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Photometric Stereo by UV-Induced Fluorescence to Detect Protrusions on Georgia O’Keeffe’s Paintings
Abstract
A significant number of oil paintings produced by Georgia O’Keeffe (1887–1986) show surface protrusions of varying width, up to several hundreds of microns. These protrusions are similar to those described in the art conservation literature as metallic soaps. Since the presence of these protrusions raises questions about the state of conservation and long-term prospects for deterioration of these artworks, a 3D-imaging technique, photometric stereo using ultraviolet illumination, was developed for the long-term monitoring of the surface shape of the protrusions and the surrounding paint. Because the UV fluorescence response of painting materials is isotropic, errors typically caused by non-Lambertian (anisotropic) specularities when using visible reflected light can be avoided providing a more accurate estimation of shape. As an added benefit, fluorescence provides additional contrast information contributing to material characterization. The developed methodology aims to detect, characterize, and quantify the distribution of micro-protrusions and their development over the surface of entire artworks. Combined with a set of analytical in situ techniques, and computational tools, this approach constitutes a novel methodology to investigate the selective distribution of protrusions in correlation with the composition of painting materials at the macroscale. While focused on O’Keeffe’s paintings as a case study, we expect the proposed approach to have broader significance by providing a non-invasive protocol to the conservation community to probe topological changes for any relatively flat painted surface of an artwork, and more specifically to monitor the dynamic formation of protrusions, in relation to paint composition and modifications of environmental conditions, loans, exhibitions, and storage over the long term.
... Normal maps produced by photometric stereo have been shown to be of higher quality than those produced by RTI (Mac-Donald 2011). The applicability and the ease of use of photometric stereo as a replacement or a complementary method to inspect surface details of fossil insects should be investigated in future studies, especially considering the emergence of easily usable free software to produce the photometric stereo models (Salvant et al. 2019). ...
The earliest known fossil beetle †Coleopsis archaica is reexamined using Reflectance Transformation Imaging (RTI). The morphological observations are evaluated with respect to phylogenetic implications and the early evolution of Coleoptera. †Coleopsis archaica belongs to an early Permian branch of beetles, outside a monophyletic unit comprising Coleoptera (in the widest sense) excluding †Tshekardocoleidae. This clade is mainly characterized by a complex of apomorphic features: elytra with epipleura and with a close fit with the posterior body, thus forming a tightly sealed subelytral space. In contrast to this, the elytra of †C. archaica and †Tshekardocoleidae cover the metathorax and abdomen in a loose tent-like manner and posteriorly distinctly surpass the abdominal apex. So far, no synapomorphies of the two taxa from the first half of the Permian have been identified. The very short and transverse pronotum is likely an autapomorphy of †C. archaica. A thorough documentation of the structural features of early beetle fossils should have high priority. RTI is a very promising tool to obtain new and well-founded morphological data, which will allow a thorough phylogenetic evaluation of Permian beetles in future studies. We extended the conventional RTI workflow by focus merging and panoramic stitching, in order to overcome previous limitations. Taxonomic rearrangements of stem group beetles including †C. archaica were suggested in recent studies by A.G. Kirejtshuk and co-workers. As they are not based on shared derived features they are irrelevant in a phylogenetic and evolutionary context.
Up to 70 % of the oil paintings conserved in collections present metal soaps, which result from the chemical reaction between metal ions present in the painted layers and free fatty acids from the lipidic binders. In recent decades, conservators and conservation scientists have been systematically identifying various and frequent conservation problems that can be linked to the formation of metal soaps. It is also increasingly recognized that metal soap formation may not compromise the integrity of paint so there is a need for careful assessment of the implications of metal soaps for conservation. This review aims to critically assess scientific literature related to commonly adopted analytical techniques for the analysis of metal soaps in oil paintings. A comparison of different analytical methods is provided, highlighting advantages associated with each, as well as limitations identified through the analysis of reference materials and applications to the analysis of samples from historical paintings. Conservation and Heritage scientists frequently face challenging issues related to metal soaps phenomena in oil paint systems. This review provides a critical overview of analytical techniques available for the identification, localisation and study of the presence of metal soaps. Advantages and disadvantages of each technique are discussed and the comparison between data collected from ad hoc and real painting samples is reported.
Metal soaps are formed in paint layers thorough the reaction of metal ions of pigments and fatty acids of organic binders. In this study, micro-ATR-FTIR spectroscopic imaging was used to analyse the formation of lead soaps in oil-based paint layers in relation to their exposure to moisture sources. The investigations were carried out on authentic samples of complex stratigraphies from cold painted terracotta statues (Sacred Mount, Varallo, UNESCO) and different IR-active lead white pigments, organic materials, and lead soaps were discriminated. The saponification of selected paint layers was correlated to the conservation history, the manufacturing technique, and the build-up of layers. The presence of hydrophilic layers within the stratigraphy and their role as a further water source are discussed. Furthermore, the modifications experienced by lead-based pigments from the core of an intact grain of pigment towards the newly formed decay phases were investigated via a novel approach based on shift of the peak for the corresponding spectral bands and their integrated absorbance in the ATR-FTIR spectra. Qualitative information on the spatial distribution from the chemical images was combined with quantitative information on the peak shift to evaluate the different manufacture (lead carbonate, basic lead carbonate) or the extent of decay undergone by the lead-based pigments as a function of their grain size, contiguous layers, and moisture source. Similar results, having a high impact on heritage science and analytical chemistry, allow developing up-to-date conservation strategies by connecting an advanced knowledge of the materials to the social and conservation history of artefacts.
