and industries, will be discussed in order to trace the history of cross-sectioning as well as note the recent developments and uses of this technique. In the following chapter, the case study will be introduced and it will be further established how the wallpaper fragments can benefit from side analysis. Next, the methodology section will outline the experimental aspect of the research, which will be followed by results and discussion. Finally the sixth chapter will conclude with a comparative analysis of all of the tested cross-sectioning methods. The final conclusion of this research will highlight the positive and negative aspects of the research method undertaken in this project and outline some of the possibilities for further research.
2. Current state of knowledge
Cross-sections have a wide range of applications and various preparation methods have been studied extensively both in the field of conservation as well as in other scientific fields. Many of the techniques traditionally used by conservators have their origin in the biomedical or material sciences and this chapter highlights some of these links in the form of a literature review.
By looking at the history of cross-section use in conservation, it becomes clear what qualities are most required from cross-sections and how these specifications differ from the needs of other sciences. Tracing the trends, and especially through looking at the use of cross-sections in paper conservation publications, it is possible to gauge what purpose cross-sections can serve, and how beneficial they could be to the paper conservator. This information will help construct the evaluation criteria for the cross-sectioning methods tested further on in this research.
Additionally, instead of formulating cross-sectioning techniques from scratch, this chapter will help identify the methods already in use with samples of historical artefacts. Techniques, which appeared most compatible to paper samples, were selected for testing in the experimental part of this research.
A lot can still be learned from the developments in cross-sectioning techniques outside of conservation practices and therefore some of the advancements, which are currently being developed, and which might find their use in conservation, will also be mentioned.
2.1 Cross-sections in conservation
Cross-sectioning of samples has a long history of use in conservation and was already being developed into a standard analytical method in the 1920s with Rutherford Gettens at the forefront of the inquest. Most vigorous development of the technique was carried out in the field of paintings conservation, and this is still reflected in the fact that most cross-sections made today are of paintings.
Initially, natural embedding materials such as dammar, Canada balsam or paraffin wax were used for embedding and the samples were always prepared into thin sections with the use of a microtome (Gettens 1933, pp. 20-28). This however presented many challenges. The natural resins could easily interfere with the samples and limited the visibility. Additionally wax was not strong enough to allow for the cutting of thin slices from harder samples held within. This was
particularly difficult with samples of low binder content which were brittle and prone to delaminating (Plesters, 1956, p.111).
In the 1950’s a variety of new polymer-based resins allowed the field of cross-sections to blossom. New embedding media such as methacrylates, polyesters and epoxies allowed for embedding and microtoming much better than waxes and natural resins.
Soon thereafter it was realised that for the study of layers in objects of cultural heritage thin sections were not necessary. The field moved away from microtoming and adapted the thick sectioning method, which substituted slicing with abrading and was significantly simpler to perform (Plesters, 1956, p.110). The 1990’s were a most recent era of large interest and development in the field of cross-section making, with many relevant conservation publications appearing at that time. Several wide-encompassing studies were undertaken in order to assess the suitability of commercially available embedding materials for use with historical object samples (Rogerson et al. 1999; Tsang et al 1991; Derrick et al. 1994).
The extensive research of the 90’s studied both embedding media, as well as techniques, most of which are still commonly used today in conservation practices. Polyester resins were highlighted with the highest suitability in those surveys and are still frequently used today (personal communication Matthijs de Keijzer, May 2018). Many of the discussed brands have, however since then been discontinued by manufacturers and limited enquiry has been made into materials available on the market today.
New developments in focused ion beam (FIB) technologies are beginning to be investigated in their potential for cross-sectioning of historical artefacts. These systems have been in development since the 70’s and the technique of ion milling has been investigated for the cross-sectioning of paintings by conservation scientists (Boon et al. 2006). For a detailed explanation of the ion milling process see section 4.4.
2.1.1 Cross-sections in paper conservation
Cross-sections never have been and still are not a popular diagnostic tool among paper conservators. The aforementioned hub of activity around cross-sectioning methods in the 90’s seems to have left a wide breadth around the field of paper conservation and up to date no research has been published, which explicitly examines cross-section making from historical paper samples.
However, despite the lack of published literature or guidelines, cross-sections of paper do appear in literature, albeit sparsely. A wide range of issues is addressed through the use of paper sections. For example cross-sections have been used to observe the effect of washing on a glaze (Mosier et al. 1992), or in order to identify materials to help ascertain historical practices and provenance (Wallert et al. 2016). Treatment plans can be formulated with the help of cross-sections, as can be observed in the case of transparent papers, which can stick together due to water damage and form complex multi-layered matrixes (Baker et al. 1989). More recently, the online reference library Graphics Atlas includes images of cross-sections of paper in the identification section of their website (Graphics Atlas.org, Identification tab).
Cross-sections are frequently studied with advanced analytical techniques such as SEM (Mosier et al. 1992), ATR-FTIR (Mazzeo et al. 2007), Raman (Pigorsch et al. 2013) and staining (Kuckova et al. 2013), which further expand the potential of this diagnostic tool beyond optical microscopy.
Lastly, a lot can be learned from the techniques used for cross-sectioning of embedded porous media and especially textiles, where similar fibres are present and same challenges of brittleness and porosity arise (Rogerson et al. 1999). Despite the number of appearances across the literature, the protocols of embedding are rarely included and it is often impossible to trace the sources of inspiration for the techniques used. The prevalence of cross-sections in the field of conservation, in spite of the lack of comprehensive guidelines or publications, proves the need for the development of this diagnostic tool as well as shows the ways in which cross-sections of paper could be used in order to aid the field of paper conservation.
