John Asmus: from Lasers to Art Conservation

interview by Rui Bordalo, January 2008

John Asmus is one of the world’s leading conservation scientists, acknowledged and often referred to as the grandfather of laser art conservation. He made history through his innovations which contributed massively to the advancement of technology applied to art  conservation. Among his outstanding professional achievements in conservation over the past years can be recalled significant works of art as the Mona Lisa and the Emperor Qin’s terracotta army.

John Asmus has been Research Physicist at the Institute for Pure and Applied Physical Sciences at UCSD since 1973. He received a Ph.D. from Caltech in Quantum Electronics and Physics, and has 130 published articles and 25 patents. He introduced holography, lasers, ultrasonic imaging, and digital image processing to art conservation and adapted gas-embedded plasma-pinch technology to the generation of intense ultraviolet radiation for surface preparation applications. Earlier work in high-energy excimer lasers was conducted at US NOL, General Atomic, Maxwell Labs, and SAIC. At General Atomic he participated in the ORION nuclear spaceship program.
The Rolex Laureate for Enterprise was awarded to him in 1990 for his restoration work on the Emperor Qin terracotta warriors of Xi’an, China. He has been a member of numerous art conservation teams throughout the world.

Rui Bordalo: As a physicist, in the 60s you were researching the application of powerful lasers within rocket technology. How did you make the switch from this to cultural heritage laser applications?

John Asmus: I began my graduate work in plasma physics at Caltech in 1958, and in 1960 one of my classmates (a Hughes Research Laboratory consultant) informed me that they had just operated the first laser. A few months later I borrowed a Hughes laser and began using it for electron scattering measurements in a plasma. Then, my part-time employer (US Naval Ordnance Laboratory) sent me to Copenhagen for two years to build a VLF (Very Low Frequency) Radar system to monitor Soviet ICBM launches and high-altitude nuclear explosions. In my spare time I travelled all over Europe visiting museums and developing a love for classical art. When I finished my graduate work in 1964 I began my employment with General Atomic on Project ORION (the nuclear-propelled spaceship for a mission to the planet Saturn). General Atomic was attractive to me as top physicists such as Freeman Dyson and Marshall Rosenbluth were there. The 1962 Nuclear Test Ban Treaty (promoted by my former chemistry professor, Nobel Laureate, Linus Pauling) put an end to any hope continuing with ORION flight tests. However, the originator of the ORION (Theodore Taylor) realized that a very powerful (non nuclear) laser could be substituted for an atomic bomb as a source of propulsion. As I was the only one at General Atomic with laser experience, I was asked to lead in the development of a laser-propelled spaceship. First, we developed a Nd:glass laser that produced 200 Joules in a 200-nanosecond pulse. We focused this gigawatt beam on small metal disks (in vacuum) and the reaction force of the ablation plasma propelled the material to a world record velocity of 20km/sec. This amounted to a terrestrial meteorite factory and we did assess the meteoroid hazard to satellites and spacecraft. All of my team were amazed that the theoreticians were correct in projecting that an energetic ablation plasma could be generated on a surface without destroying the object.

RB: You were also involved in the then recent innovation of laser generated holography which you applied in Venice in 1972. This moment is remembered as the starting point of laser cleaning application in art. Can you tell us how that happened?

