Filed under: Guest Blogger, On Location, Technology | Tags: Archaeology, biofilm removal, bronze, Lydia, Roman, RTI, Sardis
Our guest blogger, Emily B. Frank, is a student conservator on the Sardis Archaeological Expedition. Currently she is pursuing a Joint MS in Conservation of Historic and Artistic Works and MA in History of Art at New York University, Institute of Fine Arts. Thank you, Emily!
Sardis, the capital city of the Lydian empire in the seventh and sixth centuries BC, is often best remembered for the invention of coinage. Remains of a monumental temple of Artemis, begun during the Hellenistic period and never finished, still stand tall today. In Roman times, the city was famous as one of the Seven Churches of Asia in the Book of Revelation. In the fourth and fifth centuries AD, Sardis boasted what is still the largest known synagogue in antiquity. Sardis flourished and continued to grow in the Late Roman period until its decline by the seventh century AD.
Archaeological excavations at Sardis began over a century ago and are currently led by Dr. Nicholas D. Cahill, professor of Art History at the University of Wisconsin, Madison. The excavated material is vastly diverse and the conservation efforts there equally so. Conservation this season, under Harral DeBauche, a third-year conservation student at New York University, Institute of Fine Arts, supported active excavation across over 1,000 years of antiquity and addressed a number of site preservation issues. RTI greatly benefited the conservators and archaeologists in a couple of significant ways.
Significant finds with extremely shallow incised designs/inscriptions and impressions were made legible with RTI. RTI was helpful in understanding a bronze triangle recovered from the corridor of a Late Roman house (Fig. 1).
The triangle is incised with three images of a female deity and a border of magical signs (Fig. 2). Its use is likely connected with religious ritual practice in Asia Minor between the third and sixth centuries AD.
The legibility of the inscriptions, aided by RTI, reinforced the connection between the triangle’s inscriptions and material and written sources. Two comparenda for the triangle, one from Pergamon and one from Apamea, were identified, and the magic symbols on the triangle were connected with rituals described in a the Greek Magical Papyri. RTI also aided in decoding and documenting a lead curse tablet and in understanding the weave structure of bitumen basketry impressions.
Additionally, a multi-year biofilm removal project of the Artemis Temple at Sardis is currently underway, headed by Michael Morris and Hiroko Kariya, conservators in private practice. The removal of this biofilm is carried out by a six-day process. RTI was used experimentally to document the changes to the stone throughout the removal process (Fig. 3).
RTIs were taken before treatment, during treatment (day 3), and after treatment (day 7). Because the biocide continues to work for months after its application, a final RTI will be taken next summer. Initial comparison of the images showed no loss to the stone surface as a result of the biofilm removal process. All very exciting!
To find out more about the excavations at Sardis, see http://sardisexpedition.org/.
Filed under: Guest Blogger, On Location | Tags: Archaeology, knapped tools, lithics, RTI
This is the second post by our guest blogger Dr. Leszek Pawlowicz, an Associate Practitioner in the Department of Anthropology, Northern Arizona University, Flagstaff, AZ, USA. He can be contacted at email@example.com. Thank you again, Leszek! Even in the age of digital photography, archaeology still relies heavily on old-school hand-drawn illustrations for documenting artifacts, particularly for publication. It can be difficult, or even impossible, to get a single photograph that shows all of the artifact’s key details. In my area of interest, knapped stone tools, low relief in small-scale surface topography, high relief in large-scale topography, specular (reflective) materials, variations in material color and contrast across the surface, all conspire to make these artifacts difficult to photograph. A skilled illustrator is capable of creating a drawing that reveals far more detail than even a good standard photograph (Figure 1). But creating such a drawing requires experience, time, and money, often making it impractical.
Reflectance Transformation Imaging (RTI) is a great way to document and image knapped stone tools. The ability to interactively modify the lighting angle, as well as mathematically manipulate the perceived interaction of light and surface, allows the viewer to see details difficult to photograph. It’s almost the equivalent of having the object in your hands, and turning it at various angles to the light to reveal details of its structure and manufacture. However, one drawback is that you can’t embed an RTI view in a paper or publication unless it’s in electronic format. Also, unlike a drawing that can show all the critical details at once, you may have to move the virtual lighting angle in many different directions to reveal all details. Using data from the custom RTI systems I described in a previous post, I’ve been working on ways to create static images of knapped stone tools with detail comparable to that in a line drawing.
Figure 2 shows a series of images of a modern knapped projectile point fashioned from specular black obsidian, a particularly difficult material to photograph because of its shininess and lack of contrast. 2-A is an original digital photograph, with the lighting coming from the standard upper left direction; while a fair amount of detail is visible, shiny highlights obscure some details, and the overall convex shape of the point interferes with lighting on the right side of the point. 2-B shows an RTI view with lighting from the upper left; while glossy highlights have been eliminated, and more detail is visible on the right, other details are somewhat more subdued in this static view. Figure 2-C uses the RTI specular viewing mode, which imparts an artificially shiny character to the surface. This is actually a superior result for this mode on knapped stone tools – most of the time, it doesn’t look nearly this good (as you’ll see shortly). But this image suffers from lack of detail in some areas, probably because of the artifact’s overall convex surface shape. Figure 2-D uses the RTI Static Multilight mode, where the RTI data is analyzed to determine the optimal blend of multiple light angles to reveal details. Once again, this is a better result than I usually get for knapped stone tools, but some details are still hard to see because of lack of contrast in many areas. Overall, I have found that the recently added RTI Normals viewing mode leads to the best results. This mode color-codes the surface based on the perpendicular direction at every point in the image. Figure 2-E was generated using the Normals mode and a second-order HSH RTI file (standard Polynomial Texture Mapping or PTM) files produce markedly poorer results). Even in the raw colored state, the amount of detail visible across the entire surface is vastly superior to the other views. Using standard image enhancement techniques, one can further process this image to generate an extremely detailed view of the artifact’s surface details (Figure 2-F). While this modern artifact was made of a difficult material, the freshness of manufacture, lack of wear, and non-exposure to the elements make the surface features quite sharp and easy to see. What about an actual prehistoric point with a real history?
