The Shelburne Museum in Vermont is home to a renowned collection of American wildfowl and fish decoys. During renovation of the Dorset House where the decoys are usually on display, Nancy Ravenel, Objects Conservator, had the opportunity to examine some decoys more in depth. In the process, she explored the pros and cons of two type of three-dimensional x-radiography: computed tomography (CT) and volume rad tomosynthesis (VolumeRAD – a GE Healthcare trademark). Since the museum does not have its own radiography capabilities and is located in rural Vermont, there was no access to industrial imaging resources. Instead, Ravenel explored how best to maximize the capabilities from the local medical community through collaboration with the University of Vermont Medical Center.
For this exploration of radiographic techniques, the decoys proved to be excellent patients since they are somewhat simple in construction, yet personalized between makers and specific when used for hunting versus collecting. As an added bonus, they are easy to transport to the medical center. Ravenel used the Barnes swan as a case study while she looked for a maker’s mark at the head / neck joint.
Samuel Barnes,
Swan Decoy, c. 1890
Collection of Shelburne Museum, 1952-192.4
With the CT scan, Ravenel found that the metal elements cause flares, which can be distracting. Beam hardening on the image was also apparent. Since CT scanning requires specialized equipment, it is harder to schedule causing limited availability. On the other hand, CT data offers 360 degree data with options for viewing in a variety of ways. Examples of CT imaging on two ducks in the Shelburne collection can be viewed here https://youtu.be/FFjRmEat5xE and here https://youtu.be/bH3zEtzKRWs.
In contrast, the VolumeRAD technique captures data with the same equipment as standard radiography offering better accessibility. It also requires less radiation so there is less impact on the image from beam hardening. Cons to the technique include that the data is non-isotropic, the edges are not distinct, and there are fewer options for how the data is viewed. Ravenel also pointed out that it collects data of a short depth, so she has to identify where the imaging should take place, otherwise the results can be fuzzy. This can require some trial and error.
This image was taken at the University of Vermont Medical Center department of diagnostic radiology was part of a volume rad study of the joint between the neck and body of the decoy in order to locate a maker’s mark thought to be within the joint. The technique takes a series of images at set angles, thus avoiding the effect of the fasteners in the joint between the head and neck. The numeral “III” scratched into the joint is easier to see in this technique than it was in a standard posterior-anterior view radiograph.
Samuel Barnes,
Swan Decoy, c. 1890
Collection of Shelburne Museum, 1952-192.4
In the end, Ravenel felt that the VolumeRAD technique shows considerable promise and felt that she was better able to visualize the hollowing bit marks, dowels, and saw marks, which were all more distinct than in the CT scans. VolumeRAD, as a new technique, has considerable room for development and refinement.
An additional note beyond the presentation, there was some follow up discussion on viewing software. Ravenel noted in her presentation that she uses OsiriX, a DICOM viewer, for working with the data once back at the museum. An audience member pointed out that ImageJ is being widely used. Ravenel confirmed that she feels most comfortable with OsiriX and finds it to be more user friendly, while the audience member was quite happy with ImageJ and felt that it had deeper capabilities for the conservation community.
For more images of Shelburne decoys with radiographic images, visit their Flickr page here: https://www.flickr.com/photos/shelburnemuseum/albums/72157650406031226.
Tag: CT scan
44th Annual Meeting – Objects-Wooden Artifacts Session, Monday 16 May 2016, "The study of boxwood prayer beads and miniature altars from the Thomson Collection at the Art Gallery of Ontario and the Metropolitan Museum of Art” presented by Lisa Ellis
Lisa Ellis, Conservator of Sculpture and Decorative Arts at the Art Gallery of Ontario (AGO), presented collaborative work on the study of boxwood prayer beads and miniature altars from the early 16th century (c. 1500-1530). The beads and altars are very small, complex, and intricately carved artifacts whose construction has not been well characterized. Teams at the AGO and the Metropolitan Museum of Art (MMA) are exploring the carving techniques and joinery strategies using careful examination, micro-computed tomography (µCT scanning), and physical deconstruction of select artifacts to better understand how the pieces were created.