During a routine condition survey in 2007 at the Georgia O’Keeffe Museum in Santa Fe, micro-protrusions were found scattered across the surfaces of the artist’s oil paintings produced between 1920 and 1950. In many of her works, including Pedernal (1941) and A Man from the Desert (1941), lead soaps were found aggregated at the painting surface, forcing the surrounding paint to deform into pin-sized protrusions. The structure and composition of the protrusions was analyzed to determine why they formed. Microsamples were removed from four of O’Keeffe’s paintings and investigated using a combination of SEM–EDX, GC–MS, and FT-IR. The combined GC–MS and FT-IR results indicate that the protrusions contain primarily lead carboxylates. The results obtained for a series of ground samples removed from four different paintings were then compared with samples collected from a commercially pre-primed canvas roll found in the artist’s house in Ghost Ranch, NM. We identified two different types of canvases: i) a commercially pre-primed canvas and ii) artist primed canvas. This commercially pre-primed canvas roll has a similar morphology to the ground layers in her paintings that contain protrusions additionally these grounds were found to contain an excess of free fatty acids and metal carboxylates (lead soaps) based on a comparison to canvases the artist primed herself. Based on the analytical results from the four paintings and the commercial pre-primed canvas, we concluded that the artist’s use of this particular commercially pre-primed canvas between 1920 and 1950 play an important role in the observed micro-protrusions.
Lead and zinc soaps are commonly seen as signs of oil paint degradation. The pathway from pigment to metal soaps and their subsequent migration and accumulation are poorly understood, and it is unknown to what extent these processes are influenced by humidity and temperature. In order to understand fully the behavior of metal soaps in paint systems, it is first necessary to characterize the molecular structure of metal soaps in detail. This paper presents the synthesis and structural characterization of three new types of metal soaps: "basic" lead soaps, zinc soaps that incorporate sodium or potassium ions, and metal soaps containing both palmitate and stearate. The results are a first step toward more accurate interpretations in paint sample analysis and a more detailed understanding of oil paint degradation on a molecular scale.
KEYWORDS: lead soaps, zinc soaps, oil paint, degradation, XRD, FTIR
During the examination of two 19th century oil paintings by Swiss artists, the Reading pastor (ca. 1885) by Ferdinand Hodler (1853–1918) and Portrait of a young girl (ca. 1888) by Filippo Franzoni (1857–1911), it was observed that in particular locations, the surface had an unusually granular texture, given by the application of metallic particles and the presence of large translucent green agglomerates. In this paper we investigate the composition and origin of the metal particles and green agglomerates. A combination of bulk analysis (GC–MS and FTIR) and analytical microscopy techniques (X-ray tomographic microscopy, light microscopy, ATR–FTIR–FPA imaging and SEM–EDX) were used. It was concluded that the metal particles are composed of a lower quality brass (copper/zinc alloy) and ground from foil. The current appearance resulted from the reaction of the brass and its corrosion products with the fatty acids in the surrounding paint/varnish matrix. This has lead to the formation of agglomerates of zinc and copper carboxylates, the latter responsible for their green colour.
Graphical abstract:
The painting entitled Reading pastor (ca. 1885) by Ferdinand Hodler (1853–1918) was one of two paintings studied showing the formation of zinc and copper carboxylates as the alteration of brass pigment. The characterisation was carried out with combined analytical techniques including X-ray tomographic microscopy and ATR–FTIR spectroscopy.
In this paper, we propose a near-light illuminationmodel for image relighting and 3D shape recovery. Classicmethods such as used by the popular RTI software from CulturalHeritage Imaging assume that lighting is infinitely far away fromthe scene. However, this constraint is impossible to achieve inpractice: light sources cannot be too far away from the scenedue to space and power constraints. This causes non uniform illumination artifacts due to variations in the distance betweenthe light source and points in the scene. We correct this effectto provide much more uniformly lit images that yield moreappealing image for relighting applications. Furthermore, weuse our near-light model for more accurate photometric stereocalculations of surface normals, eliminating the “potato-chip”shaped surface reconstruction error that results from violating thefar-light assumption. We verify our model with both free-formcapture using hand-held flash as the illumination source, andcapture using LED lights mounted on a dome shaped surface
Starting in the 1890s the artist Paul Gauguin (1848-1903) created a series of prints and transfer drawings using techniques that are not entirely understood. To better understand the artist’s production methods, photometric stereo was used to assess the surface shape of a number of these graphic works that are now in the collection of the Art Institute of Chicago. Photometric stereo uses multiple images of Gauguin’s graphic works captured from a fixed camera position, lit from multiple specific angles to create an interactive composite image that reveals textural characteristics. These active images reveal details of sequential media application upon experimental printing matrices that help resolve longstanding art historical questions about the evolution of Gauguin’s printing techniques. Our study promotes the use of photometric stereo to capitalize on the increasing popularity of
Reflectance Transformation Imaging (RTI) among conservators in the world’s leading museums.
The Menaced Assassin by René Magritte is a major surrealist painting from the collection of the Museum of Modern Art (MoMA). A recent conservation treatment coupled with scientific analysis has characterised some puzzling and visually disturbing surface phenomena once attributed to mold growth.
