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Master Thesis
The use of advances in technologies for restoration practice
In the reintegration of lost veneer
Master Wood & Furniture
Conservation & Restoration of Cultural Heritage
Faculty of Humanities
University of Amsterdam
19-06-2018
Student: Vidar I.C. Thijssen Student number: 11121203 vidar.thijssen@student.uva.nl
Thesis supervisor: dr. Herman den Otter Second reader: Tonny P.C. Beentjes
UvA C&R Professors: prof. dr. Maarten R. van Bommel prof. dr. Ella Hendriks
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Index
Summary (English)
p. 4
Summary (Dutch)
p. 5
Introduction
p. 6
Research question and sub-questions
p. 10
Research methods
p. 10
Proposed future treatment method
p. 11
1. Veneer
p. 12
1.1 Veneer patterns
p. 13
1.1.1 Macroscopic structure of wood
p. 13
1.1.2 Spatial orientation of wood
p. 15
1.1.3 Pattern characteristics
p. 21
1.2 Veneer in furniture
p. 28
1.3 Deterioration
p. 30
1.4 Current treatment
p. 33
1.5 Conclusion
p. 39
2. Scanning techniques
p. 40
2.1 Laser scanning
p. 42
2.2 Structured light scanning
p. 45
2.3 Reflectance transformation imaging
p. 48
2.4 Infrared reflectography
p. 50
2.5 X-ray computed tomography
p. 52
2.6 Conclusion
p. 55
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3. Computing techniques
p. 57
3.1 Current digital tools
p. 58
3.1.1 General software
p. 59
3.1.2 Specialized software
p. 60
3.2 Creating specialized tools
p. 62
3.2.1 Principles of digital tools
p. 62
3.2.2 Approximating a veneer pattern (basic)
p. 63
3.2.3 Approximating a veneer pattern (complex)
p. 78
3.3 Conclusion
p. 79
3.4 Inspiration for future uses of algorithms
p. 80
Conclusion
p. 81
Acknowledgements
p. 84
References
p. 85
Figures
p. 85
Literature
p. 85
Appendices
p. 89
Appendix I. Course information of TU/e elective
p. 89
Appendix II. Code: wood pattern approximation
p. 90
Appendix III. Additional patterns (craquelure)
p. 97
Appendix IV. Code: craquelure pattern approximation
p. 101
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Summary (English)
Within the restoration of wood & furniture, the reintegration of lost veneer is a frequently occurring activity.
This research considers the possible use of 3D-scanning, -computing and -machining techniques to approximate the losses of veneer in objects. This diagnostic thesis research goes into several 3D-scanning and computing techniques; thus, creating a basis for later research into 3D-machining techniques.
The goal of this research is to illustrate the added value and potential problems of these technologies within restoration practice.
When carried out professionally, the results of traditional restoration methods of reintegrating lost veneer can be quite unobtrusive. However, there are disadvantages concerning the reversibility and aesthetics of infills. The relevance of this research concerns the possibility of reintegrating losses of veneer in a more reversible manner by leaving the original material intact and approximating the pattern of the loss to create a more visually pleasing result than can be achieved in current restoration practice.
There are different scanning techniques that could potentially be used to obtain data on the shape of the lacuna and pattern of the area surrounding the lacuna. Scanning techniques include laser scanning, structured light scanning, reflectance transformation imaging, infrared reflectography and x-ray computed tomography.
In order to approximate the visual appearance and patterns of lost veneer, digital image editing techniques can be used. Image interpolation tools have sometimes been incorporated in general image editing software. However, these interpolation techniques in general do not take into account the nature of the structure of a material and its variations. In order to overcome the previously mentioned limitations and to create digital infills with more physically correct and complex patterns, a specialized tool could be created.
There are a number of mathematical principles and type of shapes that can be derived from the patterns in natural wood. The patterns include, amongst others, straight lines, curves and parabola. These basic shapes can form the basis of a digital approximation tool specialized for wood patterns. These digital approximations on the basis of mathematical principles can also help to create other visual aspects that might characterise veneer. Other pattern characteristics can be taken into account in order to make an infill less obtrusive and visible. These could include: machine- and tool marks; forms of deterioration like scratches and compressed material; patina and craquelure patterns on a finishing layer.
The goal is of this research is to illustrate the added value and potential problems of new
technologies within restoration practice on the basis of the reintegration of lost veneer using 3D-scanning and computing techniques.
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Summary (Dutch)
Het re-integreren van verloren fineer is een vaak terugkomende bezigheid binnen de specialisatie van hout & meubelen.
Dit onderzoek beschouwt het mogelijk gebruik van 3D-scan, -digitale verwerking en -verspaan technieken om zo verliezen van fineer te benaderen. Dit diagnostische onderzoek gaat in op verscheidene 3D-scan en -digitale verwerkingstechnieken om zo een basis te vormen voor nader onderzoek naar 3D-verspaan technieken.
Het doel van dit onderzoek is om de toegevoegde waarde en potentiele problemen van deze technologieën binnen de restauratie praktijk toe te lichten.
De resultaten van restauratiemethoden kunnen best onopvallend zijn wanneer deze professioneel zijn uitgevoerd. Echter, er zijn ook nadelen met betrekking tot de reversibiliteit en esthetiek van een dergelijke restauratie. De relevantie van dit onderzoek betreft de mogelijkheid om verliezen van fineer op een reversibele manier te re-integreren door middel van het intact laten van het originele materiaal. Daarnaast is het benaderen van het originele patroon belangrijk, om zo een visueel aantrekkelijker resultaat te verkrijgen dan veelal binnen de huidige restauratiepraktijk kan worden bereikt.
Er zijn verschillende scantechnieken die mogelijk kunnen worden gebruikt om gegevens te verkrijgen over de vorm van de lacune en het patroon van het gebied rondom de lacune. Scantechnieken omvatten laserscans, structured light scans, reflective-transformative imaging, infrared reflectography and x-ray computed tomography.
Digitale beeldbewerkingentechnieken kunnen worden gebruikt om het uiterlijk en de patronen van verloren fineer te benaderen. Som zijn beeld-interpolatie technieken opgenomen in
beeldbewerkingssoftware. Deze interpolatietechnieken houden echter over het algemeen geen rekening met de aard van de structuur van een materiaal en variaties ervan. Er kan een digitaal stuk gereedschap worden gecreëerd die missende delen fineer kan benaderen die uit fysiek correctere en complexere patronen kan bestaan.
Er zijn een aantal wiskunde principen en soorten vormen die kunnen worden afgeleid uit patronen in natuurlijk hout. Deze patronen omvatten, onder andere, lijnen, bochten en parabolen. Deze simpele vormen kunnen de basis vormen van een digitaal gereedschap dat gespecialiseerd is in het
benaderen van houtpatronen.
