Stress Relief
Issue: Volume 35 Issue 4 June/July 2012

Stress Relief

More than 2,300 years ago, a noblewoman named Meret-it-es, from Middle Egypt, began her journey to the afterlife. Her body was prepared in the customary way for a person of her stature at the time, and placed in an elaborately decorated coffin that would assist her in this voyage. Various objects were placed alongside her coffin, to be used during and after her passage to the next life. The coffin itself—consisting of an inner anthropomorphic-shaped container for the body and an outer rectangular sarcophagus—was painted with representatives of deities meant to ensure her resurrection, along with intricate hieroglyphs and texts of ancient funerary spells.

The coffin set, along with the other fragile objects, are part of the funerary assemblage of Meret-it-es, acquired by the Nelson-Atkins Museum of Art in Kansas City, Missouri, and displayed in the new Egyptian galleries, part of the museum’s extensive expansion initiative. Despite having brand-new space, setting up a museum exhibit is an art form in itself—there’s lighting, traffic flow, viewpoint, and other logistics to consider. For the Meret-it-es collection, museum planners determined that the optimal position for displaying the coffin would be upright rather than flat (horizontal) due to the size of the room—in order to display the larger outer coffin, the inner piece would have to be exhibited upright. Also, the vertical placement added a desired drama to the installation.

However, there were much more important factors to consider, particularly whether the vertical placement would add undue physical stress on the precious artifact, eventually causing irreparable damage. Although the coffin has survived for more than two millennium, it is made from wood, which expands and contracts as it ages, thereby weakening or damaging the object. Would the vertical display position hasten that problem? To answer this vital question, Kate Garland, senior conservator of objects at the museum, turned to digital technology.

A Digital Solution
Garland oversees the repair, authentication, and general maintenance of the museum’s 33,500-plus art objects, and ensures that they are placed in a suitable environment. Typically, digital technology is not used in these endeavors, though she did work with Epic­Scan (Portland, Oregon) two years prior to the coffin quandary to digitally scan an 18th century room and its ornate ceiling from a locale in Turin, Italy, to verify that the dimensions of that room matched a period room installed at the museum. The CG data from the scans also will be used to construct an exact replica of the ceiling for the room at the museum.

“For us, using CG technology has been kind of an experimental thing. We normally don’t use it,” says Garland. “I was trying to think of a quick way of monitoring changes in a very graphic way so that the results would be clear to everyone.”

In the past, the museum would have used photography, or even considered photogrammetry, which is often used in the preservation world, for this endeavor. But for the issue at hand, scanning offered the most comprehensive and immediate solution, revealing even the tiniest movement in any of the components of the coffin as it was positioned upright, notes Garland. In addition, a scan of the entire surface would serve as a baseline archive should any damage occur to the object in the future; it also could be compared to a future scan to reveal any structural changes that may occur over time.

“The coffin [set] is very fragile, and we were concerned about the paint layers on top of the wood; it was already flaking off, and we wanted to document and record it in case we lost any more paint,” says Garland. “Also, the coffin had been lying horizontally since it was in the tomb, and we were concerned about how much paint we would lose if we displayed it upright due to the movement of the wood or internal stresses, as the wood itself is made up of sections—it’s not just one long piece.”

The Scanning Process
As Garland explains, the anthropomorphic inner coffin is very large (larger than life-size), contains significant gilding, and is brightly painted from head to toe. “It is in relatively good shape,” she says. “It has all these wonderful pigments, some we hadn’t known existed in the Egyptian palette.”

The body portion of the coffin is covered with designs, prayers, and incantations to the gods of the dead, and hieroglyphs. Meanwhile, the head portion of the lid contains gilding of extremely pure gold, along with areas of solid paint (for the hair), a richly painted headdress, and dark eyes and brows. A beautiful piece of carved and painted art, this part of the lid would present issues during the scan process, since laser scans typically do not work well on reflective surfaces like this. Moreover, the delicate surface of the artifact could not be touched during the process, making the data acquisition even more difficult.