2.2 Cross-sections in the paper industry
The paper industry has significantly different aims and criteria for analysis in terms of paper cross-sectioning. In stark contrast to the field of paper conservation, small sample size is not a concern as large quantities of materials are typically easily available. Furthermore, the stratigraphies present in modern papers differ significantly from what can be found in historical artefacts. Nevertheless, there is a lot that can be learned from industrial practices. The aforementioned FIB techniques have been optimised for paper samples by the industry and in particular the technique of ion milling of cross-sections offers many benefits when it comes to studying the stratigraphies of paper samples (Roede 2016).
Ion milled sections of paper are studied predominantly for reasons such as new product development or quality control. Aspects of paper morphology need to be closely monitored and therefore the analyses of starch (Pigorsch et al. 2012), filler (Roux et al. 2007) and size (Rouchon et al. 2010) distribution have been studied with the aid of a cross-section.
Adaptation and application of these analytical techniques to cross-sections of historical papers could help in determining historical manufacturing processes or object provenance and should therefore be of interest to paper conservators.
3. Case study
Two wallpaper fragments belonging to the Dutch Foundation for Historical Wall-hangings and Decorations (Stichting Historische Behangsels en Wanddecoraties) have been examined in this research project and this chapter will outline the reasons that make them appropriate for this study. The recto and verso of wallpaper fragment #1 can be seen in Fig. 1 and Fig. 2 respectively. Wallpaper fragment #2 can be seen in Fig. 3 and Fig. 4.
Fig. 1 and Fig. 2 Recto (left) and verso (right) of wallpaper fragment #1.
Fig. 4 Verso of wallpaper fragment #2.
Both fragments are presumed to have come from houses in the North Holland province of the Netherlands. They are both decorated with block printed designs on machine-made paper, which indicate that they were manufactured in the nineteenth century. Unfortunately that is the extent of available information relating to the fragments’ provenance.
Wallpapers present perhaps the most complex of layer structures that a paper conservator might encounter in their practice. Complex stratigraphies are often found in wallpapers and stem from the inherently low price of the material. With industrial revolution, came the development of efficient paper machines, which could produce cheap paper on a roll and in large quantities (Hunter 1974, pp. 341-399). Wallpapers soon after became an affordable and quick way of re-decorating interiors. Sometimes with each re-decoration layers of wallpaper would be pasted one-over-another giving rise to complex layer structures (Welsh 2004, 91-104). It is therefore possible that the layers found in such a matrix might consist of several different types of paper, coatings, paints or printing inks, varnish, dirt/alteration layers, adhesive or even wall plaster.
The two fragments in question, upon simple visual observation, appear to consist of two different wallpapers adhered to one another. Because there is the possibility for such a large assortment of elements to be present in addition to the paper layers, it was decided that the stratigraphic study of these two wallpaper fragments would allow the portrayal of a rounded experience of paper cross-sectioning. It is hoped that the results of this research will have application to other paper objects and not be limited to wallpaper alone.
4. Methodology
In order to efficiently carry out the research aims of this project, a clear methodology was established to provide a framework for both the experiments and the analysis. This chapter outlines the structure behind the individual steps taken in this research.
4.1 Research questions
Because studies of cross-sections have not focused on paper before, there were many open avenues this study could have followed. It was established that this project would attempt to answer some of the most rudimentary questions that concern paper cross-sections, in order to lay down the groundwork for further research of paper stratigraphies. Thus the main research aim was:
− To develop an efficient and accessible cross-sectioning method suitable for paper samples and to evaluate how useful this analytical technique is to the conservator.
By determining whether there is a potential for cross-sections in the field of paper conservation it can be established whether there is a need for further development and potentially spark a wide-spread use of this analytical technique, such as has been seen in several other fields of conservation, such as paintings, ceramics or stone conservation. However, before determining whether cross-sections are useful to paper conservators, it should become clear if it is possible to develop the method to the point that it can be accessible, easily performed and present a good depiction of the sampled papers. A set of sub-questions hopes to address these concerns:
− What are the most suitable sampling techniques and how do they affect the stratigraphy of the paper?
− What is an ideal sample size and where should it be taken from?
− In what ways can paper samples be prepared in order to become cross-sections? How effective and accessible are these methods?
− How do the cross-sectioning preparation methods affect the paper and the layer structures?
− How can cross-sections of paper be analysed and what information can be gained from them?
Answering this subset of questions on the practicalities of the cross-section making process will take the guesswork out of the methods and clarify what level of information can be gained through a paper cross-section.
It is important to note that the focus of this thesis is on evaluating the usefulness of cross-sectioning methods to paper conservation in general. In-depth layer analysis of the two wallpaper fragments is beyond the scope of this study, and as such, the interpretation of layers found in the sampled artefacts will be limited to presenting the potentials and/or limitations of the cross-sectioning methods. 4.2 Hypothesis
This research project was undertaken based on the supposition that cross-sections of paper would likely provide conservators with valuable information, but that the preparation methods would have a large effect on the level of clarity and representation of the sampled artefacts. Thus the research was structured around a large and wide-encompassing experiment, which observed paper’s behaviour during a variety of cross-section preparation methods and evaluated the level of layer preservation. It was hypothesized that cross-sections made using the advanced technique of ion-milling would allow for the most accurate and detailed side view of paper, and could be used to comparatively evaluate other cross-sectioning methods.