A detailed examination of the painting under the microscope has evidenced that the formation of radiating dark micro cracks is responsible for the overall speckled appearance of the painting. These web-like cracks often surround a small white crystalline particle and are generally accompanied by aggregates of very small white particles sitting on the top of the surface. The examination also revealed the presence of larger flat pustules of a translucent and softer material with no apparent visual impact. An overall whitish haze is visible as well over the areas painted in red and brown earth tones.
Samples of both white and translucent material were analysed by Fourier transform infrared spectroscopy (FTIR) and confirmed the presence of metal soaps. Past exhibition conditions of the painting and composition of the ground layer may have played a role in the formation of the soaps, in particular the coarse calcium carbonate rich priming layer and presence of kaolin as identified by FTIR and scanning electron microscopy combined with electron dispersive spectroscopy (SEM-EDX).
The formation of metal soaps is a major problem for oil paintings conservators. The complexes of either lead or zinc and fatty acids are the product of reactions between common pigments and the oil binder, and they are associated with many types of degradation that affect the appearance and stability of oil paint layers. Fourier Transform Infrared spectroscopy (FTIR) reveals that a paint sample from The Woodcutter (after Millet) by Vincent van Gogh contains two distinct zinc carboxylate species, one similar to zinc palmitate and one that is characterized by a broadened asymmetric stretch COO band shifted to 1570-1590 cm−1 . Though this observation has been made in many paintings, it was previously left unexplained. It is shown that neither variations in the composition of zinc soaps (i.e. zinc soaps containing mixtures of fatty acids or metals) nor fatty acids adsorbed on pigment surfaces are responsible for the second zinc carboxylate species. X-ray diffraction (XRD) and FTIR analysis indicate that the broad COO band represents amorphous zinc carboxylates. These species can be interpreted as either non-crystalline zinc soaps or zinc ions bound to carboxylate moieties on the polymerized oil network, a system similar to ionomers. These findings uncover an intermediate stage of metal soap-related degradation of oil paintings, and lead the way to improved methods for the prevention and treatment of oil paint degradation.
Lead soap aggregateshave been-found in lead-containing oil paint layers in paintings from the thirteenth to the twentieth century. They severely affect the stability of the paint layers and disturb the surface of the paintings. Paint cross-sectionsfrom five paintings affected by lead soaps were selected to illustrate and investigate this degradation phenomenon with the analytical imaging techniques of Fourier transform infrared spectroscopy, secondary ion mass spectrometry and scanning electron microscopy combined with X-ray analysis. Examples aregiven of lead soapsforming in a mature paint system or, alternatively, in the early drying stage of the oil; lead soapsforming from various types of lead-containing pigments or driers; lead soapsforming in multiple paint layers; and lead-containing crystallization products inside aggregates. The phenomenon of lead soap aggregatesis multifaceted, and one general scenario describing theformation of lead soap aggregatescannot explain all aspects. However, the integration of the chemical information and its distribution among the paint layers leads to the proposal that reactivefree monocarboxylicfatty acids playa key role in lead soap aggregateformation. The availability and release of thesefatty acids depends on the original paint composition, the build-up of the layers, and the conservation/environmental exposure history of the painting.
Groupe de Personnages (1938), a canvas painting created in Paris during the Spanish Civil War, required stabilization prior to exhibition. Upon examination, flaking paint and tiny spherical protrusions within those paint layers were identified. A project was initiated to determine materials that could have contributed to the current state of this painting. Because Groupe de Personnages had not undergone harsh conservation treatments this was a unique opportunity to study Miró's techniques and materials from a period of great artistic experimentation. Scientific analysis revealed the identity of pigments and the presence of zinc soaps. In addition, because Miró was a prolific writer, evidence was found in his letters, studio notes and interviews to corroborate analytical results – contributing to a more complete understanding of his artistic intentions and the conservation-related consequences of his selected materials.
We propose a novel technique called bispectral photometric stereo that makes effective use of fluorescence for shape reconstruction. Fluorescence is a common phenomenon occurring in many objects from natural gems and corals, to fluorescent dyes used in clothing. One of the important characteristics of fluorescence is its wavelength-shifting behavior: fluorescent materials absorb light at a certain wavelength and then reemit it at longer wavelengths. Due to the complexity of its emission process, fluorescence tends to be excluded from most algorithms in computer vision and image processing. In this paper, we show that there is a strong similarity between fluorescence and ideal diffuse reflection and that fluorescence can provide distinct clues on how to estimate an object's shape. Moreover, fluorescence's wavelength-shifting property enables us to estimate the shape of an object by applying photometric stereo to emission-only images without suffering from specular reflection. This is the significant advantage of the fluorescence-based method over previous methods based on reflection.
In this paper we present modern texture mapping techniques and several
applications of polynomial texture mapping in cultural heritage programs. We
also consider some well-known and some new methods for mathematical procedure
that is involved in generation of polynomial texture maps.
AA setup for reflection visible light imaging microspectroscopy (VIS-imaging) as well as its evaluationevaluation and application is described and tested. The spatial resolution of the system is about 11 fJm at a spectral resolution of 4 nm. The optical contrast between different coloured particles partsparts in the surface of a sample is optimised with a new image processing method for mapping ofof the distribution of the identified pigment particles. The potential of VIS-imaging in the study ofof paint cross-sections obtained from paintings is explored. Spectra obtained from pigment par-ticlesticles in these cross-sections result in the classification or identification of several pigments. TheThe investigated paint samples are challenging test cases, as they contain several coloured materialsmaterials with a very fine distribution. VIS-imaging can identify and map the most common traditionaltraditional blue pigments i.e. smalt, azurite. ultramarine and indigo in I7' h century oil paintings .ings. Small can be identified even after complete discoloration. VIS imaging analysis assisted inin the identification and mapping of modern synthetic red and yellow pigments in a 20''' century painting. painting.
