Christina Young

The Feasibility of Flash Thermography for the Examination and Conservation of Works of Art

Abstract This study investigates the feasibility of flash thermography for the examination and conservation of works of art: paintings, works on paper and sculpture. Thermography is a non-destructive technique for the identification of subsurface defects in materials. It is based on the propagation of surface-deposited heat through into the material. Differences in propagation between defect and defectfree areas result in a difference in the surface temperature of the material. The surface temperature is mapped over time by imaging with a mid-infrared digital camera. A xenon arc lamp is used to provide the initial source of radiation, and signal processing is typically applied to the collected data to reduce noise and to enhance key signal characteristics. This technique offers the possibility of investigating the structure of paintings and paper, particularly in cases where other non-destructive examination techniques do not provide sufficient information, for example subsurface delamination and layer structure. The results indicate that thermography is a good technique for detection of paint delamination and the degree of adhesion between layers, particularly in canvas paintings. It also successfully detected wood grain in situations where X-rays did not, although it was not effective for detecting voids or defects in wood.

Biaxial tensile testing of paintings on canvas

AbstractThis paper concentrates on the practical application of biaxial testing of canvas as the first step in gaining further insight into the mechanical properties of the complex composite structure of a canvas painting. Existing testing instruments and testing methods, directly relevant to canvas painting conservation, are reviewed. The design and implementation of a biaxial tensile tester specifically for a new research initiative in painting conservation is described, together with experimental results for raw linen canvas under uniaxial and biaxial loading. The long-term aim of this research is to aid conservators in the selection and development of conservation treatments and methodologies.

Laser Cleaning of Easel Paintings: An Overview

The application of laser cleaning to paintings is relatively recent despite its use on stone-based materials for over 30 years. The cleaning of paintings is of high importance, because it is the least reversible invasive intervention, as well as the most usual of all conservation treatments. Paintings are multilayer system of heterogeneous nature, often very sensitive and inherent difficult to clean. Being a noncontact method, laser cleaning has advantages compared to alternative techniques. Over the last decade, there have been important research studies and advances. However, they are far from sufficient to study the effects on painting materials and to establish the best parameters for each material under investigation. This paper presents a historical overview of the application of laser technology to the cleaning of paintings giving special emphasis on the research of the last decade. An overview of the current research into the interaction between the radiation and the different painting materials (varnish, pigments, and medium) is also given. The pigments mechanisms of discoloration and the presence of media as a variable factor in the discoloration of pigments are discussed.

Measurement of the biaxial properties of nineteenth century canvas primings using electronic speckle pattern interferometry

This paper reports on the use of electronic speckle pattern interferometry (ESPI) for the measurement of the biaxial tensile properties of English 19th century canvas primings and their constituent materials. Typically, such primings are comprised of a complex structure of layers, each with different mechanical properties. ESPI has been shown to be an effective technique for investigating complex composite structures and it is especially useful for understanding the behaviour of heterogeneous materials in which non-uniform strains can occur. The flexibility of canvas primings presents a more difficult application for both biaxial tensile testing and ESPI strain measurements. A series of experiments have been carried out to measure the Poissons ratio of the three main constituents of a 19th Century priming as composites and of an original 19th century primed canvas. The samples have been uniaxially tensioned on a biaxial tensile tester designed specifically to investigate the mechanical properties of paintings on canvas. Measurements of deformation have made using a two-dimensional in-plane ESPI configuration. The results have shown that Poissons ratio decreases as the constituents of a painting are built up. Preliminary tests on thermally aged and original primings suggest that for a painting without cracks it is the embrittled paint which determines the mechanical response of the painting at an relative humidity of 35–40%.

Development of a Physical Model of a Typical Nineteenth-Century English Canvas Painting

Abstract The chemical and mechanical degradation of typical nineteenth-century English canvas paintings has recently been examined. A physical model of such a painting was developed, consisting of fine Ulster linen, rabbitskin glue size, and lead white/oil ground layers. The model painting composite was artificially aged and its validity assessed by comparison with archival specimens prepared by Roberson in the nineteenth century. The failure mechanisms and selected mechanical properties of the model and the archival specimens were compared. The development of such model systems will lead to a better understanding of physical deterioration processes in paintings on canvas supports and to improved approaches to conservation.