Deze digitale benaderingen op basis van wiskundige principes kunnen ook helpen om andere visuele aspecten te benaderen die fineer karakteriseren. Zo kan er rekening worden gehouden met
aanvullende patroonkenmerken van fineer om zo de restoratie minder zichtbaar te maken. Deze kenmerken kunnen bestaan uit: machine- en gereedschapssporen; schadepatronen zoals krassen en samengedrukt materiaal; patina en craquelure patronen op een afwerklaag.
Het doel van dit onderzoek is om de toegevoegde waarden en potentiele problemen van nieuwe technologieën binnen de restauratiepraktijk te illustreren op basis van de re-integratie van verloren fineer met behulp van 3D-scan en -digitale verwerkingstechnieken.
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Introduction
The context of this research is a diagnostic master thesis into object related problems regarding conservation and restoration.
Conservation & restoration (C&R) is not merely a beautiful profession and craft, but also a discipline that requires innovation in order to achieve more reversibility, re-treatability and durability.
Important aspects of this innovation are material research and cross-overs with other disciplines, all within an (art) historical perspective. Within research into these innovations, it is very important not to lose sight of the classic material crafts themselves, for these must also be preserved.
A significant problem in furniture restoration is the difficulty in reintegrating losses of veneer. This research concentrates on creating infills for losses of veneer, for this is an often-used
restoration technique within the scope of wood & furniture. The nature of veneer and its prevailing type of application within objects often results in a partial loss of material (figure 0.01 and 0.02). Because veneer is more or less 2-dimensional, it is a relatively simple type of loss to start researching computer aided techniques for reintegrating missing parts.
Figure 0.01: a loss of veneer ad the edge of a secretaire.
P. 7 of 109 In general, difficulties concerning these types of losses in traditional restoration practice are twofold. Firstly, because of the complex shapes of a loss, it can be difficult to make particular infills when no original material can be taken away. In current practice, original material is often sacrificed in order to create a lacuna that is easy to fill (figure 0.03). However, this type of treatment is not reversible. Secondly, creating an infill from natural veneer that visually flows over into the pattern of its original surroundings can also be difficult (figure 0.04 and 0.05).
Figure 0.03: material is cut away in order to create an infill.
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Figure 0.05: an infill (left) with a pattern that does not match its surroundings.
Advances in technologies from different fields may aid the reintegration of lost veneer to become both more reversible and aesthetic. More precise: using 3-dimensional scanning, computing and manufacturing techniques to approximate losses of veneer. This paper concerns a diagnostic research into 3-dimensional scanning and computing techniques, creating a basis for later research into 3-dimensional manufacturing techniques.
The goal is of this research is to illustrate the added value and potential problems of new
technologies within restoration practice on the basis of the reintegration of lost veneer using 3D-scanning and computing techniques.
P. 9 of 109 Existing digital tools, like CAD (Computer Aided Design) or general image editing software can
already be used in order to approximate the shape and pattern of missing material. However, these cannot always achieve sufficient results, especially with more complex losses of veneer. A
specialized digital tool will potentially yield better results. In the current practice of restoration, it is quite common to adapt existing tools for specific uses; one often adapts, inter alia, a chisel, a plane, a knife or a brush for a specific task. Similarly, digital tools can be created and adjusted to fit a specific purpose. This type of ‘modern craftmanship’ can be a very useful extension to classic craftsmanship in current restoration practice. Researching new techniques inspires new methods and solutions.
The results that these specialized tools yield concerning the shape and pattern of the infill are potentially less obtrusive and visible than could be accomplished using traditional restoration methods. However, it is important to realise that these modern technologies and crafts are a similar extension of the human hand as a manual chisel or brush; classic craftmanship is still needed to realise fine results. In addition, knowledge of the structure of wood and mathematical knowledge is necessary in order to digitally approximate a veneer pattern.
A key point in the implementation of this research and possible pitfalls concerns the ethical use of new technologies; especially referring to preservation of classic crafts, authenticity of the object and durability of the used technologies & materials.
The main innovative part of this research could be the use of particular algorithms to approximate the natural patterns of veneer that seamlessly and in a physically correct way connect to the
surrounding original material. This makes the use of these technologies, in some restoration projects more interesting and ethically acceptable.
The use of these technologies may be of interest to restorers ranging from one-man workshops to large institutions. Because the techniques that could be used to approximate missing material can in principle also be used for other materials, this research is also applicable to other material
specializations within restoration. However, these techniques may not be applicable to all types of conservation & restoration projects. This research concentrates on the restoration projects where a higher level of reversibility and aesthetics is wanted.
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Research question and sub-questions
The following main-, and sub-questions can be stated: Research question:
Which advancing technologies in the field of 3D-scanning and computing techniques might be used within restoration practice to reintegrate losses of veneer?
Sub-questions:
1) How does the structure of wood result in specific patterns and of what elements do these patterns consist?
2) How does veneer deteriorate?
3) What is the current practice of reintegrating losses of veneer within the specialisation of wood & furniture?
4) Which aspects of current restoration practice concerning the reintegration of lost veneer need to be further developed, in order to achieve added reversibility and aesthetics? 5) Which advancing technologies are available to use in restoration (using the SWOT matrix)?
a. What are the Strengths of these technologies in restoration? b. What are the Weaknesses of these technologies in restoration? c. What are the Opportunities of these technologies in restoration? d. What are the Threats of these technologies in restoration?
Research methods
The goal of this research is to illustrate which advancements in technologies can be used to the benefit of reintegrating lost veneer concerning the shape and pattern of the infill. The research consists of the following three consecutive parts:
1) The first part of this research goes into the structure of veneer, the way it deteriorates, how it is currently treated and what aspects of treatment could improve in order to achieve a more reversible and visually pleasing result. This will be answered on the basis of a theoretical research into the structure of veneer and practical knowledge about creating infills in veneer.
2) The second part concerns a theoretical research into what types of scanning techniques could be relevant for digitalizing the shape of the lacuna and the pattern of its surrounding material. This part includes a SWOT analysis: assessing the strengths, weaknesses,
opportunities and threats of these technologies in restoration. A SWOT matrix is often used to analyse the use of new technologies in a particular context. It could therefore help to illustrate, inter alia, the added value and potential problems of particular technologies within restoration practice.
3) The third part of this research studies computing techniques that can be used to approximate veneer patterns. This part again includes a SWOT analysis. This theoretical research will be substantiated by a practical case study concerning the creation of a basic approximation of a veneer pattern, as a proof of concept.
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Proposed future treatment method
This proposed future treatment method illustrates how new technologies may be used to reintegrate losses in veneered objects. The first two steps will be further discussed in this thesis. ‘Step 3’ represents the manufacturing side in this treatment, which is another potential research project.
In future, treatments could be done in the following manner:
Step 1: 3D-scanning techniques
Make a 3-dimensional scan of the lacuna in an object.