Saturated monocarboxylic fatty acids (namely palmitic acid and stearic acid, which are markers for the type of oil used in paintings) can be identified with imaging secondary ion mass spectrometry (SIMS), while retaining spatial information. The P:S ratios presented were determined with negative ion SIMS in individual layers of paint cross-sections from 15th to 19th century paintings. The positive ion mass spectrum gives information about the speciation of the fatty acids (free, ester-bound or metal carboxylates), indicative of the drying stage of the oil. Studies on freshly applied multi-layered oil paint systems suggest the diffusion of oil triglycerides between layers, which will influence the P:S ratio.
Translucent white lumps or inclusions in red lead and lead-tin yellow type I paint layers are discussed in detail. These lumps are described as varying in size. Most often visible under the microscope and sometimes to the naked eye, the inclusions impart a gritty texture to paint films. Inclusions are found to be ubiquitous in European oil paintings with red lead and lead-tin yellow type I pigments. The earliest example of a lead soap inclusion has been identified in the 13th-century Westminster retable. Inclusions commonly are made up of lead carboxylates and lead carbonate and are formed by the interaction of the lead-containing pigments and oil-binding medium. Certain characteristics of the inclusions, such as the fluorescence of the particles in ultraviolet light and the reduced azelaic acid content, are proposed to be responsible for the misinterpretation of aged oil paint layers as being bound in protein. Techniques of analysis of inclusions presented in this article include energy dispersive x-ray analysis in the scanning electron microscope, x-ray diffraction, Fourier transform infrared spectroscopy, and thermally assisted transmethylation gas chromatography-mass spectrometry. The analysis of inclusions in paint layers is presented in tables that identify the paintings the samples are taken from, describe the paint layers, and list the content of the inclusions. An appendix details the preparation of lead fatty acid soaps and presents characteristic infrared frequencies for lead soaps of palmitic, stearic, and azelaic acids.
We report for the first time the formation of calcium soaps in paintings. These were characterised by multiple analytical techniques namely FTIR, CI-DTMS and GCMS. A mechanism of formation is proposed based on evidence gathered by 2D and 3D correlative microscopy using light microscopy, SEM-EDX and SRXTM. We propose that the extreme porosity of the clay mineral/chalk/glue ground promotes the absorption of triglycerides from the oil paint binding medium into the ground layer. The presence of clay slightly lowers the pH of the ground layer promoting the instability of the calcium carbonate and facilitating the formation of calcium soaps.
This paper discusses the interim results of the AHRC RTISAD project. The project has developed and tested a range of techniques for gathering and processing reflectance transformation imaging (RTI) data. It has also assembled a detailed understanding of the breadth of RTI practice. Over the past decade the range of applications and algorithms in the broad domain of RTI has increased markedly, with current working addressing issues such as large resolution capture, 3D RTI, annotation, enhancement amongst others. Capture of RTI datasets has begun to occur in all aspects of cultural heritage and elsewhere. This has in turn prompted the development of policies and methods for managing and integrating the large quantities of data produced. The paper describes these techniques and issues in the context of a range of artefacts, including painted Roman and Neolithic surfaces, examples of ancient documents in a variety of forms, and archaeological datasets from Herculaneum, ?atalh?y?k, Abydos and elsewhere. The paper also identifies on-going software development work of value to the broad EVA community and proposes further enhancements.
Polynomial texture mapping (PTM) offers advantages over traditional raking light photography for examining and documenting the surface texture and shape of paintings. It is less expensive and computationally intensive than structured light or laser-based scanning techniques. Several small paintings and mock-up paintings have been examined and it is shown that PTMs are able to record surface features including craquelure, planar distortion, wood grain, canvas weave and pentimenti. By making images before and after physical change, the PTM technique can monitor and map change to the surface texture and shape of paintings. Future development of the system is described briefly.
The tools and standards of best practice adopted by natural science (NS) and cultural heritage (CH) professionals will determine the digital future of NS and CH digital imaging work. This tutorial discusses emerging digital technologies and explores issues influencing widespread adoption of digital practices for NS and CH. The tutorial explores a possible digital future for NS and CH through key concepts; adoption of digital surrogates, empirical (scientific) provenance, perpetual digital conservation, and 'born archival' semantic knowledge management. The tutorial discusses multiple image based technologies along with current research including; Reflectance Transformation Imaging (RTI), Photometric Stereo, and new research in the next generation of multi-view RTI. This research involves extending stereo correspondence methods. These technologies permit generation of digital surrogates that can serve as trusted representations of 'real world' content. The tutorial explores how empirical provenance contributes to the reliability of digital surrogates, and how perpetual digital conservation can ensure that digital surrogates will be archived and available for future generations. The tutorial investigates the role of semantically based knowledge management strategies and their use in simplifying ease of use by natural science and CH professionals as well as long term preservation activities. The tutorial also investigates these emerging technologies' potential to democratize digital technology, making digital tools and methods easy to adopt and make NS and CH materials widely available to diverse audiences. The tutorial concludes with hands-on demonstrations of image-based capture and processing methods and a practical problem solving Q&A with the audience.