4.3 Experiment outline
An experiment was planned in order to answer questions listed in section 4.1. The experiment was constructed in order to evaluate the accessibility and efficacy of cross-sectioning methods, at producing a most true representation of the sampled paper artefact, in order to gauge the potential of the analytical technique to the field of paper conservation.
The experimentation consisted of three major pathways: the ion milling of the samples, the procurement of samples and the preparation of said samples into cross-sections. The analysed results all coalesced in the preliminary recommendations for the process of paper cross-sectioning.
In order to assess all of the methods attempted in the experiment, a clear evaluation strategy and criteria needed to be established. Preservation of the layer structures was determined as the most vital source of information to the conservator. A good cross-section was expected to have all of the layers present in the correct alignment and it should be clear where one layer begins and one ends, that is to say layer definition and the clarity of layer margins were of most
importance to this study. To a lesser extent any disturbance to the paper fibres was also analysed, as changes to the paper’s morphology could affect the compactness of the sample and disrupt the preservation of layer structures. Ion milled sections functioned as the control variable and were considered to present an undisturbed side view of a sample; it was then established how far the cross-sections made using several methods compared to this standard. It was possible to observe and evaluate the cross-section’s qualities through visual observation and photomicrographs procured via an optical microscope and a scanning electron microscope. In order to ensure comparability of results all of the imaging was performed using standardised microscopy protocols.
Accessibility of the methods was defined through the ease of preparation, elapsed time, affordability of the components including any equipment needed and lastly any health and/or toxicity concerns.
It is important to note that the experiment was carried out on a small sample size. The two wallpaper fragments presented complex layer structures and common degradation-related problems, which could be present in many other paper objects, however it cannot be assumed that the methods which worked here would be applicable uniformly to all other paper objects. Additionally, because the research was carried out on a historical object, the amount of available samples was limited and each method was attempted only once or twice, which means the study may not be representative.
4.4 Ion milling
Ion milling of samples was the first pathway of experimentation in this research. A sample taken from each wallpaper fragment was selected and sent to be ion milled by a professional paper-oriented laboratory, where paper samples are routinely polished into cross-sections. It was hypothesized that their optimised paper-specific preparation protocols would result in a cross-section with a perfectly preserved layer structure, which would allow an accurate side view at the true undisturbed paper structure.
Ion milling is a system where a focused ion beam is used to ablate materials. The beam causes the sputtering of surface material in a level and uniform way, thereby permitting the cross-sectioning of materials, which are vulnerable to mechanical abrasions necessary in traditional cross-sectioning methods. The process is said to eliminate mechanical stress to the sample and can be optimised for the observation of multi-layer films found in paper (Hitachi High-Technologies Corporation IM4000 manual).
It was suspected that paper would be very vulnerable to forces exerted during mechanical cross-section preparation methods, therefore making the ion milling process appear very advantageous. However, this sectioning method has not been previously attempted with historical paper artefacts and it could not be assumed that the protocols, which are formulated for modern papers, would automatically benefit old papers. Therefore, this experiment attempted to evaluate the effectiveness of the method and assess its potential for the analytical study of historical papers.
Paper can be categorised as a soft material, it is porous and often brittle, which presents challenges for the ion milling process. The wallpaper samples had the added difficulty of being a composite of layers, all with drastically different mechanical properties and at different states of degradation. Although less disruptive than mechanical preparation methods, ion milling could still have an adverse effect to the sample morphology. Any existing cracks could potentially be exacerbated; re-deposition of sputtered material could occur and sample surface could become marred (Weiss Brennan et al. 2014). In analytical study of historical artefacts it is important to distinguish the cracks and irregularities caused by manufacturing processes and degradation from the damages caused by the cross-section preparation methods. The extent of these latter damages will determine the potential usefulness of the technique to the field of paper conservation.
Some of the difficulties caused by the material nature of paper can be counteracted through the combination of embedding and polishing. A paper sample can be embedded in an epoxy resin and subsequently prepared with the ion beam. This method will not be attempted in this experiment as the embedding process would likely disturb the structure of the paper and obscure the layer matrix as was found in a previous study (Roede 2016). An additional disadvantage of embedding prior to ion milling is the possibility of the resin melting due to the heat released during the surface sputtering.
Heat is generated by the radiation and the bombardment by the ion beam and is dependable on the applied voltage and the incident angle. Typical polishers have a range of 0-6kV, which can be adjusted depending on the sample type, desired level of precision and time required. Temperatures could reach as high as 300oC,
which would undoubtedly destroy a paper sample and should be avoided (Kim et al. 2003). In order to minimise damage and keep the layer matrix intact in its most original state, it was decided to polish the samples without embedding and at 5kV. It was estimated that the temperature of the milling would reach around 60oC.
A sample from each wallpaper fragment was selected for the sampling and they can be seen in Fig 5 and Fig 6. The size of the samples was determined through correspondence with the ion-milling specialist. It was determined that the samples should be of at least 3mm x 4mm in size. The samples were cut with a fresh razor blade from an edge of a wallpaper fragment. They were observed under a stereomicroscope to ensure that all the desired layers were present and that the samples were representative of the fragment.
Fig. 5 and Fig. 6 Close up of samples taken from wallpaper fragment #1 (left) and #2 (right). Red line denotes the side chosen for the polishing.
