When art historians and curators study a work of art, they begin with a thorough visual examination to determine both the artist's original technique and the condition of a painting. Through careful attention to detail, art historians discover important details such as the revealing patterns of craquelure, the superposition of the paint layers, and the location of retouches and restorations. Digging deeper through the use of microscopes enables experts to verify the authenticity of a painting and its signature.
Experts use grazing light to examine paintings in the visible spectrum. Lights are set up at very shallow angles to the surface of the painting to create what is known as grazing light or raking light. Grazing light reveals details such as surface defects, distortions of the support, craquelure, and impasto with great clarity. Grazing light also increases the depth of heavy, textured paint strokes, such as found in impasto. This allows art historians to successfully study stroke patterns, making it easier to observe the manner of the stroke, the direction of the stroke, and the viscosity of the paint. You can learn a lot about the artist's technique as well as what he may have intended to convey to his viewing public by examining his brushstrokes in such great detail.
Lights can be set up in other ways to divulge even more information about a painting. For instance, paintings on canvas can be illuminated from behind, which is known as transmitted light. This can reveal severe paint loss. Transmitted light can be applied in other situations, such as in the study of signatures, overpaints, crack patterns in wooden panels, and alterations to both works of art and documentary artifacts on paper supports.
The giornata in a Raphael's fresco
If you were to use grazing light to examine the Raphael fresco above, you would see the main figures outlined with deep incisions. Looking carefully at the shadows, you could easily spot six areas. Each of these areas is called "giornata," which means "a day's work."
When painting fresco, the artist added a thinner, smooth layer of fine plaster (the intonaco) to the area of wall that he expected to complete in a day, often matching the contours of the figures or the landscape. A layer of plaster typically required 10 hours to dry; an artist would begin to paint after one hour and continue until two hours before the drying time - providing him with seven hours of working time.
Notice the differences between van Gogh's brush strokes under normal light and grazing light
van Gogh, Mademoiselle Gachet au Jardin, Louvre
What makes van Gogh's art so special? One factor is van Gogh's distinctive brush work. For evidence, look at a grazing light image of one of his paintings, which you can see by switching between the two images above.
What you can see now? Grazing light shows you that van Gogh used random strokes to suggest movement, and more direct strokes to follow forms and emphasize the roundness or solidity of objects.
Compare the differences between visible and grazing light to identify the different areas of giornata in Raphael's fresco.
Raphael, Chapel of Saint Severo, fresco, 1520, Perugia.
When you stand in a church or a noble palazzo and stare up at beautiful Renaissance frescoes, they look smooth and luminous. Saints or heroes come to life against a backdrop of blue sky, and it's easy to forget that what you see is a wall or ceiling covered with colored mortar.
What happens when you get closer? The magic dissipates and frescoes become fragile plaster. Suddenly you can see bumps, detachments, and coarse additions. Look closely enough, and a fresco will reveal how long it took for the artist to complete the work.
How it works
Both scientific cameras and commercial reflex digital cameras can collect high-resolution details in color digital files. These visible images are used as a reference in order to build up the multispectral model of the painting. Ultraviolet fluorescence, infrared reflectography, and X-ray radiography are added to the visible image. In this way, restorers and conservators are able to observe each multispectral image using the visible one as a reference. High-resolution images can be collected with a macro objective, but a binocular stereoscopic microscope allows 6x to 40x enlargements.
If you want to try this at home, you can use a reflex digital camera and common directional halogen lamps to capture a visible image at high resolution in normal or grazing light. For more detail, simply mount a macro lens onto your camera. To capture finer details, you'll need a stereomicroscope. An economical alternative is a USB microscope, which can be found on the Internet for less than $100.
Looking beyond what the eye can see
The electromagnetic spectrum is invaluable to studying art. This includes wavelengths in the visible spectrum as well as those that are invisible to the human eye. While we can only see the narrow range of light in the visible spectrum, we can use special cameras to take images of artwork using X-ray, ultraviolet, and infrared wavelenghts, which allows us to gain even deeper insights into the artwork.
One of the most effective ways to examine the condition of a work of art is through multispectral imaging. Multispectral imaging is non-invasive and non-destructive, which makes it a totally safe method for studying art. This type of imaging divides the light spectrum into a number of frequency bands and takes many pictures of the same scene, each at a different wavelength. Together, these pictures form a multispectral image.
By responding to ultraviolet and infrared light, the multispectral camera reveals information that is otherwise concealed from the human eye. Multispectral imaging can also be used to read and record faded or burnt manuscripts - even from carbonized and damaged fragments. Because books were often "recycled" in order to use the parchment for copies of other texts, what we now consider treasured antiquities were once dismounted, erased by washing off the original ink, and overwritten with new text. These washed out inks can still be read using the appropriate wavelength of light (see carbon black technical detail). This exciting scientific approach has the potential to recover significant works that have been lost to humankind for centuries.
Delving deeper into multispectral imaging
Multispectral imaging is the procedure used to observe an object using selected ranges of wavelengths in the electromagnetic spectrum. Paintings are normally observed under visible light (780-400nm), such as when we look at a painting with our eyes. In the visible range we can get two kinds of images: a plain photographic picture (VIS) and a grazing light image (GL) obtained using a raking source of light.
