As museum-goers, we often assume that artwork looks just as the artist intended it to look. This is generally not the case, as the passage of time necessarily degrades art, often in the form of dirt or cracks. The use of two technologies, laser ablation and bacteria, is helping to restore artwork to its intended form. Laser ablation involves the removal of dirt through the excitation of particles with light energy. Bacteria are used both to remove polluting materials and to fill cracks in sculpture. The trend of restoration, however, is frowned upon by some purists as an aesthetic process that is not in accordance with traditional ideas of art preservation.
We often take for granted the artwork we see at museums. We see it in its current state and assume that it has always looked the same--the way the artist intended. However, we must remember that artwork, like all of antiquity, is at the mercy of time: time brings prolonged exposure to light, moisture, dust, and other elements of nature that cause artwork to become dirty and often permanently damaged. While most museums today are designed to protect artwork from these elements, the unfortunate problem is that the damage has already been done: centuries of storage in less-than-ideal conditions have greatly damaged some of the world's most important artwork. Engineers, however, are applying technology to the field of artwork restoration with exciting results. Two of these technologies, laser ablation and bacteria, have the potential to restore safely a work of art to its original condition, as it was conceived by the artist.
Effects of Aging
In order to understand the restoration process, we must examine why it is necessary in the first place. What happens to a work of art over the course of many years? A majority of older artwork has been subjected, at one time or another, to unfavorable storage conditions. Perhaps it hung in an old cathedral where it was exposed to centuries' worth of soot from incense and candles. It might have been kept in a smoky drawing room of a wealthy art patron. Regardless, the surfaces of artwork slowly accumulate dirt and soot over time, obscuring and distorting the original colors of the work. Moreover, in paintings, the clear layer of protective varnish that artists often apply over the paint tends to yellow and darken with time and exposure to light. Once the original transparency is lost, the varnish discolors the entire painting and is largely responsible for the "aged" appearance of older paintings .
These factors, of course, are detrimental to the painting. Although the unattractive nature of dirt and soot is impetus enough for cleaning, the collection of contaminants on the surface of artwork can lead to quicker accumulation of pollutants as well as the development of mold or bacteria that can seriously harm the artwork. Fading varnish, if left unattended, can become so severe that the original painting is lost amidst a sea of dark yellow and brown . Fortunately for art lovers, new technology is being developed to reverse these effects.
Laser Ablation: Introduction
Removing the dirt, soot, and varnish from paintings has traditionally been done with solvent. This crude method entails carefully rubbing the painting's surface with a chemical compound specifically designed to remove dirt and varnish without dissolving the paint. Unfortunately, this process can potentially interfere with the chemical makeup of the art, leading to unpredictable side effects ranging from an increased oxidation rate to a breakdown of the very chemical bonds that hold together the paint (Fig. 1).
The visible light spectrum is made up of light in a certain range of wavelengths. Light whose wavelength falls outside this range (such as infrared and ultraviolet) is not visible to the human eye. Different wavelengths of light all carry different amounts of energy: the longer the wavelength, the lower the energy. High-energy light like ultraviolet can be very dangerous--it is what causes sunburns and skin cancer. Just as it is damaging to human skin, it is also damaging to artwork, so it makes sense that engineers do not want to expose art to high-energy light. This is where lasers come in.
A normal light source, like a light bulb, emits a spectrum of different wavelengths of light (including high-energy) in all directions. A laser, by comparison, emits only light of a single wavelength and does so in a tightly focused beam. Because engineers can choose what wavelength of light is emitted, the laser becomes a very powerful and precise tool, as it allows the choice of a single wavelength that will not damage the art.
The Ablation Process
Engineers choose a low-energy (large) wavelength that will interact strongly with the dirt and varnish, but not as much with the surface of the art itself. This is usually a wavelength of about 1065 nanometers, which is in the infrared range of the light spectrum . Using a laser, they then fire short pulses of this light at a point on the surface of the artwork. The majority of the light energy is transferred to the dirt or varnish, heating it to such a degree that it literally jumps off the surface. This expulsion of materials is called ablation.