For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the
analysis of scientific images. We discuss the origins, challenges and solutions of these two programs,
and how their history can serve to advise and inform other software projects.
Reflection transformation imaging offers a powerful new method of documenting and communicating numismatic cultural heritage information. The challenges of documenting numismatic material will be examined along with the limitations of traditional documentary techniques. Previous uses of structured light scanning and PTMs in numismatic documentation are reviewed and evaluated. A novel, low cost method for capturing PTMs at remote locations and subsequent data processing operations is described. Reflection transformation imaging is shown to capture more complete documentation than traditional photographic methods and communicate this information with ease through digital media. The advantages of interactive relighting of numismatic PTM images in conjunction with enhancement operations are explored along with the potential of informed choice of the most information rich illumination directions. Advantages of joint capture of structured light and PTMs are examined including the inherent registration of range and normal data, using range and normal information together to improve 3D position accuracy, and the enhanced evidentiary reliability that results.
We offer two new methods of documenting and communicating cultural heritage information using Reflection Transformation Imaging (RTI). One imaging method is able to acquire Polynomial Texture Maps (PTMs) of 3D rock art possessing a large range of sizes, shapes, and environmental contexts. Unlike existing PTM capture methods requiring known light source positions, we rely on the user to position a handheld light source, and recover the lighting direction from the specular highlights produced on a black sphere included in the field of view captured by the camera. The acquisition method is simple, fast, very low cost, and easy to learn. A complementary method of integrating digital RTI representations of subjects from multiple viewpoints is also presented. It permits RTI examination "in the round" in a unified, interactive, image-based representation. Collaborative tests between Cultural Heritage Imaging, Hewlett- Packard Labs, and the UNESCO Prehistoric Rock-Art Sites in the Côa Valley, a World Heritage Site in Portugal, suggest this approach will be very beneficial when applied to paleolithic petroglyphs of various sizes, both in the field and in the laboratory. These benefits over current standards of best practice can be generalized to a broad range of cultural heritage material.
We estimate surface normal maps of an object from either its diffuse or specular reflectance using four spherical gradient illumination patterns. In contrast to traditional photometric stereo, the spherical patterns allow normals to be estimated simultaneously from any number of viewpoints. We present two polarized lighting techniques that allow the diffuse and specular normal maps of an object to be measured independently. For scattering materials, we show that the specular normal maps yield the best record of detailed surface shape while the diffuse normals deviate from the true surface normal due to subsurface scattering, and that this effect is dependent on wavelength. We show several applications of this acquisition technique. First, we capture normal maps of a facial performance simultaneously from several viewing positions using time-multiplexed illumination. Second, we show that high- resolution normal maps based on the specular component can be used with structured light D scanning to quickly acquire high-resolution facial surface geometry using off-the-shelf digital still cameras. Finally, we present a real- time shading model that uses independently estimated normal maps for the specular and diffuse color channels to reproduce some of the perceptually important effects of subsurface scattering.
In this paper we present a new form of texture mapping that produces increased photorealism. Coefficients of a biquadratic polynomial are stored per texel, and used to reconstruct the surface color under varying lighting conditions. Like bump mapping, this allows the perception of surface deformations. However, our method is image based, and photographs of a surface under varying lighting conditions can be used to construct these maps. Unlike bump maps, these Polynomial Texture Maps (PTMs) also capture variations due to surface self-shadowing and interreflections, which enhance realism. Surface colors can be efficiently reconstructed from polynomial coefficients and light directions with minimal fixed-point hardware. We have also found PTMs useful for producing a number of other effects such as anisotropic and Fresnel shading models and variable depth of focus. Lastly, we present several reflectance function transformations that act as contrast enhancement operators. We have found these particularly useful in the study of ancient archeological clay and stone writings.
© 1978 Optical Society of America
An approach for enforcing integrability, a particular
implementation of the approach, an example of its application to
extending an existing shape-from-shading algorithm, and experimental
results showing the improvement that results from enforcing
integrability are presented. A possibly nonintegrable estimate of
surface slopes is represented by a finite set of basis functions, and
integrability is enforced by calculating the orthogonal projection onto
a vector subspace spanning the set of integrable slopes. The
integrability projection constraint was applied to extending an
iterative shape-from-shading algorithm of M.J. Brooks and B.K.P. Horn
(1985). Experimental results show that the extended algorithm converges
faster and with less error than the original version. Good surface
reconstructions were obtained with and without known boundary conditions
and for fairly complicated surfaces
Polynomial Texture Mapping (PTM; Malzbender et al 2001) uses multiple images to capture the reflectance properties of a given surface. Multiple captures may be combined in order to produce interactive, relit records of the material recorded. In addition recent research enables the capture and rendition of interactive PTMs for detailed examination of surface details. Cultural heritage examples of the technology include work on Cuneiform tablets, numismatic archives and lithic artefacts. This paper will describe the PTM data capture and processing technologies developed by the University of Southampton, with support from Hewlett Packard Labs Palo Alto. It will also identify the perceived archaeological potential of additional recording to supplement the standard PTM datasets, including the recording of the surface BRDF (bi-directional reflectance distribution function) and accurate extraction of surface normals. Such data offer considerable, under-exploited value in production of comparative conservation datasets. They also enable new forms of analysis, and the possibility for a step-change in the visual fidelity of reconstructions of archaeological surfaces. Case studies will include ongoing work on the examination of Roman wall paintings, Roman stylus writing tablets, medieval wood, bronze artefacts from a maritime contexts, Neolithic architectural plaster, excavation contexts, brick stamps and sculpture. Each of these presents particular challenges and opportunities for recording, analysis and presentation. The paper will conclude by identifying the synergies between PTM, related imaging technologies, photogrammetry and non-contact digitisation through recent case studies on African rock art and on excavated material from the Portus Project (www.portusproject.org). It will identify the ongoing challenges and proposed future developments.
