AIC PAINTINGS SPECIALTY GROUP POSTPRINTS Papers Presented at the Thirty-third Annual Meeting of the American Institute for Conservation of Historic & Artistic Works Minneapolis, Minnesota June 8-13,2005 Compiled by Helen Mar Parkin Volume 18 2006 The American Institute for Conservation of Historic & Artistic Works This publication entitled 2006 AIC Paintings Specialty Group Postprints is produced by the Paintings Specialty Group of the American Institute for Conservation of Historic & Artistic Works (AIC). © 2006 The American Institute for Conservation of Historic & Artistic Works Publication of this serial began in 1988. Except for Volume 3 (1990) all issues until Volume 16 are unnumbered. ISSN 1548-7814 The papers presented in publication have been edited for clarity and content but have not undergone a formal process of peer review. This publication is primarily intended for the members of the Paintings Specialty Group of the American Institute for Conservation of Historic & Artistic Works. The Paintings Specialty Group is an approved division of the American Institute for Conservation of Historic & Artistic Works, but does not necessarily represent AIC policies or opinions. Opinions expressed in this publication are those of the contributors and not official statements of either the Paintings Specialty Group or the American Institute for Conservation of Historic & Artistic Works. Responsibility for the materials/methods described herein rests solely with the contributors. Additional copies of this publication are available for purchase by contacting the Publications Manager at the American Institute for Conservation of Historic and Artistic Works. 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The American Institute for Conservation of Historic & Artistic Works, 1717 K Street, NW, Suite 200, Washington, D.C. 20036-5346 http://aic.stanford.edu Thomson-Shore Book Manufacturer, Dexter, Michigan; Prepress by Chris Stavroudis; Judy Mahan, Consultant AIC PAINTINGS SPECIALTY GROUP POSTPRINTS Volume 18 2006 TABLE OF CONTENTS Paintings Sessions: Digital Images in Conservation Documentation: Quality, Accuracy and Potential 1 David Saunders The ImageArchiver: Image Annotation and Metatag Manipulation Software for Conservation 16 Oliver Stahlmann Your Paintings Exposed: Negatives in the Kress Collection Archive at the National Gallery of Art 23 Joanna Dunn and Elizabeth Walmsley A Translation of the Byzantine into the Northern Renaissance: 29 Hayne de Bruxelles 'Madonna and Child, 1455 Scott A. Heffley Francois Clouet and the French Renaissance: Investigating A Lady in Her Bath 35 Pamela Betts Variants of Titian s Virgin and Child in a Landscape, A Comparative Study 43 Adelaide Izat Painting with Wax in Britain and America during the Eighteenth and Early Nineteenth Centuries 53 Lance Mayer and Gay Myers Seurat and the Making o/La Grande Jatte 67 Allison Langley and Frank Zuccari A Rubens Portrait Re-examined: How Contemporary Copies and Historical Documentation Aided in Interpretating a Reworked Portrait 76 Linnaea E. Saunders Contemporary Encaustic Techniques - Johns, Marden, Thek 84 Michael Duffy Selected Paper from General Session: The History of Conservation Documentation at Worcester Art Museum 94 Morwenna Blewett 5 slides/3 minutes: Artists'Idiosyncrasies and Treatment Peculiarities Where's the Beef? A Study of Extensive Overpaint to a Nineteenth Century Painting 108 Stephanie Grant A Note on a Discolored Oil Coating on a Nineteenth-Century German Painting 111 Elise Effmann Kees van Dongen's Reclining Nude: Preliminary Notes on the Artist's Varnish 113 and an Unrelated Surface Phenomenon Morwenna Blewett and Philip Klausmeyer Paintings without Grounds 116 Mark Lewis The Use of a New Product to Treat Canvas Irregularities: 117 Tight-n-Up, Its Effects on One Painting and the Treatment of Those Effects Niccolo Caldararo DIGITAL IMAGES IN CONSERVATION DOCUMENTATION: QUALITY, ACCURACY AND POTENTIAL David Saunders ABSTRACT - The paper defines the attributes of quality and accuracy for digital images and assesses the need for these in conservation documentation. A survey of digital imaging technologies for examining and recording paintings is illustrated with examples of infrared, X-ray, visible and raking light imaging drawn largely from developments and practice at the National Gallery, London. The advantages of digital techniques over the analogue techniques they often replace are examined, and the new possibilities opened by the manipulation and interactive presentation of images are discussed. 