The ablation process happens so quickly, however, that everything must be arranged ahead of time, and there can be no adjustments during the procedure. This means that the engineer in control of the laser must estimate the extremely small length of time needed to excite the material. If he overestimates, the laser will continue to fire, possibly damaging the artwork. Because of this uncertainty, laser ablation was not used extensively in past restorations: no one wanted to be responsible for the destruction of a masterpiece. Recently, however, the development of a new safeguard system called LIBS has made it a viable and perhaps preferable option.
Laser Induced Breakdown Spectroscopy (LIBS) is a method of examining the composition of the material being ejected from the artwork. When particles are ejected from the surface of the artwork, they give off an energy pattern specific to the material of the particle. Varnish, for instance, gives off a different energy pattern than paint. When the LIBS detects a shift in the energy pattern signaling that paint is being ejected (meaning the light has penetrated too deeply), the laser shuts off to avoid further damage . Because the process is automated and done in real-time, it can be used to control the laser pulses, totally eliminating the need for human guesswork and thus safeguarding the artwork.
With this system in place, it is only a matter of time before laser ablation becomes the standard way of cleaning surface contamination from artwork. One limitation of the process, however, is that it can only superficially restore the work. If there is an internal problem such as mold or cracking, more extreme measures must be taken. Engineers are not without answers, though, and recent developments have led to an elegant and organic solution to these challenges.
If left unchecked, dirt and soot can cause even greater problems by attracting humidity to the surface of a work of art. Humidity can cause deeper pollution in paintings and cracks in sculpture, so engineers have turned to a more adaptable tool as a solution to these problems: bacteria.
A Cleaning Agent
Engineers in Italy faced a major problem when trying to restore a medieval Italian fresco: the traditional method of using a cleaning solvent had caused much of the paint to deteriorate. Unable to continue in this manner, they turned to biology for a solution. As it turns out, engineers found that a certain strain of bacteria, Pseudomonas stutzeri, consumes many of the common pollutants found in artwork. After extensive testing, they applied small amounts of the bacterium to the fresco, and only had to sit back and watch in amazement as 80% of the fresco was cleaned by the bacteria in 12 hours, with no further damage to the pigments .
But bacteria have a far more interesting application in restoring cracked sculpture. Engineers at the Laboratoire de Recherche des Monuments Historiques in France recently started using a bacterium (Bacillus cereus) that produces calcium carbonate--a type of limestone . Since many monuments and sculptures are carved from limestone, this is quite useful. Engineers culture the bacteria to produce the same type and texture of limestone as found in the damaged artwork, and then apply a paste of the bacteria to the cracks in the sculpture. As the bacteria goes about its natural processes, it produces limestone to match the sculpture, thus filling in the crack. The team in France has had successful results in more than 60 trials and is currently attempting to seek similar solutions for different types of stone .
Conclusion: Restoration vs. Conservation
It is important to note, however, that many people are against the use of these technologies to restore artwork. These purists argue against any kind of alteration; for them, the aging process is as much a part of a painting as is the paint. Restoration, they say, takes place "for the sake of making something look good rather than fundamentally improving its failing health--the artistic equivalent of cosmetic, rather than therapeutic, surgery" . These "conservationists" would seek to only slow or stop the aging process of a painting, preserving the work's current state as much as possible. Regardless of opinion, these new technologies are available and are saving artwork from otherwise irreversible damage, allowing future generations to see masterpieces as their creators intended.
-  Smithsonian Center for Materials Research and Education. "What Makes the Paining Image Change?" Internet: http://www.si.edu/scmre/takingcare/painting_change.htm, 2002.
-  Liverpool National Museum. "Conservation Technologies: Laser Cleaning." National Museums, Liverpool Internet: http://www.liverpoolmuseums.org.uk/conservation/technologies/laserclean.asp, July 20, 2004.
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