Use particular scanning techniques in order to obtain data on the 3-dimensional shape of the lacuna and pattern of the surrounding original material.
Step 2: 3D-computing techniques
Use the information from the 3D-scan to create a digital infill.
The information from the 3D-scan will be used to digitally approximate the shape and pattern of the infill.
Step 3: 3D-manufactoring techniques
Use the digital approximation to manufacture an infill.
The digital approximation of the loss can be used in at least the following ways: - Comparison: an approximation can be compared with similar patterns from an
existing stock of wood or sheets of veneer. A material library of a stock can be used to search for the best fitting piece of material to reintegrate the loss (depending on properties such as patterns, grain direction, colours etc.). After which the infill can be cut from the sheet of veneer using for example laser cutting or CNC-milling techniques.
- Subtractive manufacturing: when the approximation of the missing material is compared with similar patterns on existing pieces of wood, the infill can be laser-cut or CNC-milled into the exact shape of the missing piece from the material.
- Additive manufacturing: 3D-print the missing pattern in the exact shape of the missing piece of material. There is the possibility of 3D-printing in a large variety of materials (e.g. wood fibres).
- Projecting: the reconstructed pattern can be projected onto the object with a beamer (reconstructing with light). This method can in some cases be preferred over an invasive physical restoration. The projection can follow any complex shapes of an object using surface mapping techniques.
Note: Ideas and inspiration concerning ‘Step 3’ will be further elaborated in chapter ‘Inspiration for future uses of algorithms in restoration practice’ (p. 80).
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1. Veneer
The first part of this research explores veneer as a material and consists of subchapters concerning: how the structure of wood results in specific patterns; how veneer was used in objects; how veneer deteriorates; how is it currently treated.
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1.1 Veneer patterns
The macroscopic structure of wood results in specific patterns. The mathematical principles that can be derived from these characteristic patterns could help to digitally approximate a piece of missing veneer. Thus, as a first step, only particular parts of wood will be described that are relevant for this research.
Besides objects made from solid wood, veneers (thin sheets of wood) have been used to decorate pieces of furniture and related objects. Specific types of veneer have been used because of their visual appearance or material value. Because visually attractive types of wood are sometimes constructively too unstable or too valuable as construction material, they are more successfully used as veneers [Rivers, 2003: p. 307-308]. Of the thousands of species of wood in existence, only a couple of hundred were used in western Europe in furniture during the past ages (figure 1.01), of which only half, or more, species were used as veneer. This research concentrates on wood or parts of wood that were likely to be used as veneer. Thus, softwoods, are less likely to occur.
This chapter discusses a series of elements that determine the pattern of wood and its
corresponding veneer, namely; the macroscopic structure of wood, the spatial orientation and pattern characteristics. The last subchapter concerns the primary and secondary visual
characteristics of the pattern in veneer. In later chapters, secondary and additional tertiary pattern characteristics will be further supplemented.
All these different elements are discussed on a macroscopic scale; this is a length scale on which objects and phenomena are still practically visible to the human eye without the use of instruments like a magnifying glass or microscope. The microscopic structure of wood, looking at the features that can only be seen under a microscopic magnification, includes elements like pit size or wall thickness of fibres [Klaassen, 2016]. These microscopic and mesoscopic elements of a wood structure are generally of less direct importance since the present research is about the visual appearance of wood to the eyes of a human spectator. However, some properties that are generally thought of as part of the microscopic structure of wood could potentially help to approximate a realistic wood pattern.
1.1.1 Macroscopic structure of wood
Wood is a material with a biological origin. Wood can be described as a cellular polymeric composite [Rivers, 2003: p. 50]. The appearance of wood is the result of the arrangement of different types of cells. These cells grow on the basis of photosynthesis. Generally, these cells have multiple tasks: they operate parts of the metabolism of a tree like water transport, storage for food reserves and lastly, serve as building blocks that give the tree firmness. Depending on their function, the cells have a specific construction. The overall composition of hardwoods is more complex than softwoods; hardwood consists of more cell types than softwoods [Klaassen, 2016]. Within species, these cells have the same shape, but in between species they have a different shape. The cells that serve mechanical and physiological functions, such as building blocks, are primarily elongated and fibre-like and lie parallel to the axis of the trunk or branches of a tree. The grain direction of a tree is determined by these longitudinal cells [Rivers, 2003: p. 307-308].
P. 14 of 109 The properties of the wood like strength, rate of expansion and shrinkage are different in each axial direction [Klaassen, 2016] [Rivers, 2003: p. 307-308]. For example, wood is less strong when cut along the cross-sectional surface, which is sporadically used as veneer. As a building material, wood is strong, yet soft. Wood that is used in furniture is generally taken from the trunk of the tree. In the case of veneer this might also be from burrs (e.g. burr walnut) and branches (e.g. oyster veneer). Because of this variety, decorative veneers consist of a large spectrum of patterns ranging from simple stripes (figure 1.02) to complex burr (figure 1.03). These patterns depend on at least the following aspects:
- Wood species - Individual trees - Parts of the tree
- The manner in which these parts of the tree are cut
Figure 1.02: veneer with simple stripe pattern Figure 1.03: veneer with complex burr. pattern.
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1.1.2 Spatial orientation of wood
The pattern of a piece of veneer greatly depends on the spatial orientation along which it was cut from a tree. Wood is usually cut in a way to show decorative patterns. There are three main surfaces (also called planes): cross section, radial and tangential (figure 1.04, 1.05 and 1.06). These are rather theoretical cuts, in practice these cuts are more intermediate.
Figure 1.04: spatial orientation of wood [Rivers, 2003: p. 52].
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Figure 1.06: structure of a tree at various magnifications (A). Microscopic view of softwood (B)(D) and hardwood (C)(E) and a macroscopic view of their growth rings, each composed of earlywood (ew) and latewood (lw). A straight-grained board (F) and a diagonal-grained board (G). Cut of the cross-section (H)
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Cross-sectional surface
The diameter and the length of a tree increases when layers of cells are added. These new layers are called year rings or growth rings, each consisting of early wood and late wood (figure 1.07). The difference in characteristics between early wood and late wood comes down to the difference in density; early wood generally has a lower density than late wood. Depending on the species, temperate woods can have distinct year rings, existing of early wood and late wood. Generally, tropical woods have less distinct growth rings. The width of these rings may vary between years. The perspective relative to the axis of the tree is also an important factor in how the patterns of growth rings are perceived; a set of growth rings that can be seen as a circle from above (0°), can under a slanted view (from an angle between 0° and 90°) be seen as an ellipse [Feijs, 2017]
Crossing these growth rings, radially outwards from the pith, are cells called rays (figure 1.5). These horizontal bands of cells are present in every species, however, only if these bands are a couple of cells wide, can they be seen by the human eye; this is not the case with all species [Rivers, 2003: p. 51-52].