In order to accommodate for potential delamination or unsuccessful attempts at the polishing the samples were cut larger than requested. Their dimensions were ± 5mm x 8mm. The extra material ensured that the procedure could be repeated in case of a failed first attempt. The samples were processed using the standard procedures and settings used for modern-day papers.
The surfaces of the samples were prepared by trimming a straight edge with a razor blade. The resulting cut sample had the dimensions of ± 2mm x 2mm. Sample #1 had to be cut twice as on the first attempt it delaminated and proved too brittle for further processing. On the second attempt the sample withstood cutting well and could be subjected to the polishing.
The cut samples were handled using tweezers and adhered to a silicone wafer with a drop of Loctite EA3430 epoxy. The wafer was then adhered to a specimen mount, a Molybdenum stub, with a hot-melt adhesive as can be seen in Fig. 7. In order to counteract the heat released during the polishing, graphite was smeared around the sample. Overall, the preparation of the sample took one hour, a large proportion of which was due to the drying time of the adhesive. Once ready, the assembly was fit into a specimen holder and into the stage of the Jeol Polisher IB-09010CP to be ion milled.
Fig. 7 Diagram showing the sample mounting procedure for the polishing. (Image courtesy of Thierry Sleypen.)
The ion-beam polishing can be a lengthy process; the exact duration is largely dependent on the thickness of the paper sample. On average for every 50μm of material the device needs 1 hour to complete the polishing. The beam additionally needs to cut through the rest of the mounting system, which can take 2 hours for the silicone wafer and ±30 minutes for the layer of adhesive.
The thickness of the sent samples was measured with a digital caliper. Sample #1 measured 450μm and the sample #2 measured 500μm. With these measurements it was possible to calculate a rough time estimate: sample #1 would take 11 hours and 30 minutes and sample #2 would take 12 hours and 30 minutes.
After successfully completing the polishing process, the sample was coated with a gold layer using a sputter coater, the JEOL JFC-1300 Auto Fine Coater. This layer measures 20-30 nm and aids the imaging process. The sample was fitted into a SEM holder and observed using the JEOL JSM-5600 SEM under a high vacuum setting of 20Pa and using a backscattered electrons detector (BSE). (Thierry Sleypen, personal communication, 12 June 2018). The resulting ion milled cross-section can be seen in cross-section 5.1.
4.5 Sampling
The second stage of experimentation examined the effect of sampling methods on the layer structures and paper fibres present in the wallpaper fragments and focused on devising a safe handling method.
It was hypothesised that different sampling techniques are likely to affect the paper fibres and the layer structures in different ways and that these changes could persist after embedding and polishing. An inadequate sampling method could have far reaching consequences and impair layer analysis of samples. For example, the application of pressure from a blade or a scissor can make layers delaminate and cause the paper to compress. Small paper samples are additionally attracted by static electricity, causing handling and storage problems, which could easily lead to disassociation. Solving these problems was crucial to the rest of the experimentation in this study and therefore the most suitable of the tested methods was used to procure samples for the embedding tests.
To begin with, the experiments were focused on the development of a safe and practical sample handling method. Often samples are taken and embedded at two separate locations and the transference of paper samples poses many risks. Furthermore, before samples can be embedded they need to be examined in order to verify the presence of all layers and determine the side best suited for cross-sectioning. Currently there are no convenient and safe methods, which could placate both concerns. Samples are typically handled with tweezers under observation with a stereomicroscope, and such excessive handling could cause undue damage. Another method found in literature is a custom sample holder, which consists of two glass slides sandwiching a sample in place (Almog et al. 2004), however, positioning of samples in such a holder poses many difficulties and is largely impractical. In order to proceed with studying the effects of sampling, this study made an attempt at designing a new type of sample holder, specifically intended for storage and microscopic observation of paper samples. After a sample holder was devised (see Chapter 5) it was possible to proceed with observation of sampling effects on paper and layer morphology. Three sampling techniques were proposed for testing:
- Cutting with a scissor (shearing motion) - Cutting with a scalpel (dragging motion)
- Pressing down with a razor blade (cleaving motion)
The three techniques are commonly used and all present different mechanical forces onto the samples and were tested and observed in this experiment. During the course of this experiment a fourth sampling method was developed:
- Cutting with a scalpel (sawing motion) 4.6 Embedding procedure
Using the least disruptive sampling technique, a number of samples taken from the wallpaper fragments were embedded, using a variety of selected resins and methods, which are described in the subsections from 4.6.1 to 4.6.9. This section will describe the resin selection and the embedding protocol, which was applied to all of the resins.
Paper samples are significantly different from other types of materials frequently embedded in conservation practices. It presents unique problems and as such the
methods which work for paintings or metal samples, might not be suitable. Paper is often brittle and has a relatively low tensile strength. Perhaps the largest difference is in the inherent porosity of paper, the spaces between the fibres create a spongy substrate, which likely has to be completely consolidated by embedding resins in order to preserve the structure and make it possible for the sample to withstand mechanical preparation methods.