Initially, the museum contacted Ryan Wood­ward of EpicScan, a service provider with whom Nelson-Atkins had contracted for the period room scan. He assessed the project requirements and obstacles, and realized that the scanner he was using—which is typically employed for digitizing extremely large objects, such as entire rooms or building facades—was not ideally suited for an object such as this. So he contacted Hexagon Metrology Services, the parent company of Leica Geosystems, which manufactures his Epic­Scan equipment. Not only did Woodward receive more appropriate hardware for the job, but also the services of Hexagon application engineer Rina Molari, who assisted him with this delicate documentation project.

“The type of equipment Ryan was using was not sufficient for the accuracy and point density required for an object this size,” says Molari. “We have other equipment for applications when tighter tolerances are required, like in this instance.”

According to Woodward, the technology used for this type of scan occurs in a very small window—4 by 4 feet or smaller. However, the coffin was much larger (over 7 feet in length) and would have required multiple scans that then would have to be stitched together, thus increasing the possibility of introducing errors as the scans were fit together.

As a solution, Woodward and Molari used a two-pronged approach: a high-precision Leica laser tracker with a mounted camera that uses photogrammetry to read orientation around the target, and a non-contact Leica T-Scan hand scanner. The laser tracker reads the position of the laser scanner; the camera on top of the tracker reads the orientation of the scanner in space. The scanner, meanwhile, captures all the point-cloud information.

“We wanted very dense data spread out over a fairly large envelope,” says Woodward. “With the laser tracker and T-Scan combination, we could leverage the large measurement volume accuracy of a laser tracker and the point density of a scanner.”

Nevertheless, there were still a number of obstacles that had to be considered—specifically the coffin’s multi-colored, reflective surface that could add noise to the data and affect the scan’s accuracy. According to Hexagon Metrology, the T-Scan is an optical device that is nearly impervious to varying light conditions. As a result, it can handle a wide range of black and shiny surfaces without the need for a surface-coating spray, which was obviously out of the question here. (The face of the coffin contains a great deal of matted black coloring for the hair and shiny black paint for the eyebrows, both of which are in close proximity to the gold.)

In addition, Woodward had to take into account the limited space he had to perform the scan. The workable space around the coffin was close to 20 by 20 by 10 feet, which was within the 30-foot envelope placement of the T-Scan to the laser tracker. The team also used the Leica HDS 6000 as a supplemental measuring tool that would enable them to compare both sets of data and document the room where the coffin was housed.

Molari and Woodward spent two and a half days digitizing the outer portion of the object—half of a day to set up and two days for scanning it in both a flat and upright position. The ultra-high-speed hand scanner gathers millions of points, with an accuracy of .00079 of an inch.

The exterior of the coffin had mostly smooth contours, with some areas around the face and lower portion containing sharp angles and surface complexity. “The act of scanning is similar to using a paintbrush, except you are painting light onto the surface and making sure you have overlaps and proper coverage,” explains Molari. “But scanning a delicate object without touching it is certainly a human challenge. You can’t lay your hands on the object, nor brush against it.”

Once the digitizing was complete, Molari and Woodward had two resulting point clouds: 32.5 million points for the object in the horizontal position, and 18.8 million points for it in the vertical placement. After processing the data, Woodward imported it into Geomagic’s Studio, where a model of each object was created so the surface of the coffin in each position could be compared graphically.

“We created mesh data to generate an error map, which is color-coded, showing different gaps,” says Woodward. “The analysis showed that the main body of the coffin was very stable, and there was no movement in the body. In the upright position, we saw a shift in the foot, which was expected.” As he explains, that’s because the feet are semi-detached and have dowels (not original), holding the section in place due to previous damage.

The ornate coffin(right and up) was digitally scanned using a high precision Leica T-Scan laser tracker and a non-contact Leica T-Scan hand scanner(above).

Reviewing the Results
Indeed, Garland was not surprised that the scans showed that there was movement at the foot region, indicated by gaps in the data. “We knew this [movement] was probable due to the way the wood had been assembled,” she says. “But there were no other surprises or problems.”

In fact, soon after the object was acquired by the museum, an ultrasound and X-ray were done to reveal the density of the wood and to show voids and insecure areas. CT scans were considered, but no local scanning units were large enough for the job. This initial imaging provided the team with a firm indication that there were areas of weakness or places where the wood might bend or move over time. According to Garland, it is the back section (with a number of defects) that is most concerning, not the much larger and top-heavy front.