These cross-sectional patterns have been used in veneer, however, not often.
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Longitudinal surface
Decorative patterns are mainly to be found on the longitudinal surfaces of wood [Rivers, 2003: 49-54]. According to the direction and angle at which the wood is cut, the longitudinal surface can be divided into the radial and tangential surface (figure 1.08 and 1.09). The patterns on the radial and tangential surfaces are often used as veneer on objects.
Radial surface
The width and intervals of the growth ring pattern on the cross-sectional plane can also be seen on the radial plane and partly on the tangential plane. On the radial plane the growth rings can be seen as relatively straight lines (figure 1.08 A). These decorative line patterns are commonly used in furniture.
In addition to these line patterns, in some types of wood, like oak, mahogany and sycamore, rays are visually present on the radial surface.
Tangential surface
Although growth rings are rather circular when seen on the cross surface, when wood is cut under a slight slope on a tangential surface, the growth rings create a shape that is similar to an ellipse or a parabola in a ‘∩’ shape (see figure 1.08 B); depending on the way the wood is cut, this can also be a ‘U’ shape. This type of pattern is, besides a stripe pattern, a type of decoration used in furniture. This pattern existing of ‘∩’ and ‘U’ shapes is sometimes used in the form of so-called ‘ailes de papillon’ or butterfly wings. Due to many varieties and irregularities of wood and the way it is cut, these stripe and parabola-like patterns can occur interchangeably (figure 1.10 and 1.11).
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Figure 1.09: different directions of cutting wood and corresponding patterns on the longitudinal surface [Kohl,
2007: p. 273-274].
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1.1.3 Pattern characteristics
Primary pattern characteristics
The general structure of wood refers to the orientation of its cells, which results in a specific pattern. The variations in wood and subsequent structures and patterns can be divided into the following contributing elements: grain, surface roughness, topography, colour and irregularities.
Grain
The grain of a piece of wood is the longitudinal alignment of cells; the relative direction of the fibres to the axis of the tree or the longitudinal edges of an individual piece of wood [Rivers, 2003: p. 49-54].
A number of variations in the type of grain can be distinguished [Rivers, 2003: p. 49-54] (figure 1.16): - Straight
In straight wood, the fibres are relatively parallel to the vertical axis of the tree. Although this is a relatively simple pattern, it is used as decorative veneer (figure 1.02).
- Spiral
In spiralled wood, the fibres follow a spiral in a left- or right-handed course at a slight incline relative to the vertical axis of the trunk.
- Irregularly inclined
In irregularly inclined wood, the fibres are inclined at varying and irregular angles to the vertical axis of the three (figure 1.12). Apart from knots, irregular grain can also be the result of knot-like elevations which result in blister figures and depressions in the growth rings, result in bird’s eye figures (figure 1.13).
Figure 1.12: irregular grain of black Walnut (sometimes called ‘feather crotch’) [The wood database, 5 July 2018].
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Figure 1.13: lightly figured bird’s eye maple [Wood magazine, 14 June 2018].
- Interlocked
In wood with an interlocked grain, the fibres of successive growth layers are inclined in opposite directions. This can be seen on quarter-sawn surfaces in the form of ribbon- or stripe figures, for example in satinwood. This is a characteristic of mainly tropical hardwoods and less common of temperate woods (figure 1.14).
Figure 1.14: a ribbon stripe figure on Narra (Pterocarpus indicus) which is formed by an interlocked grain spiralling in different directions in the tree’s trunk [The wood database, 5 July 2018].
P. 23 of 109 - Wavy
In wavy wood, for example in maple, the direction of the fibres is constantly changing in a regular wave or sinusoid (figure 1.15). Because of these variations in grain direction, the reflection of the light results in a decorative pattern; also known as a ‘fiddle-back figure’. Furthermore, wavy- and interlocked grain can occur simultaneously, on a radial surface. This results in a broken ripple, also known as a ‘roe figure’.
Figure 1.15: a wavy pattern on mahogany.
Figure 1.16: different types of grain [Wood magazine, 14 June 2018]. Note: the terminology of these drawings differs between sources.
P. 24 of 109 Surface roughness
The roughness of a material such as wood can be described as the relative size and its variations of its cells [Rivers, 2003: p. 49-54], this also depends on the way the wood has been processed and finished (planed, sanded, etc.). The surface roughness is a singular value that expresses both characteristics of the wood species and the finishing methods.
Topography
The topography or spatial shape of a veneer describes the entire ‘landscape’ of a surface with all its irregularities: curvature, traces of tools, damages, etc. Because the surface roughness of a material is expressed in a singular value, this research needs additional data that describes the entire shape of an object. The shape of the surface of a veneer, and the negative shape that is the results from a piece of missing material (also called a lacuna), is therefore described as the topography of a surface. In terms of hard-woods, the following can be stated [Klaassen, 2016]:
- Ring-porous woods: uneven topography - Diffuse-porous woods: even topography
Colour
The colour of a piece of wood is mainly caused by the arrangement, orientation and concentration of different cell types. The way light falls onto the surface of the wood can influence the perceived colour, luster etc. In some types of wood this effect is more accentuated and known as a ‘moiré effect’.
The colour of the wood can change over time because of various factors: light, air, heat, etc. Also, specific chemical reactions with other materials can change the colour of the wood, like the discolouration of oak by iron [Rivers, 2003: p. 49-54].
Irregularities
Apart from the previously discussed variations in the structure of wood grain, a tree can possess a number of irregularities and distortions of the grain. Some examples are:
- Burrs
P. 25 of 109 Figure 1.17: walnut burr
- Knots
Branches grow together with the stem. When a branch dies, the stem will overgrow the branches surface (figure 1.18). Knots also affect their surrounding wood structure and corresponding pattern.
Figure 1.18: small knot (left) and large knot (right).
- Reaction wood
Apart from species of tree that have a natural tendency to grow in a non-circular way, growth rings can also have different shapes because of other factors. In the situation when a tree grows on a slope, in order to grow straight upwards, a tree’s stem makes reaction wood, in this reaction wood (compression wood in softwood and tension wood in hard wood) growth rings generally have deviant shapes.
- Cracks
P. 26 of 109 - Branches
The parting of the trunk in separate branches gives a characteristic ‘V’ shape pattern. An example is crotch mahogany which can often be seen on objects (figure 1.19).
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Secondary pattern characteristics
In addition to the previously mentioned primary elements of which a pattern consists, there are also a set of secondary elements that influence the visual aspects of veneer.