An ideal embedding resin for paper samples should fulfil these eight requirements:
− Wide availability − Low toxicity − Translucency
− Consistency between batches − Low viscosity and good infiltration − Uniform polymerisation
− Minimal shrinking
− Suitability for mechanical polishing
The embedding procedure selected is the half-block, two-step embedding method, which is well established and frequently used by conservators. In this method the prepared resins were first poured to fill the moulds halfway and cured, making it possible to place the samples and labels on the half-block. The rest of the resin is then poured on top to seal the sample and is cured to create a full-block (Derrick et al 1994; Khandekar 2003; Wachowiak 2004). In some cases a fixation agent was used (see section 4.6.8). The samples were oriented with the edge selected for cross-sectioning facing upwards, around 1mm away from the top edge of the cube. This embedding protocol is illustrated in Fig 8.
Side view
Top view
Fig. 8 Diagram illustrating the embedding method used in the experimentation. 1. Empty embedding mould, 2. Resin is poured to fill the mould halfway, 3. Sample and label are placed on
the half-block, 4. The rest of the embedding mould is filled with resin to create the block. Each sample was accompanied by a label detailing the wallpaper it came from, and which embedding resin was used. This was done with a small piece of paper with the information written in pencil. Pens were avoided because of the possibility of ink being dissolved into the resin. The labels prevent disassociation and are vital for storage of samples.
4.6.1 Embedding moulds
Selecting an appropriate embedding mould was a crucial step in setting up the experiment. Many distributors of embedding media offer moulds specifically catered for embedding of samples and using such moulds is likely the best and most efficient option. However, the high price point and the nature of this experimentation made other mould types more appropriate. Instead, silicone moulds were self-made and a polyethylene ice-cube mould was selected, which can be seen in Fig 9. Because each tested resin had different curing specifications, it was more practical to make moulds fit for individual cubes.
Sample
Label with sample information Half-block
Fig. 9 Embedding moulds used for the experimentation. Silicone (left) and polyethylene ice-cube mould (right).
The silicone mould was made using addition silicone of shore hardness A40, which was selected because of its high durability and chemical stability. More information about the silicone can be seen in section 5.2.1 and the steps of making the silicone mould are described in Appendix I.
The polyethylene mould was selected because of its high availability and practicality as well as low price point. The ice-cube mould was broken into individual cubes, which could be used as disposable breakaway moulds. The plastic can be chipped away with pliers to free an embedded sample in the case of resins that release with difficulty.
4.6.2 Poly-Pol PS230
Polyester resins have been frequently recommended for the practice of cross-sectioning in conservation (Derrick et al. 1994; Plesters 1956; Tsang et al. 1991; Wachowiak 2004). The resin consists of unsaturated polyester dissolved in styrene and is initiated with methyl ethyl ketone (MEK). The polyester resin Poly-Pol PS230 was selected for the experiment; it is a cheap and multipurpose resin readily available in the Netherlands. Embedding in polyester has many advantages. The resin is crystal-clear and cures relatively fast at room temperature, over the period of 6 to 8 hours. The resin can be both polished and microtomed in order to create a cross-section.
In the experiment, 10g of resin were mixed with 0.1g of MEK. The amount of the initiator varies depending on the ambient temperature, and can be between 0.6-1% w/w. Too little initiator will result in a block too soft and too much will cause
yellowing and brittleness (Derrick et al. 1994). The viscosity of this resin resembles that of corn syrup and the mixing was performed slowly, in a figure-eight pattern, in order to minimise the amount of trapped air bubbles. The relatively low viscosity of the resin allows for de-airing, however, as the resin begins to gel and thicken within the first 15 minutes of mixing, there may not be enough time for all bubbles to escape. The resin should only be used for embedding samples before it has begun to gel and colour is a good indicator of the level of cross-linking. Soon after initiating the resin will turn yellow and as it sets, the resin once again turns crystal clear. The lightening of the yellow colour signals that the viscosity of the resin is likely too high for embedding.
The prepared resin was poured into a silicone mould to create the half-block and was left to cure overnight. The following day two samples taken from wallpaper fragment 2 were positioned and a label was placed. The resin was set and hard, however, the surface remained sticky, which helped to hold the samples and label in place. Another 10g of resin were mixed and the first sample was covered with resin to fill the rest of the cube.
It was observed that the sample floated in the resin and as it penetrated the paper, air bubbles were released. With a toothpick the sample was repositioned again, although precision was limited, due to limited visibility through the resin. The air bubbles were mostly cleared away with the toothpick and over time most of them de-aired before the resin set fully. To prevent this from happening again, the second sample was dipped in the resin for 30 seconds before being placed into the mould. It was anticipated that this would help the penetration of the resin and prevent bubbles from releasing once the sample was put in place. Once the sample was placed in the mould and covered with the resin it did not float or change position and no air bubbles were released. The covered samples were left to set overnight. The finishing steps are described in section 4.6.9 and the resulting cross-sections can be seen in section 5.3.
4.6.3 Technovit 2000 LC
Technovit is a brand name for a wide range of acrylic embedding materials formulated with methacrylate monomers and cured with blue light. Technovit 2000 LC is a resin specifically designed for metallography sample embedding, however, in conservation it is used for other types of materials as well, including painting samples (de Fonjaudran 2008, pp. 77-86). The one-component resin is ready to use straight out of the bottle, which is of great convenience. The resin has a very low viscosity, slightly above that of water, and produces crystal-clear blocks.
However, the resin also has some disadvantages. The curing temperature can reach as high as 90oC, and even though it can be lowered to 50oC through an
intricate embedding process, it is too high for paper samples and can cause shrinking and distortion of the sample.