JA: In 1968 the Gulf Oil Company purchased General Atomic. Much of the laser, plasma physics, and nuclear work came to an end at that time. I accepted a position in Washington, D.C. as a Defense Department advisor on the high-energy (STARWARS) laser program. In 1970 the President’s science advisor (Edward David) asked me to lead his JASON summer-study group in formulating a National Laser Program. During the study I happened to give a lecture on the holographic plasma diagnostics that had been performed at General Atomic. A geophysicist in the study group (Walter Munk) suggested that this type of pulsed insitu holographic 3D recording could be used to produce high-resolution archival images of crumbling Venetian marble sculpture before they disappeared entirely. (Munk had been working in Venice on developing a computer model of the tidal action in the Adriatic that leads to the "acqua alta" floods.) We wrote a grant proposal to the Italian Government offering to determine whether it was feasible to produce large format in-situ holograms of life-size statues in Venice. Then, an Italian resident (Dennis Gabor) won the Nobel Prize for physics and endorsed our proposal. Consequently, our proposed study received funding. Second, a pioneer in pulsed holography, Ralph Wuerker of TRW, agreed to participate in the program in Venice. Third, the president of TRW (Simon Ramo) offered to loan us the most powerful ruby holographic laser then in existence at no cost.
In January 1972 the laser was delivered to Venice and it functioned perfectly in spite of numerous logistical impediments. By the end of February we had produced over 50 holograms (later displayed at the Academia Museum) and NDT (NonDestructive Testing) holographic interferograms (revealing hidden defects in artworks). In March the restorer of monuments in Venice (Giulia Musumeci) described to us the great difficulties in successfully cleaning crumbling marble sculpture. Based on the ORION experience I estimated appropriate conditions for ablating the black crusts from crumbling marble, with minimum damage. The ruby holographic laser was then focused to an appropriate fluence and self-limiting radiation divestment of friable marble was demonstrated. Musumeci, her mentor (Kenneth Hempel of the Victoria and Albert Museum), and Sir Ashley Clark (Director, Venice in Peril Fund) were delighted with the laser cleaning and began promoting the process to their patrons, associates, and the Venetian officials. At that point the holography program became a laser statue cleaner development project.

RB: The first cleaning attempts were made with a ruby laser but its use was not continued, later being substituted by Nd:YAG lasers. What disadvantages did the ruby laser present and why did it turn out to be an obsolete technology for the conservation of artworks?

JA: The Venetian officials (Francisco Valcanover, in particular) convinced the S.H. Kress Foundation and the Smithsonian Institution to fund a program at UCSD (Walter Munk’s organization, University of California – San Diego) to evaluate the "safety" and "utility" of laser treatment of all materials found in artworks. Between 1972 and 1974 it was found that laser cleaning can be made to work on marble, limestone, oolite, sandstone, stucco, concrete, terra cotta, most metals, leather, velum, paper, cotton, wool, silk, moleskin, and wood. The cleaning of stained glass, ceramics, paintings, and frescos appeared promising, but problematical. The final phase of the program was the development, construction, and delivery of a laser statue cleaner for Venice (1975). This laser employed a Nd:YAG, rather than a ruby, laser crystal. During the  intervening years the laser industry had succeeded in perfecting large, high quality YAG laser rods that led to systems higher in both efficiency and repetition rate than ruby, as well as lower in cost and smaller in size. (Ruby lasers continue to be favored for pulsed holography as the wavelength in the visible is commensurate with the spectral sensitivity of photographic emulsions, and high coherence is readily achieved, as well).

RB: Among your numerous works, you were involved in a project designated to uncover the lost mural painting "The Battle of Anghiari" by Leonardo da Vinci. Can you give an insight into this fascinating challenge?

JA: Carlo Pedretti of UCLA located historical Florentine documents suggesting that the Medici may not have destroyed the remains of Leonardo’s great "Battle of Anghiari" mural, but covered it over with Vasari paintings, instead. Upon hearing of the laser work in Venice, a Florentine official (Umberto Baldini) asked that we make holograms of the Vasari murals in order to determine whether there were "remains of the Leonardo mural beneath the surface" and then "uncover the Anghiari through laser divestment". A UCSD multidisciplinary team concluded that the Vasari and masonry materials of the Hall of the Five Hundred could be more successfully explored through ultrasonic imaging.
In 1976 we designed, built, and tested a 1MHz ultrasonic digital imaging system with an automated stepping-motor-controlled transducer scanner. In 1977 we ultrasonically mapped the masonry strata beneath the Vasari paintings. On the East wall we located a 2mx5m plane "island" 7mm beneath the surface through ensemble averaging of the tens of thousands of ultrasonic echoes. As this area appeared to be the most likely location for any remains of the Anghiari mural, we suggested using a laser to drill a very small (1mm) hole into the wall and employ LIBS (Laser Induced Breakdown Spectroscopy) to watch for any sign of pigment residues at the 7mm depth. The search was then turned over to a local Florentine "art diagnostician" and we continue (after 30 years!) to await any announcement as to the results of that investigation.