These tests also showed that the coffin was made up of various pieces of wood that are pinned together with wooden pegs—which is typical construction for this type of object since wood was scarce in Egypt. Unfortunately, many of these shaped pieces of wood have separated, causing paint loss.

This issue was detected in the digital scan as well. As Garland points out, the construction has splits in the wood and gesso fills that are no longer flush with the surface, particularly in the shoulder and knee areas, indicating that significant movement of the wood had already occurred. “Voids are accompanied by patching, dowels (modern and original), and multiple pieces of wood, all of which represent potential areas of instability,” she explains. “However, none of these areas are part of the load-bearing structure. The load would primarily be born down the sides of the figure and concentrate on the feet, which appear to be structurally sound.”

More concerning was the split in the center of the main load-bearing wood that follows the wood grain: Splits and checking such as this significantly reduce the structural integrity of the wood. As the weight of the coffin is shifted during reorientation to a vertical position, a force bearing down on the tip of a split will be distributed over the split’s surface since the interior of the crack is not able to support any weight and resolution of the forces over the surface will cause the split to open. In addition, the top of the split is cut off at a saw cut (which occurred prior to the museum’s acquisitioning), making it easier for the forces to pull the split open, thus weakening the structure over time.

The early imaging, as well as the more recent digital scanning, delivered some good news: Despite its age, the coffin was in great shape.

“The ultrasound showed that ideally [the 400-pound coffin] should be presented flat. But there was a strong sense within the museum that we wanted a very powerful effect [in the presentation],” says Garland. “We also had so many beautiful accoutrements and objects that we wanted to display within this small space, which was another reason why we wanted to show [the coffin] upright.”

Display Decision
Before the museum could make a final determination as to how the coffin would be displayed, some preliminary work had to be done on the coffin. As Garland explains, the flaking paint had to be consolidated since the results would have an effect on the orientation of the mount. If the paint could not be satisfactorily reattached without altering the appearance of the matte paint, then the inner coffin would not be displayed upright.

Once that had been accomplished and with the scan data backing up their decision, the group at the Nelson-Atkins Museum of Art hired a structural engineer to design a mount—no easy task, considering that the priceless object could not be physically altered in any way in order to attach it to the armature, nor could the mount be enduring despite the fact that the exhibit itself is permanent. “We have to be sure that anything we do can be undone,” Garland says. “We spent a long time evaluating how to do the mount that would hold the coffin in place. The scans were also useful for this purpose.”

Today, the mount is holding the coffin upright, but it is tilted slightly backward about five degrees, thus easing the downward stress exerted on the wood of this fragile artifact since the force is distributed over the surface along the foot rather than being concentrated only at the front. Inclining the bottom section also reduces the stresses on the load-bearing crack in the bottom center since the forces are shifted toward the back.
This tilted position will also help preserve the paint on the wooden surfaces. “Egyptian paint is very fragile and delicate, and we didn’t want to lose any more of it due to movement in the wood [because of how it was positioned],” says Garland.

Despite having the digital data to help allay their fears, Garland nevertheless was nervous when it came to lifting the coffin into its new display setting. “It was so nerve-racking lifting that piece up. We didn’t know if the coffin and the wood structure would be damaged, if it would collapse,” she says. “We were pretty sure it would be fine, but you just never know, and this stuff is irreplaceable.”

The placement went without a hitch, and today this beautiful object and the rest of the assemblage of Meret-it-es’ funerary objects are on public display in the new museum wing. And should the museum opt to do so, the digital data from the scan can be turned into a realistic, interactive 3D model that, unlike the actual artifact, can be handled without worry.

Meanwhile, the long-term effects of re-orienting the coffin may not be known for a long time. Indeed, there was no structural shifting during the vertical placement, but it is difficult to predict with accuracy how the ancient wood will hold up over time, contends Garland. But with digital methods, the museum will be able to continually check on this millennia-old treasure to safeguard it well into the future.

Karen Moltenbrey is the chief editor of Computer Graphics World.