In the experiment the resin was poured into a polyethylene mould and placed in a blue light polymerisation unit (Technotray POWER) for 5 minutes in order to make the half-block. The surface remained very sticky, which aided in the placement of samples and labels. The sample was then covered with more resin and cured in the polymerisation unit for 20 minutes to create the full cube. Technovit resins reach their full hardness after cooling down to room temperature, when they can be polished, which is described in section 4.6.9. The sticky residue on the surface was wiped with ethanol. The results can be seen in section 5.4.
4.6.4 Methyl Methacrylate/N-Butyl Methacrylate
It is possible to formulate a custom methacrylate embedding resin through the mixing of base components in a specific ratio. For this experiment methyl methacrylate and n-butyl methacrylate monomers were selected and benzoin methyl ether was used as the UV initiator (Mendiratta et al. 1975). This custom embedding system has the advantage of a low price-point and more control over the physical qualities of the embedding process as well as the resulting blocks. By adjusting the ratio of n-butyl to methyl methacrylate the hardness of resulting blocks can be adjusted with a larger proportion of n-butyl methacrylate resulting in more flexible blocks (Glauert 1992, p. 154). The resin is crystal clear and has very low viscosity as compared to polyester resins.
Methacrylate based embedding systems also posses many disadvantages. The resin typically does not polymerise evenly throughout the block and significant shrinking can happen during curing. Methacrylates have a long and established history of use in microscopy sciences since the early 50’s, however they are becoming less prominently used due to these concerns (Glauert 1962, p. 269). In the experiment the ratio of 4:1 n-butyl and methyl methacrylates was selected and 1.5% w/v of benzoin methyl ether was added. The mixture was poured into a silicone mould and placed under an UV polymerisation unit for two hours. A reaction occurred, either due to released heat or chemical interactions and the mould was damaged. In its place, the polyethylene mould was used to make the half-block and no problems were experienced. After curing for 2 hours with UV irradiation, the surface of the block remained slightly tacky. The sample and label were placed and the cube was cured for another 2 hours. After cooling to room
temperature it was possible to process the block into a cross-section using the method described in section 4.6.9. The results can be seen in section 5.5.
4.6.5 LR White
LR White, otherwise known as London Resin, is an acrylate based one-component resin, which can be cured either with UV irradiation or with the use of an accelerator. Both of the methods were attempted in this research. The resin has very low viscosity and produces translucent cubes. The low toxicity of this resin as well as cold curing is what makes this product stand out among embedding media (Hess 2003, p. 44).
In the first test, 10ml of the resin were mixed with 1 drop of the accelerator. The extremely low viscosity facilitated easy mixing and no air bubbles were introduced. The prepared resin was poured into a polyethylene mould to form the half-block and was left to cure at room temperature. The block was hardened within 30 minutes, and a film of liquid resin remained on the surface and sides of the block. This liquid resin aided in the infiltration of the sample during the positioning. More resin was mixed with the accelerator and the sample was covered. The low viscosity of the resin made it very easy for the sample to move and it had to be re-positioned with a toothpick. The resin was fully hardened after another 30 minutes. The film of liquid resin on the surface of the cube helped with de-moulding and was wiped away with tissue paper. The accelerator caused the resin to turn lightly yellow and this colour persisted in the finished block. The second method involved pouring the resin directly into the mould and curing in a UV unit for one hour to create the half-block. The sample and label were then placed and covered with the resin and the mould was placed back in the UV unit for two hours. The resulting block is crystal clear in colour. Finishing steps are described in section 4.6.9 and results can be seen in section 5.6.
4.6.6 Unicryl
Unicryl is a more recently developed acrylate based resin similar to LR White (Hess 2003, p. 51). The one-component resin is formulated with polymers of similar molecular weights, which helps ensure even penetration and polymerisation. The resin is typically used for microtoming of sections and has been formulated to have unique cutting properties (Electron Microscopy Science, product catalogue). The resin has low viscosity and produces crystal clear blocks. In the experiment the resin was poured into a polyethylene mould and placed in a UV unit for 12 hours. However, it did not appear to cure and remained liquid so
instead a silicone mould was used. The manufacturer warns against the use of soft moulds, however in this instance it appeared to work well. After 6 hours of UV exposure the half-block was formed. The sample and label were placed and covered with more resin. The low viscosity made it very easy for the sample to move and so it had to be re-positioned with a toothpick. The block was polished into a cross-section following steps from 4.6.9 and the results can be seen in section 5.7.
4.6.7 Dr. Spurr’s Low Viscosity
Dr. Spur specially formulated this epoxy for the embedding of samples in 1969 and the resin has a long and established use in microscopy sciences (Hess 2003, p. 45) This four-component resin has a viscosity similar to that of corn syrup and is yellow in colour.
In this test the components were mixed in the ratios: 18ml of ERL-4221 (Cycloaliphatic Epoxide Resin), 14ml of DER-736 (diglycidyl ether of polypropylene glycol, flexibiliser), 48 ml of NSA (nonenyl succinic anhydride, hardener) and 0.6ml of DMAE (dimethylaminoethanol, accelerator). Air bubbles were trapped during the mixing, however the low viscosity allowed for de-airing. The resin was poured into a silicone mould and placed in an oven at the temperature of 40oC. It took a week for the resin to set and form the half-block.
The recommended polymerising temperature by the manufacturer is 70-80oC,
however this temperature was deemed to high for the samples. It was also attempted to see how long the resin would take to cure at room temperature, however after a month the resin remained sticky and malleable and this test was dismissed.