Manufacturing veneer
The way veneer is manufactured has an effect on the visual properties. Veneers vary in thickness and were during the past centuries roughly cut between 0.5 mm to 5 mm. The orientation in which the veneers are cut, influences their pattern. Multiple production methods have been used to create veneer, the use of which depends on, inter alia things, the period. Sawing and slicing wood to create veneer can result in any pattern that is present in a tree; rotary peeling creates a very specific pattern. Each manufacturing method also has its limitations concerning the dimensions of the sheets of veneer that can be cut, for example; early sawing methods yield smaller sheets of veneer than more modern rotary peeling methods.
Besides the orientation in which the veneer is cut and its corresponding patterns, various parts of manufacturing methods can leave subtle traces. Examples include tools like saws (figure 1.20), planes, chisels and various sanding equipment, which can all leave their characteristic marks on macroscopic level.
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1.2 Veneer in furniture
Veneers with their characteristic patterns have been used to decorate objects. Particular patterns of different species have often been combined to form a decorative composition of veneers; ranging from figurative marquetry to geometric parquetry compositions. Concerning abstract parquetry compositions, there are a couple of common patterns in which veneers are laid out:
- Mirroring
Because the size of sheets of veneer are limited by the dimensions of the saw and the cutting technique, they are often mirrored (book-matched, figure 1.21), quartered (figure 1.22) or laid out in more complex shapes to cover the entirety of the surface of an object. The mirror effect is formed because the sheets of veneer are cut side by side from the same piece of wood; the pattern may vary slightly because of an elapsing grain direction. Veneer in objects with such a repetition can be used as a reference to approximate the pattern of missing veneer.
- Herring-bone
Bands of veneer that are laid out in the pattern of a herring bone. - Cross banding
General term for bands of veneer.
Figure 1.21: mirrored veneer Figure 1.22: quartered veneer. (also mirroring along the edges).
P. 29 of 109 These compositions of veneer have been glued onto objects, often using animal glues. Apart from the decorative effect of the pattern itself, veneer also hides the underlying carcass. Together with the structure of the material itself, the construction plays an important role in the deterioration of veneer, see chapter ‘Deterioration’ (p. 30).
Since the broader goal of this research is to give the ability to approximate the whole range of veneers that have been used within the furniture trade, in principle, any veneered object can be used for this research. Especially veneers with a clear pattern are relevant for this research.
Secondary pattern characteristics
Finishing layersEach type of finishing layer has a characteristic effect on the visual perception of wood. This concerns the refraction and reflection of light, which is specific for each combination of type of finishing layer and type of wood. Transparent finishing layers can enhance specific elements of a pattern or give the illusion of depth (figure 1.23 and 1.24). Colourants can have an additional effect on the perception of a pattern.
Figure 1.23: infill without finishing layer.
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1.3 Deterioration
Under stable conditions of both environment and substrate, veneer can remain in place for long periods. When veneer deteriorates and comes loose, or lifts from its substrate, this is mainly caused by the difference in grain directions, hygroscopic sensitivity and the rates of response between the veneer and the substrate. When the veneer and its substrate move in different ways due to different grain directions, this may result in veneer that is too large or too small for the underlying
construction which is susceptible to mechanical damage or dissociation during routine handling and cleaning [Rivers, 2003: 307-308].
Besides missing veneer, there can also be a shortage of material due to the movement, and in particular shrinkage of the underlying construction, resulting in ‘missing’ material in the form of a split. In contrast to a piece of veneer breaking away or splitting, it can also relatively slowly disappear due to wear. The way the substrate of the veneer is constructed, greatly influences the deterioration of veneer.
Missing material often occurs along the edges of a piece of furniture. However, material can also get lost in the centre parts of a piece of veneer. This can happen because of, among other things, mechanical shock in the form of an impact, blisters, wear or incorrect restoration.
Adhesives (often animal glues) can lose their adhesive power and therefore hasten the deterioration of the veneer because of various factors like a high relative humidity or incompatibility between the substrate and the veneer. This incompatibility could be the result of a difference in grain direction, stiffness of the material or factors that make the veneer and the substrate stick less well onto each other like greasiness.
The dimensions of missing veneer can vary greatly, generally from a few square millimetres to dozens of square decimetres. Similarly, splits generally vary from a few square millimetres to dozens of square centimetres.
Thus, there are multiple ways in which veneer on objects deteriorates. Specifying these according to the ‘agents of deterioration’ from the Canadian Conservation Institute, these are as follows:
- Dissociation: this research concentrates on losses of veneer, this is therefore the main agent of deterioration; which is the result, and often a combination, of the following agents:
o Incorrect relative humidity (too low, too high and fluctuations) [Michalski, 2013]: ▪ Glue softens at a high relative humidity, after which the veneer can detach. ▪ Cross grain veneer movement; the wood grain of the veneer has a different direction in respect to the underlying wooden frame, because of which, the veneer can come loose.
o Water: similar to an incorrect relative humidity. o Physical forces:
▪ Breaking away loose veneer [Macron, 2014].
▪ Breaking away lifting veneer that was still (partly) glued to the underlying wooden frame [Macron, 2014].
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Tertiary pattern characteristics
In addition to the primary and secondary, tertiary pattern characteristics of veneer refer to the types of deterioration that visually influence the pattern. These characteristics are often present on older objects, examples are:
- Scratches
A marking on the surface of an object made with a sharp object, resulting in torn wood fibres (figure 1.25).
- Compressed material
Compressed wood or dents could occur in both patches and scratch-like shapes. However, these are not actual scratches since the wood fibres are not torn (figure 1.26).
- Water marks
Water marks can leave a distinct pattern on wood often known as tide lines. This is the result of the chromatography of finishing layers, dirt and other materials on and slightly under the surface of veneer (figure 1.27).
- Patina
Similar to finishing layers, the deposition of dirt can also have a significant contribution to the visual perception and topography of the pattern of veneer (figure 1.28).
- Craquelure
Craquelure patterns in the finishing layers change the visual appearance of the veneer pattern (figure 1.29).
Figure 1.25: scratches (filled with i.a. Figure 1.26: compressed material. dirt and finishing layers).
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Figure 1.27: water marks. Figure 1.28: patina.
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1.4 Current treatment
Firstly, the goal is to give an understanding of the spectrum of these treatments. Secondly, deciding what aspects of these treatments can be further developed on the basis of the current ethical values and additional wishes within this field. These findings will guide the later stages of this research that concern the digital approximation of lost veneer.
There is a range of techniques that are used to reintegrate losses of veneer. Exactly what technique is selected depends the type of loss and its location on the object. Other factors include the diversity of objects and contexts of restorers; ranging from the self-employed to large multidisciplinary workshops and from small to large museums and institutions. Besides the knowledge, personal preferences in working methods and corresponding ethics of a restorer, restoration techniques depend on the context of the restoration. Important factors are: object, complexity of the treatment, budget & timeframe, client, etc. Because of the extent of all these factors and subsequent amount of attention, not all treatments are conducted in a manner that might be
considered fully ethical. This means that some treatments are not reversible, re-treatable or durable. However, many treatments fall into a grey area; techniques are often used interchangeably, even within objects. This makes it difficult to say to what extent techniques are actually used and how reversible, re-treatable and durable treatments actually are.
