Hyperspectral Imaging

Setting up for imaging

On Friday December 16^th, I had the pleasure of assisting with the hyperspectral imaging of three paintings in the Cantor Arts Center’s collection, including Andre Derain’s Still Life With Fruit (1938),  Dirck van Delen’s Solomon Receiving the Queen of Sheba (1642), and Hieronymus Bosch’s The Last Judgment (1500s).  Joyce Farrell and Torbjorn Skauli of the Stanford Center for Image Systems Engineering led the advanced imaging procedure, which employs a high-resolution MCT (mercury cadmium telluride) detector that produces digital, high-resolution true-color renderings, performs near-infra-red scans to uncover any underdrawings, and provides information about the pigmentation and other materials in the medium.  Both the Louvre and the National Gallery have also been using this system to study and photograph their collections. (See http://www.hyspex.no/pdfs/HySpex_Art_scanner_web.pdf.)

Curators, professors, conservators, and scientists collaborating at the Cantor Arts Center

What is hyperspectral imaging?  First developed for remote sensing applications, hyprspectral imaging collects information about a scene or an object from across the electromagnetic spectrum.  It is is a type of spectroscopy that collects information as a set of images. Each image represents a range of the electromagnetic spectrum, known as a spectral band.  Hyperspectral imaging deals with taking snapshots using narrow spectral bands over a continuous spectral range to produce a visual rendering of the spectra of all pixels in the scene.   To examine art, the hyperspectral imager gleans optical information  over visible (400-700nm) and near-infrared wavelength ranges (700nm to > 1micrometer), as some oils and pigments don’t fluoresce in the visible light range.  Because certain objects and materials leave unique fingerprints across the electromagnetic spectrum, the imaging technique has been applied in a multitude of ways in the fields of geology, agriculture, resource management, and medicine – to image faces, large fields, forested areas, and even pig organs.

Lubert Stryer, Professor of Cell Biology, pays a visit to the Cantor Arts Center

To measure the spectral data of the paintings, we needed two lights to shine onto the paintings, however this caused some concern when the lights began to raise the temperature of nearby paintings in the gallery above the recommended temperature of 72 degrees Farenheit.  By altering the height of the lamps and turning them off when not in use, we managed to avoid setting off any temperature alarms.

Holding a white board over the work to calibrate the light levels of the imager

The processing of these high-resolution, spatially-resolved data sets is rather complicated.  In fact, Brian Wandell and Joyce Farrell are teaching a class next quarter (http://white.stanford.edu/~brian/psy221/syllabus.html), and they hope some students from the class will be interested in analyzing the images.

Torbjorn collecting data using a portable computer

Does art want to be conserved?

Several months ago, WJT Mitchell from the University of Chicago delivered a lecture and a follow-up seminar at the Stanford art department which discussed part of his book: What Do Pictures Want?  In it, Mitchell speculates that the words idol, fetish and totem can be used to classify the entire spectrum of images: from the Mona Lisa to Ronald McDonald to your family’s holiday card.   While the historical and philosophical twists and turns of his argument go beyond the scope of this blog post, the gist of his talk is thought provoking for art conservation.  Indeed, I’ve been stewing over it for weeks now.  By categorizing all images within the ostensibly magical spectrum of idol, fetish and totem, by proxy Mitchell endows all art works with a degree of agency.  Intriguing!  In the practice of art conservation we are used to thinking about what all the different agents of art – the artist, the curator, the public, etc. – would want for the object’s material condition.  Very rarely – if ever – do  we  consider that the object itself might want something.

Totem?

Fetish?

Idol?

For instance, the word totem is derived from an American Indian word that means something like “member of my family” or “relative of mine.”  Given the great lengths we go to in order to care for Western paintings and sculptures and the outrage and sense of loss that occurs when a so-called masterpiece is destroyed, perhaps “member of my family” is a more accurate definition for such objects than “work of fine art.”

The words fetish and idol can be used in similar ways.  The former is traditionally defined as a object with magical properties while the latter implies a manifest, god-like entity.  While I usually think of them as describing a foreign culture or distant time, could they also apply to Western art objects?  Take the Poussin painting above as an example (the one on the far right).  Perhaps my belief in its ability to teach us about the past makes it more of a “fetish” than a work of “fine art”?  What is “fine art” anyway?  And exactly how and why is it so good at teaching us about the past?  It is after all an inanimate object that cannot actively teach anything in the literal sense of the word.  In other words, perhaps my belief in Western painting’s ability to teach has more to do with my belief in its magical properties than in its distinguished history.

