AIC's 53rd Annual Meeting

Dendrochronology is a method of dating the age of wood, based on the measurement of tree ring widths followed by statistical comparison with master chronologies. Dendrochronology is used widely in the study and authentication of artworks, particularly furniture, panel paintings, and sculptures. In traditional dendrochronology this involves destructive surface preparation, macro photography, and manual ring counting. On painted surfaces or inlaid panels however, the tree rings are often not accessible to retrieve this information. X-ray imaging has therefore been used with success to obtain virtual cross-sections of objects without invasive methods. A proven method is CT scanning, which provides a three-dimensional image of the object, which can be virtually 'sliced' to obtain images of the tree rings. Recently, fast digital radiography detectors with high resolution have become available that can be combined with commercially available digital turntables to rapidly generate large numbers of radiographs of a single object at fixed angular increments. With these, it is now possible to generate full X-ray tomographic reconstructions (CT scans) using equipment available in many museum radiography laboratories. This eliminates the need for dedicated CT equipment or the transport of artworks to specialized facilities.

Large flat panels of wood, such as those used in furniture and panel paintings, pose difficult problems for conventional tomography because it can be difficult or impossible to rotate the entire panel within the field of view, and because the thickness of wood presented to the X-ray beam varies so dramatically as it is rotated.

In this presentation, we will present a solution to the challenge of large panels by obtaining X-ray images in a limited angular range. The resulting tomographic reconstruction has lower resolution in depth, but tree rings are still clearly resolved. The creation of a full 3D reconstruction means that obscuring elements such as marquetry, paint, and cradles can be virtually stripped away. We take an extra step by then using imaging processing methods to automatically measure the tree rings along the full 3D volume, thus averaging thousands of measurements and yielding precise and robust measurements. This method was developed using only the in-house X-ray imaging equipment of the J. Paul Getty Museum – which consists of a digital detector and a simple computer-controlled turntable. We demonstrate the method and prove that it works on test planks first. The obtained measurements are compared to traditional measurements made by three dendrochronologists. We then continue to apply the method on a case study object from the J. Paul Getty Museum. This method should allow many more artworks and architectural elements to be dated by dendrochronology than ever before.

We combine the expertise of dendrochronologists, conservators, X-ray scientists and computer scientists in a highly interdisciplinary project. The project is a collaboration between the J. Paul Getty Museum (Los Angeles) and the Center for Mathematics and Computer Science (Amsterdam).