Current restoration treatments of lost veneer are generally conducted as follows [Rivers, 2003: p. 307-308]:
1) Find a veneer that matches the original surrounding veneer. When choosing a veneer, it is important that the patterns of the infill and the surroundings of the infill are similar and that the directions of their grain align.
Readily available veneer can be searched for, or else, veneer is often cut from solid wood in order to create an infill that has a matching pattern and grain direction to that of its
surrounding material. The species of wood of the infill does not necessarily have to be the same as its original surroundings.
2) Reintegrate the loss (either a or b):
a. Cut away some original material in order to create a shape that can easily be seamlessly filled. These shapes depend on the shape of the lost veneer in order to minimalize loss of original material. These cuts can leave a straight, slightly conical or rounded outline (figure 1.30). When cutting away original material, create a joint running with the grain and preferably on a dark line in a pattern (figure 1.31, 1.32 and 1.35). In addition, avoid large cross-grain joints. Or;
b. Create an infill by leaving all original material intact and cutting the infill to fit. This method takes more time, when possible at all (figure 1.34 and 1.35).
Methods 2a and 2b illustrate the range of current treatments in restoration practice (1.35). In addition, a previously made infill can be replaced with a piece of veneer that is visually less obtrusive (figure 1.36).
3) Cut the infill level with the surrounding original veneer after gluing (figure 1.32 and 1.34) or level the back of the veneer before gluing. When the height of the infill is reduced to fit its surroundings with a chisel or plane, the colour and pattern can slightly change when the direction of the grain is not similar through the depth of the veneer.
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.
Figure 1.30: different outlines of cuts to straighten damaged veneer: straight, slightly conical or rounded.
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Figure 1.32: treatment according to 2a; in this case the original was cut straight onto which the infill was glued.
Figure 1.33: treatment according to 2a; in the finished result, the straight edge between original material and the infill is still visible and their patterns do not naturally merge into each other.
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Figure 1.34: treatment according to 2b; the infill was made to fit the surrounding material; no original material was taken away.
Figure 1.35: the difference between the treatments. Left: material is cut away in order to be easily filled. Right: the infill is made to fit the original veneer.
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Figure 1.36: old mismatching veneer (top); veneer is removed and the underlying construction becomes visible (middle); a piece of veneer is placed in the lacuna that has a better matching pattern (bottom) [Germond,
P. 38 of 109 There are additional techniques in respect to the treatment methods explained above that are used to reintegrate losses of veneer.
In the case of a split within a piece of veneer, arisen from movement of the underlying construction, multiple restoration methods are used:
- Loosening one side of the veneer and shifting this side to close the gap of the split. This treatment is more successful when most of the material along the sides of the split is still present and cleanable. When shifting the veneer, it is possible that a lacuna arises at the edge, in this case, a strip of similar veneer can be added to that side.
- Use moisture and heat to expand one side of the veneer and make it more pliable, followed by gluing the expanded veneer down, closing the gap.
- Filling the gap with a filler material and retouching the infill to match it surroundings. - Cutting away original material in order to make an infill using matching veneer.
In the case of relatively small lacunas, (or complex losses with no allowance to take away original material,) filler materials can be used to reintegrate lost veneer. This is done with materials such as animal glue (often in combination with fillers such as wood powder or scrapings or micro balloons), shellac, wax, ‘Modostuc’, epoxy, etc. These filler materials are often coloured through with
colourants to fit the background colour, or the dominant colour of its surroundings. In addition, these infills can be glazed or overpainted. With this type of manual inpainting, the wood grain can be interpolated or extrapolated to create a continuous pattern between the infill and the original surrounding veneer.
All in all, there is a range of techniques that are used to reintegrate losses of veneer. When carried out professionally, the results can be unobtrusive and even quite invisible. The disadvantages of currently used restoration techniques mainly concern two aspects. Firstly, because of the complex shapes of a loss, it can be problematically difficult to make particular infills when no original material is to be taken away. That is why original material is often sacrificed in order to create a lacuna that is easy to fill. However, this type of treatment is not reversible. Additionally, filling materials are used to reintegrate small losses, but these do not have the structure or pattern of natural veneer.
Secondly, creating an infill from natural veneer that visually flows over into the pattern of its original surroundings can be problematic. In some contexts, the reversibility and aesthetics of currently used restoration techniques could be improved.
These shortcomings form the basis of the next chapters in which opportunities are discussed that may form a solution.
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1.5 Conclusion
The structure of wood results in a characteristic pattern that depends on factors like wood species, individual trees, parts of the tree and in which orientation these parts of the tree are cut. These patterns range from simple stripes and curves to complex burrs. For this research, important primary characteristics of a pattern are the grain, surface roughness, topography, colour and irregularities. In addition, secondary pattern characteristics involve the traces that tools like saws and chisels leave on the surface of the veneer.
A variety of different patterns from different species has been used to decorate furniture. This is done by gluing (often with animal glues) veneer onto an object. Under stable conditions of both environment and substrate, veneer can remain in place for long periods. However, because of an incorrect relative humidity the underlying construction and difference in grain direction between the substrate and the veneer causes the object to deteriorate. When veneer loosens from its substrate, it becomes susceptible to mechanical damage or dissociation during routine handling and cleaning. Besides missing veneer, there can also be a shortage of material in the form of a split due to
shrinkage of the underlying construction. In contrast to a piece of veneer breaking away or splitting, it can also relatively slowly disappear due to wear. In general, the dissociation of veneer is mainly caused by a combination of an incorrect relative humidity and physical forces.
When carried out professionally, the results of traditional restoration methods of reintegrating lost veneer can be unobtrusive and even quite invisible. However, the disadvantages of currently used restoration techniques mainly concern two aspects. Firstly, because of the complex shapes of a loss, it can be problematic to make particular infills when no original material can be taken away. That is why original material is often sacrificed in order to create a lacuna that is easy to fill, however, this type of treatment is not reversible. Secondly, creating an infill from natural veneer that visually flows over into the pattern of its original surroundings can also be difficult. This is mainly relevant for pieces of veneer with a clear pattern.
All in all, in some contexts, the reversibility and aesthetics of currently used restoration techniques could be improved.
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2. Scanning techniques
This chapter discusses a range of scanning techniques that could be relevant in the process of creating a digital approximation of missing veneer.
The contribution of these techniques lies in what type of information they can yield concerning the loss. Two steps are important. Firstly, scanning the shape or topography of the lacuna in order to make a precise infill. Secondly, scanning the pattern of the area surrounding the infill in order to digitally approximate the pattern of the infill itself.
