Imaging and Technical Analysis
An important component of treating works of art is understanding as much as possible the materials and techniques used by the artist. Specialized imaging techniques can reveal alterations made during past restorations and their affect on the appearance of the painting enabling the conservator to undertake the most prudent and appropriate conservation treatment. Knowledge gained through examination and analysis may also help answer questions of attribution and dating.
Ultraviolet (UV) Induced Visible Fluorescence
Ultraviolet light, which is invisible to the human eye, excites electrons in the molecules of certain materials on a painting's surface causing them to emit visible light. Materials that are not excited will absorb ultraviolet radiation and appear dark. The characteristic fluorescence or absorption of the various materials enables the conservator to discern varnish layers present on the surface and the degree to which they have degraded. By contrast, restorations typically absorb ultraviolet illumination and appear dark against the varnish and underlying paint layers. With this examination technique non original additions, alterations and damages are revealed.
Normal light (left) and ultraviolet induced fluorescence image (right)
Infrared (IR) Digital Photography
Infrared imaging is a technique used by conservators to examine what lies beneath the visible paint surface of a work of art. Using a high resolution digital camera modified to capture images in the near infrared range, the method can reveal what is otherwise invisible to the human eye. The technique is especially useful for detecting carbon-based under-drawings which absorb infrared radiation and appear black against an IR-reflective ground layer. In addition, inscriptions, compositional changes made by the artist and later restorations may be revealed.
Infrared image (left) showing under-drawing lines defining the hand and tablecloth and visible light image (right)
Reflectance Transformation Imaging (RTI)
Reflectance transformation imaging (RTI) is a computational imaging process that creates an image of a surface that can be virtually illuminated from multiple angles. A sequence of photos is taken with a high resolution camera from a fixed position and for each image the flash is placed in a different location at a predetermined distance from the painting. Specialized software allows for viewing of a painting's features in raking or specular light using an interactive 2-dimensional image. Brushstrokes, paint texture, color and condition problems such as lifting paint and cracks are revealed. For more information visit: http://culturalheritageimaging.org/Technologies/RTI/
RTI still image of a paint surface (specular enhancement mode)
The composition of pigments and medium can be studied by taking a tiny sample from a painting using cross-section analysis. The sample is embedded in a casting resin which, after hardening, is polished to reveal the layer structure of the ground, paint and varnish layers. The sample can then be studied under the microscope to identify pigments and binders used by the artist and to differentiate between the original paint layers and later restorations.
When required, samples are analyzed and interpreted by Conservation Scientist Erin Mysak, PhD.
Cross-section of a paint sample containing several layers of paint seen in visible light (top) and ultraviolet light (bottom)