General results of the metal soap aggregation survey done between 2002 and 2005 are presented. From the data collected, it is clear there are several degradative phenomena associated with metal soap formation: aggregate formation, efflorescence and changes in transparency. We present a case study of localised darkening due to increased transparency as a result of metal soap formation on the wood grain in a leadwhite-
containing imprimatura layer in a panel painting. Reduced scattering, as a result of the dissolution of the lead
white particles, explains the darkening observed.
Technology in microscopy advances rapidly, enabling increasingly affordable, faster, and more precise quantitative biomedical imaging, which necessitates correspondingly more-advanced image processing and analysis techniques. A wide range of software is available-from commercial to academic, special-purpose to Swiss army knife, small to large-but a key characteristic of software that is suitable for scientific inquiry is its accessibility. Open-source software is ideal for scientific endeavors because it can be freely inspected, modified, and redistributed; in particular, the open-software platform ImageJ has had a huge impact on the life sciences, and continues to do so. From its inception, ImageJ has grown significantly due largely to being freely available and its vibrant and helpful user community. Scientists as diverse as interested hobbyists, technical assistants, students, scientific staff, and advanced biology researchers use ImageJ on a daily basis, and exchange knowledge via its dedicated mailing list. Uses of ImageJ range from data visualization and teaching to advanced image processing and statistical analysis. The software's extensibility continues to attract biologists at all career stages as well as computer scientists who wish to effectively implement specific image-processing algorithms. In this review, we use the ImageJ project as a case study of how open-source software fosters its suites of software tools, making multitudes of image-analysis technology easily accessible to the scientific community. We specifically explore what makes ImageJ so popular, how it impacts the life sciences, how it inspires other projects, and how it is self-influenced by coevolving projects within the ImageJ ecosystem. Mol. Reprod. Dev. 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
To characterize more fully the metal soaps found in paint films or on metal surfaces, several metal soaps were synthesized and their X-ray diffraction pattern and Fourier transform infrared (FTIR) and Raman spectra measured. Metal soaps were obtained from four different fatty acids found in drying oils - two saturated (palmitic and stearic acids) and two unsaturated (oleic and linoleic acids) - and from copper, zinc and lead, three metals that are typically found in metal alloys and paint systems. Data are reported for the following compounds: palmitic acid, stearic acid, oleic acid, linoleic acid, zinc palmitate, zinc stearate, zinc oleate, zinc linoleate, copper palmitate, copper stearate, copper oleate, lead palmitate, lead stearate and lead oleate. Features that are characteristics of specific compounds were observed. Soaps obtained from different fatty acids with the same metal ion show differences, as do soaps obtained with the same fatty acid but with different metal ions. Identification is key to understanding how and why metal soaps form on actual objects, and this may lead to preventive measures.
The paper introduces a novel technique called photometric stereo. The idea of photometric stereo is to vary the direction of the incident illumination between successive views while holding the viewing direction constant. This provides enough information to determine surface orientation at each picture element.
In an on-going study of the materials and techniques of twentieth-century Canadian painters, similar conservation issues in oil paintings by various artists have been noted. These include delamination and lifting paint, zinc soap protrusions and surface efflorescence or accretions. Examples of these phenomena are presented. Delamination in an oil painting from 1956 was found to be related to an underlayer with a high concentration of zinc fatty acid salts (zinc soaps). In two paintings that date from 1936 and 1937, zinc soaps have aggregated and formed protrusions that have broken through the paint surface. The protrusions were analysed using a combination of SEM-EDX, GCMS and FTIR. The FTIR spectra were compared to those of synthetic zinc palmitate, stearate, azelate and oleate. The combined GCMS and FTIR results indicate that the protrusions contain primarily zinc palmitate and stearate. Peak splitting in the FTIR spectrum, which is not observed in synthetic zinc palmitate, stearate or binary palmitate-stearate salts, is likely due to structural distortion. The final example describes a disfiguring surface accretion on a 1952–1954 painting caused by the reaction of zinc with a low molecular weight carboxylic acid (2-hydroxypropanoic or lactic acid).
Zinc oxide is a prevalent pigment in twentieth century oil based paints. Its reactivity with fatty acids has consequences for paint film properties and stability. Resulting zinc carboxylates are implicated in a variety of deterioration phenomena affecting late nineteenth and twentieth century paintings. Naturally aged artists’ oil paints and reference paint film draw downs are investigated using scanning electron microscopy, and conventional attenuated total reflectance and synchrotron transmission Fourier transform infrared spectroscopy. This study reports results for soybean oil paints and Naples yellow hue pigment mixtures with zinc oxide in addition to previously reported findings for linseed and safflower oil based paints involving zinc oxide alone and in combination with lead and titanium whites. A solvent model examining interactions between zinc oxide and stearic acid or aluminium stearate supports observations made in paint films. The significance of aluminium stearate as a stearic acid source in paint formulations is corroborated and is more significant than oil type or pigment combination for formation of high concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates.