After curing, the surface of the half-block was slightly tacky, but not enough to help anchor the sample and label, which moved slightly while the rest of the resin, was poured into the mould. They had to be re-positioned using a toothpick. Again, the mould was placed in the oven at 40oC for 7 days, when the resin
hardened enough to be mechanically prepared into a cross-section. The section was ground according to 4.6.9; for results see 5.8.
4.6.8 Barrier methods
Because paper is a porous and fragile material it was hypothesised that good resin infiltration would improve the quality of the cross-sections. The resin would strengthen the sample through consolidating it and therefore allow for mechanical means of polishing, which otherwise the samples might not have been able to withstand. However, infiltration of the resin could additionally have
some adverse effects. For example, it could mask presence of a binder or solubilise certain layers. Polyester is especially known to dissolve waxes, some organic dyes and natural resins (Derrick et al 1994, p.45). The role of a consolidant is to form a barrier between the samples and resin so that no interaction can occur between the two. Two sealants, a cyanoacrylate gel adhesive and cyclododecane, were selected and attempted in combination with accelerated LR White resin and Poly-Pol PS230. These resins were selected because of their fast curing times and low setting temperature, which would minimise the risk of the consolidant melting during the embedding.
In the first experiment a cyanoacrylate, commonly know as superglue, was used. Bison Colle Seconde Lijm adhesive was used, which is a widely available gel cyanoacrylate and has an open working time of 30 seconds, which is longer than that of typical cyanoacrylates. First the gel was placed on the half-block with a toothpick in a dollop slightly larger than the sample. The sample was then positioned in the gel and pressed into it slightly. More gel was added on top of the sample to encapsulate it fully. The medium viscosity of the gel and fast setting time were supposed to prevent the infiltration of the adhesive and encapsulate the sample (Chang et al. 2002, p.58). After the gel was cured fully, the rest of the resin was poured to fill the mould.
In the second experiment cyclododecane was used in order to seal the sample. Cyclododecane is a saturated cyclic alkane that sublimates at room temperature and is used as a temporary and reversible protective coating in conservation practices (Rowe et al. 2008 p.18). In the context of embedding, cyclododecane was hoped to fully infiltrate the paper sample and thereby prevent contamination by the resin (de Fonjaudran et al. 2008, pp. 77-86). It was hoped that the infiltration of the alkane would enable mechanical polishing of the sample and that upon sublimation the paper sample would resurface undisturbed by the cross-sectioning process.
Because cyclododecane has a melting point of 60.8oC, in the experiment it was
dissolved in toluene instead, in the ratio of 4:1 w/v, in order to avoid the high temperature. A drop of the solution was placed on the sample and then the sample was positioned for the embedding after the solvent evaporated. For results see section 5.9.
4.6.9 Finishing steps
There are two general ways of manually preparing cross-sections: microtoming, which produces thin sections and grinding which produces thick sections. Thin sections are most commonly made in general, however, in conservation thick
sections are predominantly used. Early on, in the practice of embedding painting samples, microtoming was found too harsh for friable media and was henceforth largely dismissed as a cross-sectioning technique in conservation (Plesters 1956, p.110). It is possible that the technique would work better with paper samples than painting samples, as it is a more porous material. It is likely that the high penetration of an embedding medium would strengthen the sample sufficiently enough to allow for the making of thin samples. Microtoming of sections, however, is outside the scope of this study and therefore only the thick section finishing methods will be discussed. All of the samples embedded in the experiment were finished using the same mechanical polishing steps as described below.
After fully curing, the embedded sample cubes were ground and polished using both wet and dry polishing methods. It is advised to use a lubricant while grinding the cross-sections in order to prevent the ground resin and sample particles from inserting themselves back into the cross-sections and to limit heat generation from the mechanical action (Wachowiak, 2004 p.217). An additional benefit of wet polishing is the entrapment of ground dust in the liquid. This fine dust could otherwise end up being inhaled and causing health concerns.
Initially, the samples were ground on a mechanical grinder with a constant stream of water. Water worked well in washing away ground particles and prevented excess heat from generating. Water-resistant abrasive silicon carbide sheets were used in progressively finer grits starting at FEPA#320. The coarsest sheet was used only on the resin in order to bring the sample closer to the surface. Grit #800 was then used in order to breach the sample and progressively finer grits were then used ending at FEPA#4000. The sections were cleaned between each change of abrasive paper with water in order to avoid the transference of grit. The sections were also checked under a microscope in order to assess the level of polishing and to prevent overgrinding.
Dry polishing is sometimes recommended in order to avoid the risk of certain components solubilising (de Fonjaudran, 2008, p.80) and the cross-sections were then polished dry and by hand on progressively finer textile backed silica carbide sheets in grits up to FEPA#16000. When progressing onto new sheets, the samples were wiped free of ground dust with lens paper. It is recommended that manual grinding is performed on a perfectly smooth surface, such as glass, and that the sheet is adhered to the substrate to prevent movement, which could damage the surface of the section.
Was hed ou t la yer
These sections were then ready for analysis, however many problems were observed. As can be seen in Fig 10 where a water-based layer, likely of adhesive, was washed away and the resulting groove accumulated grinding debris.
Fig. 10 SEM photomicrograph showing detail of the washed out layer and accumulation of debris. This grinding method additionally caused contamination with grinding material as is illustrated in Fig 11.
Fig. 11 SEM photomicrograph showing detail of contamination with silica carbide particles.