Through the use of specialized technology, we can capture the images of artworks in ranges of wavelengths that are much shorter or much longer than what we can see with the naked eye. The UV range (400-360nm) produces ultraviolet fluorescence images (UVF) and ultraviolet reflected images (UVR). The infrared electromagnetic radiation most close to the visible range (1100-780nm) can be detected by a modified photographic camera, which is used to collect infrared CCD images (IRCCD) and infrared false color images (IRFC). More sophisticated cameras can detect infrared light even further down the range (1100-1700nm), producing infrared reflectography images (IRR). An x-ray, which is light at much shorter wavelengths, produces an x-ray radiograph (RX).
Those who study manuscripts and works of art glean a wealth of information from each method. Taken together, these methods are complementary, because each spectral range interacts with a different layer of a painting. For instance, UV light is blocked by the varnish. However, visible light can go through the varnish, where it is then reflected by the painting's film. Infrared light can pass through the painting film and "see" the underdrawing. X-ray light passes through the entire painting, as well as through thick wood supports. All of these methods combine to generate an illuminating view into the structure and history of an artwork or manuscript.
Although professional analytical instruments are prohibitively expensive for laypeople, you can easily conduct certain kinds of multispectral imaging on your own. The technology inside consumer-range digital cameras and webcams can be modified with easy-to-find, inexpensive items that will enable you to discover the layers of information contained in any painting.
Ultraviolet light, Ultraviolet Fluorescence (UVF) and Ultraviolet Reflectography (UVR)
Organic compounds can easily be detected under UV light. This effective tool detects later restorations that appear darker than the aged original varnish layers. UVF can reveal the presence of natural resin varnishes, as these often fluoresce under UV light. It is also possible to identify any retouchings on top of an aged varnish, since oil paint and newer varnish do not fluoresce under UV. Retouchings therefore appear as dark patches on the varnish surface.
If you go to the picturesque village of Trevi, Umbria (Italy) and look at the Magi figures painted on a Renaissance fresco, they appear dressed as humble kings. That seems odd. Where are the insignia of royalty?
If you shine a black light over these figures, you'll reveal decorative attire suited for these famous kings. Did the painter intend to keep these regal details concealed? Not at all. The passage of time caused the painter's materials to fade, making them invisible to the naked eye. The black light unveils the artist's true intent.
How do art restorers and curators determine if a painting's brushstrokes are completely original, or if they have been retouched? The older the painting, the more likely it is that early "restorers" were hired to paint over damaged or faded areas - without much regard for Old Masters.
If you were to shine a black light near this 16th century Madonna, you would immediately notice a pale, milky blue fluorescence coming from the aged varnish. A few seconds later, you would spot darker areas of linseed oil and newer varnish - solid evidence of retouchings.
How do painters achieve their effects? How do they mix or overlap pigments in order to get a particular result? Each painter develops his or her own techniques based on the materials available as well as the contemporary practices of their time. Discovering the techniques of master artists throughout history is beneficial to both art historians and contemporary artists. Art historians are eager to learn about old artists' techniques because the information enables them to authenticate a work of art. Contemporary artists crave this information because they would like to recreate the effects of Old Masters' secret techniques.
Let's try to understand how Elisabeth Chaplin, in her 1930 painting, Mercato, rendered the highlighting for a white shirt on an elegant street trader. Chaplin succeeded in making an already white shirt a bit more white when you look at it from different angles, which was no easy task. How did she do this?
An infrared false color (IRFC) image typically distinguishes different pigments that appear the same to the naked eye. In this case, the tones of light blue reinforce that the shirt is painted with the same pigment. However, a UV false color (UVFC) image, using a black light, reveals the artist's pigments. You clearly see two different pigments for the left and right side of the shirt. The artist has in fact used zinc white, which appears bright yellow in the reflected UV false color restitution, as a base color, and has instead highlighted the areas most in relief with another white pigment, which remains white in reflected UV image.
3D laser scanning technology is particularly well-suited for recording landscapes, especially when making detailed inventory records that need to be accomplished rapidly. 3D models are generated during evaluation, as well as the usual contour line maps and detailed records of the vegetation, trees, rocks, etc in the area. Additional information such as aerial feature photographs and geophysical investigations can also be integrated into the 3D model.
Accurate plans of monuments and old buildings are notoriously rare. Laser scanning provides an economical solution for making basic surveys (ground plans, cross sections) as well as for recording the complex measurements of distorted buildings. Exterior walls and interior spaces can be completely documented in three dimensions and with an accuracy of 0.5 to 2.5 cm. Ground plans and cross sections can be created at any point throughout the entire building. All of this is made possible by the use of a combination of total station surveying (polygons), systematic laser scanning and, when required, additional photogrammetry recordings. During the computer-supported evaluation of the survey data, CAD plans are created which can incorporate ground plans and cross sections. The data can be digitally transferred into all standard formats. It is also possible to generate a three dimensional model of the building that is being documented. This method of surveying 3D data is much more rapid and economical than any other system. Laser scanning also unlocks huge potential in the field of post-processing. When appropriate 3D software is used, real color information can be incorporated into 3D models to create accurate, photo-realistic, three dimensional illustrations of the surveyed objects.