Because of their depth and small size, traditional photography has been inadequate to capture the various layers in focus within one image, making distance sharing and comparative work impossible. In order to better share between institutions and scholars, the AGO set out to photodocument these artifacts with high resolution images that are in focus throughout the depth of the artifact. In order to do this, they are taking a series of photos at various focal depths, then stacking the images to maintain sharpness. The image quality is profoundly improved from the old hazy images that made it impossible to understand the detail.
Prayer bead, AGOID.29365. Detail showing “The Coronation of the Virgin.” The Thomson Collection of European Art © Art Gallery of Ontario.
Through preliminary x-radiography, they found that the artifacts can be grouped in to two broad classes: artifacts created in simple relief and artifacts created with a complex design. The complex artifacts were then µCT scanned, revealing the multiple elements joined together. Using medical imaging software, they were able to better understand the parts and see that the beads were created in layers. With the software, the various layers could be virtually deconstructed so that each layer could be examined and stacked, as if each piece were separate. At the MMA, Pete Dandridge, Conservator and Administrator, was able to disassemble a bead to physically see the pieces, which further helped to interpret the µCT data and reinforced the understanding of the layers. Since not all artifacts can be taken apart, the µCT scans provided to be invaluable in examining the construction and documenting the process. One example showed a bead attached to a rosary that had multiple roundels set into the main structure. The roundels could be virtually removed with the µCT scans and software, revealing a numbering system beneath.
In addition to examining the construction, they also looked at the limited polychromy present on some beads. Although most pieces were unpainted, a few pieces had painted details in blue, black, or red. These elements, along with adhesives and coatings, are being analyzed at the MMA and the Canadian Conservation Institute (CCI) with a suite of techniques.
These artifacts and findings about them will be presented in an exhibition, Small Wonders: Gothic Boxwood Miniatures, opening in Toronto on Nov. 5, 2016. The exhibition will feature over 60 boxwood carvings from institutions and private collections across Europe and North America. Following its debut at the AGO, the exhibition will open at the The Cloisters at The Metropolitan Museum of Art on Feb. 21, 2017, before travelling to the Rijksmuseum on June 15, 2017. For more details about the exhibition and related programming visit www.ago.net and follow #miniAGO on twitter and instagram.
For images and further details on the work being carried out at the AGO, visit this link at the CODART eZine: http://ezine.codart.nl/17/issue/45/artikel/investigating-miniature-boxwood-carving-at-the-art-gallery-of-ontario-in-toronto/?id=119#!/page/1
Investigation on these materials have been on-going. For some background on earlier work that started this process, visit this link on the AGO website: http://www.ago.net/idea-lab
Other collaborators not mentioned above include: Alexandra Suda (AGO), Andrew Nelson (Sustainable Archaeology, Western University), Barbara Drake Boehm (MMA – Cloisters), Elizabeth Moffatt (CCI – retired), Jennifer Poulin (CCI)
42nd Annual Meeting- OSG, May 31, "Restoration by Other Means: CT scanning and 3D Computer Modeling for the Re-Restoration of a Previously Restored Skull from the Magdalenian Era by J.P. Brown and Robert D. Martin"
After collaborating with JP at the Field Museum on rendering CT scans a few years ago and seeing his article about this work in the spring MRCG newsletter, I was excited to see some images about this in person. JP has been working with CT scanners since 2006 starting out by taking advantage of the kindness of local hospitals and more recently renting a portable unit that came to museum on a truck.
As many of us know, CT scanners can look inside objects non-destructively and provide accurate images with 3D geometric accuracy. JP started the talk be reviewing some of the physics of getting a CT scan done, the benefits, and limitations. Here’s a run-down:
1. The scanner has a donut shaped gantry consisting of a steel ring containing the X-ray tube and curved detector on the opposite side, so your object has to fit within the imaging area inside the steel ring.
2. On each revolution you get lots of images scanned within 30 seconds to 5 min- this is very fast.
3. The biggest logistical challenge is moving objects to and from the hospital safely.
4. During the scanning you immediately get slices, which are cross-section images from three different directions. Volumetric rendering is done from the slices and there is free software for this.
5. Apparently it is relatively easy to do segmentation, segment out regions of interest, and extract wire frame models, just time consuming. From there you can get images of the surface and texture and can even print the models. It is relatively easy to go from slice to wireframe, but harder to achieve a manufacturing mesh to produce a 3D print, which can be expensive in comparison to traditional molding and casting.