This type of contamination is particularly confusing as it can misleadingly impact visual analysis as these particles could have easily been misnamed as pigment or filler. Thanks to EDX elemental analysis, these particles were confirmed to consist of Silicone and therefore could be ruled out as a grinding side-product (for full elemental report see Appendix II).
Debris
It was later discovered that the stage of the mechanical polisher was damaged with several indents, which potentially could have caused enough shock for some abrasive silica grains to be released from their substrate and embed themselves into the surface of the cross-section.
Several parameters were changed in order to prevent these issues from occurring. The sections were manually re-ground using Shelsol T as lubricant in order to avoid the water based media from solubilising (personal communication, Merel van Schrojenstein Lantman, April 2018). Because the samples were already breached, only grits of #800 up to #4000 were used. The sections were cleaned by submersion in the solvent. The sections were then dry polished using the same method as previously.
Using this altered method resulted in samples that were intact and no layers were washed out. It is interesting to note that when wet polishing with water, it was only the layer with a water-sensitive medium that was negatively affected and that the paper fibres appeared entirely intact. Based on these results, it might be safe to polish paper samples with water as lubricant, as long as there is no water-sensitive medium present.
4.7 Analysis of cross-sections
There is a large variety of analytical methods which can be used in combination with cross-sections. For example, techniques such as staining or FTIR for size and binder analysis or XRF for pigment examination can be successfully applied (Kuckova et al. 2013; Rouchon et al. 2010). However, because the focus of this research is on the evaluation of different preparation methods, the selected type of analysis reflects on the physical qualities of the cross-section on the whole, rather than on the layer analysis alone. Therefore the protocols used are not recommended for the purpose of typical layer analyses, but rather in order to evaluate the surface quality, topography, penetration of the embedding medium and the overall integrity of the sections.
Visual analytical methods were deemed most suitable and optical microscopy and SEM imaging were selected. The protocols used in order to obtain consistent and comparable images with these systems are outlined in the following subsections.
4.7.1 Optical microscopy protocols
It was theorised that optical microscopy would be the most accessible and informative of analytical techniques that could be used in order to evaluate the quality of the cross-sections.
A Leica DM2500 M microscope, with a Zeiss Axiocam 105 colour camera, was used for the observation of all of the cross-sections and images were taken using the Zeiss Zen software. Observation of thick sections cannot take place with transmitted light and therefore HAL reflected light lamp and a HBO-100W Fluorescent lamp were used.
In order to allow for observation, the sections were mounted onto microscope slides, using UHU White Tack malleable putty. A sheet of lens paper was placed on the surface as a protection layer and the sample was pressed in a levelling press. The lens paper was removed and the slide was mounted on to the microscope stage. Observation of the sample began at x50 and proceeded to x100, x200 and finally x500.
Three protocols were used in order to create consistent and comparable photomicrographs of the sections, which are detailed in Table 1.
# Light Polarising filter Reflector turret Analyser
1 Reflected ✓ POL (4) ✓
2 Reflected ✗ DF (1) ✓
3 UV ✗ A (3) ✗
Table 1. Optical microscopy protocols.
The first protocol was designed to show the layer structure present in the samples. The colours were bright and layers were easily discernible.
Protocol #2 follows the same settings with the exclusion of the polarising filter. This causes the light to reflect more prominently from the surface of the cross-section and highlights the surface topography.
The last protocol allows for observation of any fluorescing elements in the samples and was meant to complement layer interpretation.
Ideally, all of the cross-sections should be observed using these protocols, however the ion milled samples were sputter coated before colour images could be taken. This was a great disadvantage to this study and an attempt should have been made at removing the coating (Leslie et al. 2007, p.1459). Alternatively the
samples should have been re-polished for best comparative analysis. This was unfortunately not possible due to time constraints.
Colour plays a large part in the analysis of layer structures and is perhaps the most useful of indicators of adhesive, dirt, varnish and size layers. Optical microscopy should always be the first step of cross-section analysis and is a necessary pre-requisite for SEM analysis. It is incredibly useful to compare colour photomicrographs of the samples to the SEM imaging, which does not differentiate well between organic materials. All OM photomicrographs were taken by the author of this thesis.
4.7.2 SEM protocols
The advantage of using SEM for the imaging of cross-sections is the high magnification and resolution, which allow for the study of sample topography in great detail. This section will describe the SEM settings used in this research project as well as outline the challenges and reasoning behind them.
Overall, due to time constraints and limited availability, three different SEM devices were used. This complicated the drawing of straightforward conclusions from the photomicrographs because all three microscopes had different calibrations and detectors. The devices used were:
− JEOL JSM-5600 SEM − JEOL JSM-5910 SEM − Nova NanoSEM 450
Although similar results can be obtained with each one, there will always be visible variances in the resulting photomicrographs, which could interfere with comparative analysis of the images.
This discrepancy was further exacerbated by the fact that the ion milled sections were sputter coated with a gold layer, while the rest of the cross-sections were not. Coating of samples with a thin layer of conductive material is common practice in SEM laboratories and is done to moderate the charging of surfaces as well as to promote the emission of secondary electrons (Leslie et al. 2007, p.1459). This allows for better imaging, particularly in the case of paper samples which present many challenges for SEM imaging. Paper has a highly irregular and porous structure, which causes local variations and is highly prone to charging because it is a composite of insulating materials (Roede 2016, p.68). This negative