One of the primary benefits of thinking through art conservation with this triad is precisely that it approaches the art object from a non-Western perspective.  It is very easy to assume that we know what art is and  what we are supposed to do with it: we display it, look at it, enjoy it, learn from it, etc.  And our conservation decisions stem from this understanding.  However, following Mitchell, I want to suggest that these thoughts wash over a dynamism that is potentially at work within every art object.  If art history has taught us anything in the past 50 years it is that the relationship between viewing subject and object viewed is far from static.

Such thought exercises are useful to art conservation because they help us elaborate our reasons and make us more aware of our actions.  Our museum setting rose out of a Western tradition and continues to be dominated by Western objects and ideas.  And if art conservation is to be a truly world wide practice we have to think through these exercises and honestly come to terms with them.  By doing so future conservators might readily ask: Does the object want to be conserved or do we want to conserve it?

Protecting Patina on “Large Torso: Arch”

Henry Moore’s “Large Torso: Arch” is one of the many bronze sculptures we have in the collection here at Stanford.  Like most bronzes, it has a chemical patina that was applied by the foundry that cast it: in this case, Noack Foundry in Berlin.  Moore chose to have his bronzes fabricated by Noack at least partly because of their patination abilities; indeed they are one of two foundries that cast the vast majority of his work.  For whatever reason, Moore judged Noack’s finishing techniques and their results to be appropriate for his sculpture.

One of the main obstacles conservators face is the mystery behind these techniques.  For the foundry, such secrets keep them in business; for the conservator, the fact that these details are kept a secret makes work difficult.  Even though you might be able to guess what was used to create a certain patina – Ferric Nitrate, Cupric Nitrate and Sulfurated Potash are all commonly used – the variables that contribute to the final appearance are endless.  For instance, the concentration of the solutions, the duration of application and the surface temperature of the bronze can all change the end result.  Conservators often solicit artists directly about their patination process so as to to anticipate potential future conservation needs.  While this has its advantages and has proven invaluable here at Stanford, its drawbacks are that opinions on patina often change and that people sometimes forget how a certain effect was produced or what the original parameters of production were.

"Large Torso: Arch" in the early 1970s

The lack of definite information about the exact chemicals used and the details of the patination process leaves conservators without a direct path to follow.  If they want to maintain the artist’s original intention throughout the years, they have to protect the mystery patina that is already there.  If it’s not protected the bronze will quickly corrode and the surface will begin to look more like it’s been covered in teal spray-paint than layer upon layer of subtle, chemically patinated color.  A common misunderstanding about bronzes is that the bright green corrosion that can develop over time is the patina.  In fact, the secret patinas of most foundries are almost always composed of layers of translucent color on top of each other: brown, green, black, and sometimes even blue, red and orange.  The combination of layers of colors like these builds up a surface that reacts to changing light conditions and highlights different areas of the surface.

Of course, the artist could express that they want the patina to corrode.  In fact, Henry Moore is known to have expressed this exact sentiment for certain sculptures.  However, it’s also known that he sometimes insisted on complete repatination because of a corrosion.  Moore’s fascination with and insistence upon the relationship between his sculptures and their environment supports this modifiable aesthetic.  He didn’t have a monolithic approach to maintaining his sculptures because he didn’t see them as isolated entities that could stand on their own, but rather as object that were deeply enmeshed in their environment.  In other words, the record seems to indicate that Moore’s opinion on patina depended upon the sculpture’s context.

"Large Torso: Arch" in 1998 before conservation

So the million dollar question is: what did Moore think of this particular cast in this particular location?  Unfortunately we do not have a definite answer.  However we do know that he liked the site and that he liked the appearance of the patina when it was originally installed.  So all we can do is try to preserve that appearance as best as we can.  This is why we apply a wax coating to the surface.  Otherwise, the patina would corrode entirely away.  This is also why we don’t apply a thicker, more protective and more opaque coating.  Because we know he like the sculpture as it looked when it was first installed.

"Large Torso: Arch" in 2003 after conservation