6. PROs of scanning and printing: there is no contact with the object, complex geometry is not a problem, the scans and volumetric rendering are dimensionally accurate, you can print in lots of materials; prints can be scaled to make large things handleable or small things more robust for handling or increase visibility; subtractive manufacture, in which you can use a computerized milling machine to cut out a positive or negative, is also a possibility.
7. CONs of scanning and printing: printing is slow, the build volume is limited, a non-traditional skill set is required of conservators to produce the final product, and only a few materials age well. The best material is sintered nylon, extruded polyester may also be safe, but it doesn’t take paint well; it is hard to get the industry to think about permanence.
The object at the center of this project was a Magdalenian skull. The skeleton itself is of considerable importance, because it is the only magdalenian era skeleton of almost completion. A little history: it was excavated, quite professionally, in 1911 when they lowered the floor of the site. Unfortunately the burial was discovered when someone hit the skull with a pickax. Needless to say, the skull did not come out in one piece. In 1915 the full skeleton was removed in two blocks. My notes are a little fuzzy here, but basically at some point between the excavation the skull was restored and then went from being 2 pieces to 6 pieces, as it is documented in a 1932 publication by von Bonen. It appears that at that point the skull was also skin coated with plaster. Thankfully (?) those repairs have held up. Great, so why, did they need to scan and reconstruct the skull? Well according to Dr. Robert Martin, JP’s colleague at the Field Museum, the skull doesn’t look anatomically correct. Apparently during the time period when it was put together there was an interest in race and the skull fragments could have been lined up incorrectly accentuating cultural assumptions.
A previous x-ray showed that two fragments in the forehead are secured with a metal pin. In 2012, when the mobile CT scanner came to the museum, they were all geared up to start with the Magdalenian skull. Unfortunately there was not much difference in attenuation between bone and plaster making it tricky to define between the two materials in the scans. JP consulted a cranial reconstruction group and asked them to pretend this was a pediatric car crash victim with a cranial injury; they asked, why aren’t you using the mimics software package?
JP and his team also imaged the skull with a micro CT scan that has a 0.1 mm resolution versus the normal modern setting of 0.3 mm. They had previously identified 36 fragments of bone from the previous scan. It was hard to tell if some of those separations were just cracks or actual breaks between fragments. The hope was that the micro CT scanner could better define these areas. The micro CT scanner works opposite to the industrial/medical scanner. As you can see in the image to the left, the tube and detector are fixed, while the sample is rotated. Other differences are that it is slower, one scan takes 30-90 minutes and because of scanner geometry the skull had to be imaged in two scans . Because of this, JP used the previous scan to mill out a contoured support to hold the skull in the exact position. JP noted that digitally filling in the holes of the skull to create the support was the most time consuming part of that process and suggests using different radio-opaque marker dots to identify left and right for orientation during the later stitching process. With the new scans at least three separations were identified as cracks vs. breaks.
Now for the virtual reconstruction… the biggest obstacle in this stage was how to achieve something more anatomically correct using the virtual fragments when they have no boundaries. The fragments don’t push back in the computer- and the fragments can easily move into each other. With the software JP used mostly the translation and rotation functions and the free animation software Blender (which has a high learning curve and took several days to get accustomed to) to create hierarchical parent child relationships between the fragments as he joined them together. Just like putting a vessel together, right? In the virtual world at least there is no worry about lockout. They had a 3D printed of the final skull reconstruction and had an artist do facial reconstruction, which JP thinks always look related to Jean Luc Picard… So how successful was this? From a conservation perspective- awesome, it’s fully reversible! Scientifically though, it’s decent, well documented and scientifically justifiable- However, someone else could go through the same process and come up with a different reconstruction because of their reliance on left right symmetry for this reconstruction…
So what did I take away from this talk? This was a very cool project and if I have a question about CT scanning and 3D renderings, I will call JP! The scans can be extremely informational and there seems to be a lot of potential in their use for mount-making, crates, and storage, and possibly virtual reconstructions. Hopefully at some point in the future the software will become more intuitive and easier to use so that more of these types of projects can be done.