1. INTRODUCTION Twenty years ago, a handful of conservators were using computers and no-one outside the science departments of the largest museums and galleries was using digital imaging techniques. These two decades have, therefore, seen an immense change in this pattern of use; for better or worse, computers and digital cameras have become as much a part of the conservation studio as easels and pigments. This is an interesting point at which to survey the advances in the field, as something of a watershed has been reached with relatively inexpensive amateur digital cameras - these are now capable of recording as much detail as the best 35 mm slide films used routinely to document treatments in the past. This does not mean that no further improvements are possible; far from it, as the purpose of this review is to assess the requirements for conservation documentation, look at how far current techniques fulfil these needs, and what future developments might be desirable to meet conservators' expectations. The examples draw heavily from the author's experience at the National Gallery, London, but many have parallels in other museums and galleries worldwide. However, before detailing the practical application of imaging techniques, it is worth looking at the issues of quality and accuracy in images that are relevant to these discussions. 2. QUALITY AND ACCURACY The suitability of the digital images recorded in museums to fulfil the variety of different purposes they are intended to serve, for example as permanent documents in a conservation archive or to assist in the technical examination of paintings, depends on both their quality and accuracy. The quality of digital images is often quantified by their resolution and their bit-depth. The former gives an indication of the level of detail that can be resolved in the image and the latter can point to the ability of the image to represent the range of colours present in the image (or levels of grey in the case of monochrome images). The example offered in Fig. 1 shows the effect of image resolution. Both images cover the same area of a painting (A Young Girl by Jan Gossaert; National Gallery, London, No. 2211). The image to the left is extracted from an image that has a resolution of approximately 20 pixels per millimetre on the painting surface; that is, each square centimetre of the surface of the paintings is divided in to 200 x 200, or 40000 individual points. The fine detail, including the craquelure, is clearly visible. In contrast, in the right image, which has a resolution of 2 pixels per millimetre, this information is lost. Another factor that contributes to the quality of an image is the bit depth. The number of bits or bytes needed to represent the colour of a pixel was a crucial factor in the early days of digital imaging, as memory and storage capacity were limited and standard computer monitors were unable to display images with high bit depths. At some stage, the 16 million colour image started to become prevalent, the figure of 16 million arising from the common method of coding colour in a red, blue and green channels, each of which has 256 levels; 256 x 256 x 256 gives over 16 million colours. With advances in imaging, most images now have at least 16 million colours, so the image format and bit depth are rarely issues. David Saunders, The British Museum, Great Russell Street, London WC1B 3DG, UK. At the time this paper was delivered the author's address was: The National Gallery, Trafalgar Square, London WC2N 5DN, UK. 1 AIC PSG Postprints 18 (2006) 1 Fig. 1. Detail from A Young Girl by Jan Gossaert (National Gallery, London, No. 2211). The image to the left has a resolution of 20 pixels per millimetre on the painting surface, that to the right a resolution of 2 pixels per millimetre The second criterion to be considered is accuracy and, again, how accurate an image needs to be depends on purpose. Taking the example of the low-resolution image intended as an aide memoir on a web site, accuracy is not critical, as the object represented merely needs to be recognisable from the image. At the other extreme, an image intended to serve as permanent record of the state of preservation of an object, against which future records will be evaluated, needs to be detailed (high quality) and accurate, so that comparisons can be made in the future. In some respects the boundary between quality and accuracy is poorly defined. A more meaningful division might be made between those attributes of an image that are largely subjective, and those that can be assessed against a particular standard, usually with respect to the object which has been imaged. Attributes of image resolution, geometry and sharpness can be measured, but are normally assessed subjectively: is the image of sufficient resolution to see fine detail that is considered interesting or important; is the image distorted in one direction or 'curved' at the edges; is the image in focus? However, the factor that is most often assessed against an external standard is the colour quality of an image and here a more objective numerical comparison between the object colour and the colour in the image is possible as a complement to the subjective judgement of whether a colour 'looks' right. Naturally this only applies to visible images, as technical images are mostly monochrome and, if coloured, the colour is notional rather than representational.