A recent visual survey of Abstract Expressionist-era paintings in the collection of the Hirshhorn Museum and Sculpture Garden (HMSG), Smithsonian Institution revealed a particular type of paint layer separation. Earlier work by the authors showed that zinc oxide in oil paint is a contributing factor to the problem. Ten samples from five Abstract Expressionist-era paintings as well as twenty-three samples eight years or older from the Smithsonian Institution’s (SI) Materials Study Collection were analyzed by pyrolysis – gas chromatography – mass spectrometry (Py-GC-MS), and unexpectedly significant amounts of oleic (cis-octadecenoic) acid were detected in samples containing high proportions of zinc oxide (25 % or greater by weight). In a typical fully cured oil paint, the oleic acid is oxidized to azelaic (nonanedioic) acid. Although the formation of zinc soaps in oil paints is well-known, the detection of zinc oleate in paints by Py-GC-MS has never been described. The close-packing of the oleate chains in the plate-like structure of zinc oleate prevents the oxidation of the cis-double bond, and therefore prevents the formation of azelaic acid. The detection of zinc oleate in paintings is an indication that the paint layers are at risk for future separation.
Beyond day glow highlighters and psychedelic black light posters, it has been estimated that fluorescence is a property exhibited by 20% of objects. When a fluorescent material is illuminated with a short wavelength light, it re-emits light at a longer wavelength isotropically in a similar manner as a Lambertian surface reflects light. This hitherto neglected property opens the doors to using fluorescence to reconstruct 3D shape with some of the same techniques as for Lambertian surfaces – even when the surface's reflectance is highly non-Lambertian. Thus, performing reconstruction using fluorescence has advantages over purely Lambertian surfaces. Single image shape-from-shading and calibrated Lambertian photometric stereo can be applied to fluorescence images to reveal 3D shape. When performing uncalibrated photometric stereo, both fluorescence and reflectance can be used to recover Euclidean shape and resolve the generalized bas relief ambiguity. Finally for objects that fluoresce in wavelengths distinct from their reflectance (such as plants and vegetables), reconstructions do not suffer from problems due to inter-reflections. We validate these claims through experiments.
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Lead(II) carboxylate soaps of two fatty acids, palmitic (C15H31COOH) and stearic acids (C17H35COOH), and a dicarboxylic acid, azelaic acid (HOOCC7H14COOH), have been synthesised and characterised by FTIR spectroscopy. These acids are all encountered in aged traditional oil paint, the azelaic acid resulting from the oxidative degradation of unsaturated fatty acids in the oil. Lead(II) azelate synthesised by hydrothermal methods was characterised by single crystal structure determination. This has a 3D polymeric structure with lead(II) ions linked by carboxylate bridges to form an infinite stack of (PbO4)n units. These layers are connected to adjacent layers by an infinite number of parallel C(CH2)7C chains arranged perpendicularly to the stacks. The lead(II) ions display an unusual 7-fold coordination. The first direct evidence that the ‘protrusions’ encountered in aged traditional lead-containing oil paints contain lead soaps is reported. Their mechanism of formation is discussed.
Zinc oxide is a prevalent industrial-age pigment that readily reacts with fatty acids in oil-based paints to form zinc carboxylates. Zinc stearate aggregates are associated with deterioration in late nineteenth and twentieth century paintings. The current study uses both conventional and synchrotron Fourier transform infrared spectroscopy (FT-IR) to investigate metal carboxylate composition in a range of naturally aged artists' oil paints and reference paint film draw-downs. The paints contain zinc oxide alone or in combination with lead white, titanium white, and aluminum stearate and are prepared with linseed and safflower oils. Attenuated total reflectance (ATR)-FT-IR using the conventional source identifies marked differences in carboxylate profiles between exposed and protected surfaces in a large number of samples. Synchrotron FT-IR microspectroscopy of thin paint cross-sections maps metal carboxylate distributions at high spatial resolution and resolves broad concentration gradients and micrometer-scale phase separation of carboxylate species. Aluminum stearate, a common paint additive, is found to influence the distribution of zinc carboxylates more strongly than pigment composition or oil type. The presence of aluminum stearate results in higher concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates in the margin of paint nearest the polyester substrate. The presence of aluminum stearate in association with zinc oxide has a clear influence on zinc carboxylate formation and distribution, with potential implications for long term stability of vulnerable paintings.
The potential of fibre optic reflection infrared spectroscopy for the non-invasive identification of artists’ pigments is presented. Sixteen different carbonate, sulphate and silicate-based pigments are taken into account considering their wide use during the history of art and their infrared optical properties. The infrared distortions mainly generated by the specular reflection are discussed on the basis of experimental
measurements carried out on reference samples. The study on pure materials permitted the definition of marker bands, mainly combination and overtone modes, enhanced by the diffuse reflection component of the light, functional for the non-invasive pigment identification in real artworks.
Several case studies are reported, including wall, easel, canvaspaintings and manuscripts from ancient to modern art demonstrating the strengths of the technique on the identification of pigments even in the presence of complex mixtures of both organic (binders, varnishes) and inorganic (supports, fillers and other pigments) compounds.