RB: You made as well an analytical study of Leonardo’s Mona Lisa. Can you tell us the story of this interesting study and of the surprising results you achieved?

JA: When Carlo Pedretti and the late Lord Kenneth Clark learned of our ultrasonic digital image processing results they asked that we attempt to employ computer enhancement to clarify Leonardo’s "Mona Lisa". They hoped to see a simulation of the painting’s original appearance: free of restorations (paint losses as well as layers of brown varnish) and unfiltered by webs of cleavage and craquelure. In 1980 I was interviewed by TV anchor, Walter Cronkite, on the Anghiari project for his new "Universe" program. This resulted in his agreeing to fund the Mona Lisa image enhancement effort, which included his "renting" the Leonardo piece for a day of inspection and analysis at the Louvre conservation laboratory. With additional  funding from IBM and free use of their supercomputer we produced a beautiful version of the Mona Lisa that was free of the brown "smoggy atmosphere" and without the cracks. We employed gain-bias adjustments together with FFT, principal component, and bi-scatter digital filtering to attain the final "restored" image. Various science museums display 1m enlarged prints showing the various steps in the enhancement. We were able to map areas that had been over cleaned and clarify some details in the background.
The most dramatic and unexpected result was the discovery of pentimenti revealing that Leonardo had first painted the female image with a necklace and then hid it with over paints. (Ten years later, after first questioning the validity of our discovery, the Louvre’s laboratory made new X rays of the painting and confirmed the existence of the necklace.) Another revealing area of the enhanced image is the river and bridge at the lady’s left elbow. This exact scene appears in works by other artists and is clearly the Tiber River "Near Perugia". This observation together with the necklace evidence lend weight to the longheld idiosyncratic theory that the original subject of the painting was Costanza d’Avalos.

RB: In your articles you often refer to laser cleaning of works of art as "laser divestment". What is the origin of this term and why do you prefer it?

JA: When I first began treating black crusts on marble with lasers I called it "cleaning". Upon meeting several prominent conservators (e.g., Hempel, Organ, Rinne, Stout, and Buck), I was told that the correct term is "divestment" which means the removal of a specific stratum (viz., cloak) to uncover the "body" within. Subsequently, I have observed that many conservators do call it "cleaning".

RB: Your projects in the 70s and 80s were funded by several institutions, from the Getty Foundation to unexpected sources as Metro-Goldwyn-Mayer (MGM). Compared to those times how do you see the funding of scientific projects applied to works of art nowadays?

JA: At Caltech there was a legend that Carl Anderson (one of our Nobel Laureates) had received funding from a motion picture studio for his discoveries of the positron and antimatter. Naturally, it seemed to me that I should try to do likewise. One year, when we began working on stone, the motion picture "Clockwork Orange" did very well at the Venice Film Festival. It was easy to convince Jack Warner that he should share a portion of his profits for the conservation of the city. As I retired in 1988, I haven’t had to approach any funding agencies for a long time. In the beginning very little funding seemed to be available from museums and we were supported by foundations such as the Kress, agencies such as the National Museum Act, and private conservation companies such as Arecon of Padova. Now that conservation with lasers is accepted and widespread it seems that a great deal of the work is supported by museums.

RB: Laser cleaning application started on stone but later research focused also on other supports such as paintings, paper, paleontological materials and even on unusual materials such as feathers or egg shells. However, commercially there is still only equipment designed for stone cleaning. Why do you think this is? Do you think technology is still not well developed, that research stagnated for a long time, or that the conservation world is just too conservative?

