I’ve finally been able to more closely examine the Canaletto to gain a better understanding of its present overall condition and treatment history. Generally, the picture is in better shape than I anticipated. To come to that conclusion I used a variety of evaluative and analytical techniques to assess the painting’s structural and aesthetic state. These non-destructive techniques range from looking closely at the picture in bright and varied lighting to bathing the picture in ultraviolet or x-radiation. The use of a stereomicroscope is also useful under these illumination conditions to very closely assess the paint surface.
Some of the simplest evaluative techniques used to assess the condition of a painting are bright lights placed at various positions relative to the picture to highlight different aspects of the surface. These include:
- ‘Normal’ illumination is the use of two light sources placed on each side of the picture. The light from the two sources evenly covers the picture while minimizing shadow and glare. Bright, ‘normal’ illumination is particularly helpful at assessing darkly painted passages of a painting, often making visible subtle color and tonal changes within the paint itself.
- Raking light is the use of one light source placed in the same plane as the picture. The light’s location causes the high points of the surface to cast shadows, thereby emphasizing the picture’s topography and texture.
- Specular examination involves placing a light source directly in front of a picture or at a 90-degree angle from one’s point of view (oblique specular).
- Specular illumination reveals changes in surface reflectance and subtle topographical changes often not seen in raking light.
Other analytical tools used to assess the condition of the picture are ultraviolet and x-radiation. Both ultraviolet and x-radiation are invisible forms of energy and when they come into contact with a paint surface, they force the materials that comprise the surface (i.e. oil, pigment, resins) to react in interesting ways, thus allowing conservators to begin to characterize and identify them. Fortunately we know that many artists' materials respond in unique ways in the presence of ultraviolet radiation:
- Natural resin varnishes appear greenish and shellac radiates a bright orange.
- Pigments like zinc white auto-fluoresce a lemon yellow.
- Also, many restoration layers, especially those most recently applied, will appear very dark and stand out in contrast to the original paint surface.
Ultraviolet radiation helps a conservator map out old damages and restorations.
While ultraviolet radiation helps to characterize the upper layers of a picture, x-radiation penetrates the surface giving conservators look underneath. X-radiation is a very high-energy source. It is an effective tool because the energy readily passes through less dense material and is adsorbed by more dense material. The image created is a representation of the differing densities of the materials examined. Like a skeletal x-ray at the hospital, bone is visible because its density is greater than the surrounding tissue and x-ray imaging captures that difference. The same principal is applied to paintings and works of art: x-rays pass through less dense artists’ materials (natural resins, many pigments) and are absorbed by those that are more dense, pigments like lead white and wood support structures, thereby creating a contrasting image with remarkable detail.
I’ll discuss my findings in the next post.
This post is one in a multi-part series by Denver Art Museum staff on a long lost painting located in our collections. Find links to more from the series.
Image credit: Giovanni Antonio Canal, called il Canaletto. Venice: The Molo from the Bacino di S. Marco, about 1724. Oil on canvas. Bequest of Charles Edwin M. Stanton, 2009.336.