Fabrics for the twenty-first century: As artist canvas and for the structural reinforcement of easel paintings on canvas

Abstract The requirements for, and assessment of, suitable fabrics for artist canvas and for the structural reinforcement of easel paintings on canvas are presented. Crucially, the aesthetic and kinaesthetic, as well as physical properties of the canvas fabrics have been taken into account. To evaluate the fabrics the following properties were measured: stiffness, ultimate tensile strength, moisture response, crimp, drape, and lustre. Fabrics investigated include cotton, linen, polyester, polyamides, and carbonized fibres. Although polyester is yet to match linen or cotton kinaesthetically or aesthetically, overall it exhibits the best combination of properties. However, the results have shown that even when raw fibre material has suitable properties the finished woven fabric may not. This is because of the strong influence of the woven geometry on the final behaviour. It has been found that drape and lustre are very good properties to quantify some aspects of the feel and look of fabrics, in the context of conservation, and to provide a common language to ensure that the specification incorporates these aspects.

Towards a better understanding of the physical properties of lining materials for paintings: Interim results

Abstract This paper reports on a preliminary investigation into the biaxial properties of a selection of traditional and modern materials which could be used in the lining of paintings on canvas. It describes their load response to high levels of moisture and their load‐extension behaviour when under biaxial restraint. All the woven lining materials tested were found to be anisotropic and to have load‐extension curves with characteristic features: non‐linear load‐extension, hysterisis and creep. The role of crimp in determining mechanical properties and anisotropy of these materials is discussed. Preliminary observations on the application of these findings to practice are given.

A Preliminary Study into the Suitability of Femtosecond Lasers For the Removal of Adhesive from Canvas Paintings

The numerous attempts to use lasers in different fields of conservation reflect awareness of their potential as non-contact tools which may be tailored to specific cleaning problems. Ultrashort laser pulses, in the order of pico- and femtoseconds have been used for precision ablation of biological tissue and have been shown to eliminate collateral damage normally caused by thermal diffusion of the laser pulse. Thermal damage and photochemical changes have both been reported as by-products of laser cleaning experiments with Excimer lasers and thus femtosecond lasers potentially offer significant benefits in this application. In the work reported here a specific problem within painting conservation has been addressed to assess the suitability of femtosecond lasers; namely the removal of glue-paste adhesive from the reverse side of a painting on canvas. This study is based on a collaborative project between the Conservation Departments of the Courtauld Institute of Art, the Tate Gallery, and the Femtosecond Optics Group at Imperial College of Science, Technology and Medicine. In this study 10ps, 830nm pulses derived from a Ti:Sapphire laser were used to ablate marks with a diameter of 5□m and a depth of 10□m in a glue-paste layer on canvas. The surface characteristics of the ablation marks on glue-paste, canvas and priming were investigated using microscopy. Ablation occurred irrespective of material type and no thermal damage was observed.

Purely Decorative? Technical Analysis of a Fifteenth-Century Northern European Parade Shield

Abstract A fifteenth-century decorative parade shield (1863,0501.1) held at the British Museum is a finely crafted, three-dimensional object thought to be of Burgundian origin. Little was known about its painting technique, construction, and function before the study presented in this paper. Concerns over the objects stability and specific mounting needs for its inclusion in the recently refurbished medieval galleries prompted the detailed research undertaken collaboratively between the Courtauld Institute of Art, London and the British Museum. There were three areas of focus: a study of the original materials and techniques, a survey of the conservation history and materials, and recommendations for display. Analysis revealed that the painting method was analogous to fifteenth-century northern European easel painting practice and the structure similar to that reported in parade shields of the period and region. The study also revealed extensive damage to the wooden core. Further damage to the shields edges, and degradation of the numerous repair materials, have left these areas vulnerable to environmental fluctuations. These findings prompted development of a supporting mount, identification of optimum environmental conditions and recommendations for future conservation. The accompanying stylistic research supported a fifteenth-century origin and suggested links to the ‘Master of the Princely Portraits’. It is likely that the shield was used for display purposes, perhaps during parades at court festivals, or as a tournament prize.

Micromechanics models for predicting tensile properties of latex paint films

The mechanical properties of latex paint films containing different volume fractions of TiO2, CaCO3 and kaolin were measured in uniaxial tension over a broad range of temperatures and crosshead speeds. Young’s modulus results in the glassy region are first compared with several micromechanics theories for particle-filled composites containing elastic phases. It was found that the Mori-Tanaka theory [1, 2] slightly under-predicted the modulus enhancement, while the Lielens approach [3, 4] provided very accurate results. A nonlinear viscoelastic material model involving a Prony series and the neo-Hookean hyperelastic function was used to represent the tensile data up to relatively small strains (3-4%). Using the experimental data, the material model was calibrated and the parameters of the model were determined. The derived parameters were then used to re-construct time dependent shear modulus plots which were compared with the approximations given by Clements and Mas [5, 6] for the viscoelastic Mori-Tanaka theory in the time-domain. It was found that the experimentally observed modulus enhancement was much stronger than the predicted values in the rubbery region. This is attributed to the small particle size, the high level of constraint posed on the latex matrix at the particle interface or possibly the formation of a particle network.