Input: 3 MLIC Images Our Results: Enhanced Shape and Surface Detail Figure 1: The Multi-Light Image Collection for this chard leaf contains 3 images taken under varying lighting conditions. The shading in each input image reveals different aspects of its shape and surface details. We combine the shading at multiple scales across the input images to generate the enhanced results. The result on the left exaggerates surface details by eliminating shadows, but yields a flat look. The result on the right is less extreme and includes some shadows to increase the perception of depth, at the cost of reducing some visible detail in the shadow regions. Abstract We present a new image-based technique for enhancing the shape and surface details of an object. The input to our system is a small set of photographs taken from a fixed viewpoint, but under varying lighting conditions. For each image we compute a multiscale de-composition based on the bilateral filter and then reconstruct an en-hanced image that combines detail information at each scale across all the input images. Our approach does not require any informa-tion about light source positions, or camera calibration, and can produce good results with 3 to 5 input images. In addition our sys-tem provides a few high-level parameters for controlling the amount of enhancement and does not require pixel-level user input. We show that the bilateral filter is a good choice for our multiscale al-gorithm because it avoids the halo artifacts commonly associated with the traditional Laplacian image pyramid. We also develop a new scheme for computing our multiscale bilateral decomposition that is simple to implement, fast O(N 2 log N) and accurate.
Oil paintings are complex hybrid materials, made of organic binders associated with inorganic minerals, susceptible to evolving
over centuries. In particular, interactions of oil with lead compounds may give rise to the formation of lead soap aggregates,
so-called protrusions. This phenomenon is studied here via X-ray and FTIR micro-analysis of an ancient painting dated from
1610. In complement, the synthesis of modern preparations, reconstructed from ancient recipes was assessed. Molecular and
atomic images are obtained by combining synchrotron-based FTIR and X-ray fluorescence microscopies. Protrusions are identified
in both ancient and modern samples, more particularly, in the ground layer of the paintings, below the colored layer. These
observations imply that lead oxide, introduced as a siccative and not as a pigment, may be the element mainly responsible
for the protrusions formation, and that this degradation may appear very rapidly on paintings.
A user friendly program for X-ray fluorescence analysis has been developed at the European Synchrotron Radiation Facility. The program allows interactive as well as batch processing of large data sets and it is particularly well suited for X-ray imaging. Its implementation of a complete description of the M shell is particularly helpful for analysis of data collected at low energies. The code is platform independent (Linux, Windows, MacOS X, Solaris …) and it is freely available for non-commercial use. Description of the algorithms used and practical examples are presented.
Max Beckmann's "Pierrette und Clown" (Pierrette and Clown), 1925, is undoubtedly one of the major works in the collection of the Kunsthalle Mannheim, Germany. In this painting, damages can be seen, described as protrusions. These are blisters and crater-like holes, filled with metallic soap aggregates. Painting samples and cross-sections of the ground layer have been examined by environmental scanning electron microscopy (ESEM) and by confocal synchrotron radiation micro-X-ray fluorescence (CSR[small mu ]XRF) in order to get compositional and spatial distribution information about the components and their behaviour. The analyzed samples of the unpainted ground layer consist of lithopone (ZnS[middle dot]BaSO4), as several bulges resembling small blisters, presumably protrusions in the nascent state. Direct CSR[small mu ]XRF measurements on closed blisters showed that these blisters have a high concentration of zinc in the centre with surrounding layers of barium and strontium. Relatively small amounts of lead have been recorded. In order to determine the composition of the organic compounds (e.g. fatty acids) and of the inorganic pigments (lead white, zinc white, lithopone), Raman spectroscopy has been successfully applied to flakes of an opened protrusion. The obtained Raman spectra could be assigned to derivatives of fatty acids and lithopones. These results allow conclusions to be made about the chemical behaviour by measuring the identical sample spot of the protrusion with CSR[small mu ]XRF as well as Raman spectroscopy
Polynomial Texture Mapping is an image capture and processing technique that was developed by HP Labs in 2000. It enables the recording and representation of subtle surface details using a standard digital camera and lighting, and software that is free for non-commercial use. Cultural heritage applications have been associated with the technology from its earliest stages, including examples in areas such as cuneiform, numismatics, rock art, lithics and Byzantine art. The paper begins by outlining the technical principles involved. It then brings together the extant work in the field. Through examples developed by the University of Southampton in partnership with a range of UK and international bodies it demonstrates the benefits of the technology in the areas of archaeological analysis, conservation and representation. Finally it considers the future possibilities of this technology and ongoing developments.
The potential of attenuated total reflection Fourier transform infrared (ATR-FTIR) imaging for the characterisation of the chemical components of paint cross sections from old master paintings was investigated. Three cross sections were chosen to cover a variety of the analytical problems encountered in samples from paintings. The binding medium and degradation products in a green paint sample from a fifteenth-century Florentine painting were imaged, as well as a thin layer within a cross-section from a fifteenth-century German painting, and multiple thin surface coatings on a painting of the 1760s by Peter Romney. The application of chemometric methods for further analysis of the large data set generated for each sample was also explored. The study demonstrated the advantages of ATR-FTIR imaging, which allowed images to be obtained with high spatial resolution (ca. 3-4 microm) without the need to microtome the sample. The gain in sensitivity in detecting trace materials and the information derived from the location of these compounds in the sample was especially valuable, improving interpretation of the FTIR analysis and extending knowledge of the sample composition beyond that obtainable with other analytical techniques.
Metal soap degradation of oil paintings: aggregates, increased transparency and efflorescence
- P Noble
- J Boon