Several scanning techniques are selected on the basis of their suitability or relevance for this research; only these will be further discussed. Their relevance is defined by their ability to visualize data of patterns and lacunas. Generally, the dimensions of these lacunas and surrounding patterns range from less than a square centimetre to multiple square decimetres.
The scanning techniques should provide information about, some of, the following aspects: - Shape of a surface
- Pattern on a surface
- 3-dimensional structure of the wood grain within the material
Scanning techniques that will not be further discussed are those optimized to scan larger or smaller area's (from room- to cellular level), scanning techniques that do not yield sufficient data about the visual appearance of a material and scanning techniques that provide data about material
identification. Although striving for completeness, it is possible that not all scanning techniques that are potentially relevant for this research are included. The scanning techniques will be discussed in a general sense. Thus, because of rapidness in which these technologies change, particular brands or types of equipment will not be mentioned.
The subchapters of the individual scanning techniques are classified according to the technique they are based on. This generally concerns the type of radiation that is emitted by the source. The sub-chapters include how the scanning techniques work, on which technologies they are based and how they are currently used in restoration practice. Additionally, it is important to weigh the advantages and disadvantages of specific types of scanning equipment in order to make the right judgement on which techniques to choose for a restoration project. Thus, each sub-chapter includes a SWOT-analysis, filled in where relevant.
Relatively much is known about scanning techniques, also within the conservation and restoration sector. However, within the context of wood and corresponding finishing techniques that are common in wood restoration, relatively less is known. This makes it more difficult to assess which scanning techniques may actually be relevant for this research.
Most of the scanning techniques that are mentioned in this chapter can be used together in one piece of apparatus. These combinations can make it difficult to judge upon which methods a technology is based. In addition, care should be taken in interpreting the technical capabilities that are stated by companies who sell scanning equipment for this knowledge could have been adopted by academic literature. For example, the accuracy of a scanner can be very high, however, when only the accuracy of the sensor is high while the quality of the glass lens in front of the sensor is not able to pass along that visual information, the final accuracy of the scan does not necessarily have to be equally high. Or, differently put, the equipment is as good as its worst part.
P. 41 of 109 In addition, some restoration related literature falls back on the use of a limited amount of scanning techniques and rules out the use and potential benefits from other techniques. This means that in some restoration contexts there are more scanning techniques that may be useful than are initially obvious.
Furthermore, terminology may not always be used consistently in literature. For example, it is important to distinguish the difference between accuracy and precision. The accuracy indicates the range of error of a measurement. The precision concerns the repeatability of a measurement.
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2.1 Laser scanning
In short:
- Digital 3-dimensional model
- Surface scan (does not include deeper layers) - Point detection
- Possibility of adding colour images
How does the scanning technique work, on what technology is it based?
3-Dimensional laser scanning is a scanning method of which multiple types exist. In general, this technique produces a digital 3-dimensional model of the surface of an object. The different types of laser scanners are based on the same principle; from a source, a laser beam is emitted onto the surface of an object and the reflected beam is recorded by a sensor [Payne, 2012: p. 17].
Equipment: laser emitter - object - sensor - software
The objects’ surface is scanned point by point. In each point information about the 3-dimensional structure of the object is captured. This results in a point cloud (set of data points in space) in which data points together form a 3-dimensional model of the surface of the object relative to specified XYZ coordinates. Laser scans can be combined with photographs (figure 2.1). in order to create a model with colours on which, inter alia, veneer patterns can be seen It is not always clear if the colours on a model are the result of combining a laser scan with photographic imaging techniques; other techniques might also have been used.
In some systems, rotating mirrors are used to direct the laser beam over the object's surface. Types of scanners that use a mirror (similar to laser cleaning systems) generally produce scans with a high resolution [Piening, 2012: p. 97].
The group of 3-dimensional laser scanners consists of different subtypes. Differences lie, among others, in the power and wavelength of the laser beam which result in quality differences of the 3-dimensional models. Each method has its own advantages and limitations. At least the following laser scanning methods can be distinguished:
Triangulation-based laser scanners
Laser, object and sensor form a triangle. This triangle can be used to calculate the X,Y and Z coordinates of individual points because there is a fixed, known distance between the laser source and sensor [Bryan 2006: 164] [Payne, 2012: p. 17]. In this way the distance of the object to the scanner can be calculated. From the data, a topography of the surface can be created [Factum Arte].
Time-of-flight-based laser scanners
In this laser scanning method, the distance of one point on the object to the scanner is calculated by measuring the time it takes for the laser to be reflected back to the scanner for the speed of light is known. This laser scanning method is also known as ‘Licht Detection and Ranging’ or LiDaR [Bryan 2006: p. 165-166].
P. 43 of 109 How is the scanning technique currently used in restoration practice or other fields?
Laser scanning techniques are widely used in different fields. Examples of applications range from archaeological mapping to continuously monitoring the state of heritage related objects. Short-range systems are generally used for recording the shape and surface of smaller objects, for example, for investigating deterioration or for making a replica. In comparison to stationary scanners, handheld scanners can have added benefits, such as, when scanning objects that cannot be moved or objects with undercuts [Piening, 2012: p. 97]
a. What are the Strengths of these technologies in restoration?
- Triangulation scanners have a high accuracy, typically 50μm [Jones, 2007: p. 7-8] [Payne, 2012: p. 17].
- Time-of-flight scanners are not restricted by a fixed distance between object and scanner. They are therefore more portable [Jones, 2007: p. 7-8] [Payne, 2012: p. 17] and able to create a scan at a longer distance.
- Systems that use mirrors are faster because mirrors are light and rotate easier than the entire apparatus.
b. What are the Weaknesses of these technologies in restoration?
- Because laser scanners record the object point by point, there is a possibility of distortions arising when there are movements or vibrations during the scanning process.
- Triangulation systems have a restricted operating range (<1m) and their operation is disrupted by bright sunlight [Bryan 2006: p. 165-6]
- Time-of-flight scanning is less accurate (circa 3–6mm) than triangulation [Jones 2007: p. 7]. Due to the high speed of light, timing the round-trip time (light source - object - sensor) is difficult and the accuracy of the distance measurement is relatively low, on the order of millimetres. Therefore, for most restoration applications, tri-angulation-based scanning is more appropriate [Payne, 2012: p. 17].
c. What are the Opportunities of these technologies in restoration?
- Instead of creating point clouds that result in a relative loss of information, 3-dimensional data may be stored in the form of raw tonal video which enables the possibility of re-processing the data at a higher resolution when technologies are further developed in the future (figure 2.2). It is also possible to combine infra-red, x-ray, colour, historic pictures and other types of information with the data from the laser scan [Factum Arte]. In addition, it may be possible to obtain some data on the wood structure from under paint layers using these techniques.
d. What are the Threats of these technologies in restoration? - The high intensity light source may deteriorate the object.