JA: Fortunately, it turns out that removing black crusts from marble is very easy as it is black (absorptive) on white (reflective). That is where laser conservation began. In addition there are large amounts of stone to conserve as it’s such a popular building material and the existing technologies were unsatisfactory. It is also an accident that one of the most popular and high performance of laser types (Nd:YAG) works very well on most stones. The situation is far different in other fields of art conservation. Many traditional techniques work quite well. There is a great diversity in materials and their colors and properties. One could speculate that a different laser may be needed to optimize a treatment for each conservation problem. There certainly are not the financial resources in art conservation to enable the invention of a new laser type to solve each individual problem. Still, there are other commercial laser types such as excimer, carbon dioxide, and erbium that have found limited use in art conservation. The bottom line is that much of the entire laser industry is awaiting a breakthrough much as took place in the  semiconductor/ computer industry some decades ago. What everyone awaits is a low cost, high power, versatile, semiconductor laser that can be mass-produced.
The first conservators I met told me "conservators are conservative". That has not bothered me very much. Many medical physicians have complained to me that some medical laser systems have been "over sold". They were sold before they were proven.

RB: Laser cleaning started over 30 years ago but it is still not a commonly employed technology. Do you think this is due to conservators’ lack of trust in new technology or to other reasons such as the lack of information, insufficient research and dissemination, high costs, etc.?

JA: One of the first conservators I met was David Rinne of the Getty museum. He predicted that lasers would become commonplace in conservation "when the last of the old guard dies off". That may be partially true. However, when I worked on the terra cotta warriors (1990) in Xi’an I was amazed at the performance and diversity of the art conservation lasers they had developed on their own.
About 25 years ago there was an accidental breach in security, and I learned that I had been nominated for a McArthur ("Genius") Award. I understood that there was one unfavorable evaluation of my work which stated that the laser in conservation was "bogus". The writer held a prominent and powerful position in one of the major US museums. It is also interesting to note that (in contrast to the US) virtually every EU country has a significant laser conservation program. The establishment in 1994 of LACONA is responsible for this in large measure. I’m also encouraged by recent increases in communications, visitors, and visiting scholars from such places as Australia, South America, India, and Egypt.

RB: You were involved in the restoration of the Emperor Qin terracotta army. How was your experience in China?

JA: I visited, toured, and worked in China several times between 1980 and 1990. They made incredible technical progress in those ten years. By the end of that interval my host, Mr. Wen Rei Tang, was building and using laser statue cleaners higher in performance than any others in the world. Further, he accomplished this using primitive components such as Nd:glass, rather than YAG (which was not available to him). Mr. Wen had come from a military weapons background and was highly experienced and well trained. Our collaboration ended in 1991 when he was reassigned to a different project involving commercial products. I am very surprised that a Chinese laser statue cleaner has not appeared on the international market.
My three favorite memories involve the wonderful dining, the High Lama of the Fa Men Temple granting me "one hundred additional years of great good fortune", and the vast amount of art in need of conservation.

RB: Based on your extensive experience that has successfully combined science, technology and art, how do you see the future of this technology in conservation of artworks?

JA: One of my mentors at UCSD is James Arnold of the Chemistry Department. His thesis at the University of Chicago was the discovery of radiocarbon dating. Several years ago I told him of my disappointment that the widespread use of lasers in art remained a dream. He responded that to his chagrin his first publication on dating with isotopes was greeted with "a great big yawn". He counseled patience. In a larger sense I look upon my work in the arts as a continuation of Prof. Arnold’s example and tradition. I have been gratified at recent conferences to see others continuing to apply still newer  technologies to the service of the arts.


We thank Dr. John Asmus for his kind collaboration to this interview, carried out by email in January 2008, by Rui Bordalo.


From left to right:
Image 1. On the Hawaiian Island of Nihaau (1956) to make measurements of radiation from a high-altitude nuclear test over Johnston Island.
Image 2. Test cleaning a stone block from the US Capitol Building (1980). This test was done at the request of Mr. George White (Capitol Architect), which then arranged for John Asmus to laser clean some stones from the Egyptian Temple of Philae.
Image 3. John Asmus applying pioneering laser divestment methods to remove dirt from a marble relief of "The Last Supper" at the entrance (Porta della Carta) to the Palazzo Ducale in Venice (1981).
Image 4. Fiber Optic (Nd:YAG) laser statue cleaner being tested on a contemporary iron sculpture in San Diego prior to delivery to Arecon (Padova, Italy) in 1994. (Built by UCSD student volunteers at low cost: US$10,000).
Image 5. John Asmus employing computer enhancement to reveal hidden details of the image of Mona Lisa.






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