- When combining data from different scanning methods (e.g. photography), warping might affect the correct alignment of the 3-dimensonal shape with the photograph. This should be
P. 44 of 109 taken into account when taking digital measurement (e.g. on which to base an
approximation of missing material) which might subsequently also be affected.
Figure 2.1: a digital model of an Egyptian foot cover which is the result of a laser scan [Payne, 2012: p. 21], possibly in combination with photographic techniques.
Figure 2.2: a 3-dimensonal rendering of a panel painting using techniques based on raw tonal video on which the structure of the wood is partly visible from under the finishing layers [Factum Arte].
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2.2 Structured Light Scanning
In short:
- Digital 3-dimensional model
- Surface scan (does not include deeper layers)
How does the scanning technique work, on what technology is it based?
Structured light scanners produce a digital 3-dimensional model of the surface of an object. This is based on interpreting the deformation of a specific light pattern that is projected onto that object.
Equipment: pattern projector - object - sensor - software
Structured light scanners scan a surface by projecting a light pattern over an object. The pattern deforms on the surface and this deformation is registered by one or more cameras (figure 2.3). Software is used to calculate the 3-dimensional shape of an object based on these deformations [4D Lab, 25 May 2018]. A wide range of patterns is possible to use in this scanning method, these are, amongst others based on stripes, dots, grids and colours. The type of pattern influences the accuracy and speed of the system [Bell, 2014: p. 181-213].
How is the scanning technique currently used in restoration practice or other fields?
This technique is, inter alia, used to make replications, ranging from surfaces and parts to entire objects, for example, with chairs from the Parker Collection [Piening, 2012: p. 106],
What advances are recently made in this technology?
Recent innovations have overcome some problems in this scanning method: scanners can move more freely and object size is less of a limitation [4D Lab, 25 May 2018]
a. What are the Strengths of these technologies in restoration?
- Structured light scanning can perform scans in just milliseconds [Bell, 2016: p. 181-231]. - The method creates scans of high precision because they scan the surface of an object
several times [4D Lab, 25 May 2018].
- Generally, structured light scanners are able to acquire higher precision than laser scanners [4D Lab, 25 May 2018].
b. What are the Weaknesses of these technologies in restoration?
- Some types of scanners are bound to the size of the projected pattern. It can therefore be difficult to scan larger objects. A solution may be to use photogrammetry. Photogrammetry is the science of taking 2- or 3-dimensional measurements from photographs. These
measurements can be a goal in itself or they can be used to stitch multiple photographs together to form a digital 3-dimensional model.
P. 46 of 109 - Strong exterior light conditions might influence the projected pattern, reducing their
contrast. Controlled light conditions might therefore be a requirement [4D Lab, 25 May 2018].
White light fringe projection
White light fringe projection is a frequently used subtype of structured light scanners.
How does the scanning technique work, on what technology is it based?
The technique is based on using white light. The fringe projection concerns the type of pattern that is projected onto the object consisting of alternating lighter and darker bands caused by e.g. overlapping of two set of parallel lines.
a. What are the Strengths of these technologies in restoration?
- Can be used to scan objects of various sizes [Piening, 2012: p. 97].
- Different resolutions of the data are possible, depending on the distance between the object and the scanner. The shorter the distance, the higher the resolutions and the precision of the scan (18-400 μm) [Piening, 2012: p. 97].
- Relatively flat surface structures are possible to be scanned [Piening, 2012: p. 97].
b. What are the Weaknesses of these technologies in restoration?
- By scanning 3-dimensional objects with a lot of undercuts, the position of the scanner has to be changed frequently in order to fill in 'black points'. In practice, this may significantly increase the time for scanning and post-processing the obtained data [Piening, 2012: p. 97].
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Figure 2.3: The different phases of a 3-dimensional capture using a multiwavelength phase-shifting method with a calibrated structured light system (a-h) and the resulting digital 3-dimensional model (i-l)
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2.3 Reflectance Transformation Imaging (RTI)
In short:
- Digital 2-dimensional image
- Creates a digital interactively illuminated image - Colour image
- Surface scan (does not include deeper layers)
How does the scanning technique work, on what technology is it based?
Reflectance transformation imaging creates a digital 2-dimensional photographic image of an object that can be interactively illuminated and examined.
Equipment: movable light source - object - camera - software
The underlying principle consists of making multiple photographs of an object from a fixed camera position and illuminating the object from different directions. Software processes the multiple images into a single file [Smithsonian, 2-6-2018]. The different illumination directions and RGB info are encoded in per individual image pixel [Payne, 2012: p. 18]. In the resulting image it is possible to interactively, artificially, illuminate the surface from any direction, for example, in order to reveal texture (figure 2.4) [Payne, 2012: p. 18]. Besides controlling the light direction, it is possible to zoom in and out, or enhancing the image by increasing sharpness and contrast [Smithsonian, 2 June 2018]. The setup of scanning equipment can be very simple.
The RTI technique can also be performed with the additional use of a microscope and also with wavelengths outside of the visible spectrum; for example, with cameras that can capture IR of UV [Schroer, 2012: p. 40].
How is the scanning technique currently used in restoration practice or other fields?
In the restoration field, this scanning technique can be used to monitor the state of an object. It is also possible to study tool marks (on furniture), brushstrokes and other subtle surface details such as traces of wear [Schroer, 2012: p. 41]. With the use of infrared light, it is possible to disclose hidden surface features below painted surfaces or finishing layers [Schroer, 2012: p. 40]. Another interesting project in which this technology is used, is reading letters in the rolled-up Dead Sea Scrolls
[Cosentino: p. 71].
a. What are the Strengths of these technologies in restoration?
- Possible to examine a variety of object sizes in a large range of materials.
- Possibility to interactively re-light and examine a digital model of the surface of an object, which enables to see the topography and colour of an object in detail [Schroer, 2012: p. 38]. Thus, patterns and other surface characteristics of veneer can be visually enhanced.
- There is generally no data loss due to shadow and specular highlights; as is the case in conventional photography [Cosentino: p. 71].
P. 49 of 109 - It is possible to perform scan on macro but also in micro scale in the order of hundreds of
microns [Cosentino: p. 71], even with commercial cameras. This makes it possible to gain high scanning accuracy with relatively simple equipment [Cultural Heritage Imaging, 1 June 18]
- Method of RTI is relatively easy to learn [Schroer, 2012: p. 39].
b. What are the Weaknesses of these technologies in restoration?
- Because the direct output of this scanning technique is 2-dimensional, the usage is limited.
c. What are the Opportunities of these technologies in restoration? - Possibility to couple data to a 3-dimensional model.
d. What are the Threats of these technologies in restoration?
- Due to the changing direction of the light source, it may be possible that shapes are slightly distorted. This should be considered when taking measurements or combining the data to form a 3-dimensional model.