Figure 3 shows a series of similar photographs for the Molina Spring Clovis point, collected about 10 years ago in the Apache-Sitgreaves National Forest. Clovis points are approximately 13,500 years old; this one in particular has seen a lot of wear on the flake scars, making them difficult to make out. What’s more, the point’s material (chert) is slightly glossy and very light in color, minimizing contrast in surface details. Figures 3-A through 3-F show the same image processing sequence as Figures 2-A through 2-F; note in particular that the Specular and Static Multilight modes (3-C and 3-D) do not produce especially useful images for this point. The raw Normals image (3-E) brings up details difficult to impossible to spot in the previous images, and enhancing the Normals image (3-F) makes those details even easier to see. For Figure 3-G, a Matlab script was used generate a modified view based on slope; this brings out certain details not immediately visible in the original normals image. In my opinion, images like 3-F and 3-G are superior to standard digital photographs of knapped stone tools and could well be used instead of line drawings for publication purposes. Even in cases where line drawings are preferred, these images could provide a useful background basis for tracing artifact details, instead of drawing them freehand. Beyond static images, the normals data can be used to generate a full 3D representation of the artifact’s surface. Some examples of this are visible at my website, along with downloadable 3D files and RTI data files for several projectile points. The 3D surfaces aren’t yet fully accurate, as they can be affected by inaccuracies in the normals calculation, and error accumulations in the surface fitting. However, these results are already significantly improved from my initial efforts, and I will be working on improving them further. Even in the current form, I believe that it should be possible to extract usable information than can be used to quantitatively characterize knapped stone tools.
Filed under: Equipment, Guest Blogger, On Location | Tags: Archaeology, canon, capture, dome, guest blogger, lighting array, lithics, Reflectance Transformation Imaging, RTI, stone tools
Our guest blogger is Dr. Leszek Pawlowicz, an Associate Practitioner in the Department of Anthropology, Northern Arizona University, Flagstaff, AZ, USA. He can be contacted at firstname.lastname@example.org. A longer version of this post can be seen at http://rtimage.us/?page_id=27. Thank you, Leszek!
When I learned about Reflectance Transformation Imaging (RTI) back in 2009, one of my first thoughts was that it could be a useful tool for imaging and analyzing lithic archaeological artifacts, flaked stone tools in particular. Not an original thought even back then, and over the next four years I’ve seen the occasional RTI lithic image pop up on the web, demonstrating how useful RTI could be in this application. Early in 2013, I started experimenting with RTI on some modern replica projectile points using Highlight-RTI method. Though I got usable results with these experiments, I decided that Dome-RTI was a more appropriate method because of the reduced data acquisition and processing times.
So began a two-year process of building my first Dome-RTI system and refining it. After multiple iterations of the lighting system, controller, and camera/dome stand, I wound up with an 18″-diameter acrylic dome that produces excellent results and is useful for RTI on larger artifacts. However, it’s grossly over-sized for most of the artifacts I’m interested in documenting. Most flaked stone lithic artifacts in the American Southwest are less than 3 inches in length, and an 18″ dome is easily capable of imaging artifacts of at least 4.5″ in maximum dimension (I’ve gotten useful results on artifacts up to 6″ in length). What’s more, these artifacts are housed in scattered locations (museums, government facilities, universities, etc.), and the large size of the dome and stand make transportation and setup of this big system cumbersome. So, applying lessons learned from the first system, I built a second system with an emphasis on portability and speed (Figure 1):
- Dome diameter is 12″, and sits on a stand that is 13.5″ square; total weight of the dome + stand + camera is less than 4 kg. The small size lets it fit into a Pelican case for easy transport.
- The controller box automatically lights 48 3W LEDs in sequence for the light sources; maximum current is 1 amp, and can be set as low as 150 milliamps. The camera shutter is triggered automatically in sync with the LEDs using either a wired remote cable, an IR remote signal, or a Bluetooth HID transmitter; a manual shutter mode is also available.
- Data acquisition time is about 3 minutes with a Canon S110 camera (12 MP, native 12-bit RAW), about one minute with Canon/Nikon DSLRs. A custom GUI front-end for the PTM and HSH fitters reduces data processing time to 1-3 minutes after the photographs are transferred to a computer.
- Dome is mounted on a hinged stand, which allows artifacts to be swapped in/out in about 10 seconds.
- Entire system is powered by 9-12V DC, either from a wall transformer or appropriate battery power supply.
The system can fit securely in a standard camping backpack with room to spare, with a total weight of less than 5 kg. The option of battery power makes this a truly portable, field-ready RTI system (Figure 2).
When recording archaeological sites out in the field, it is often not possible to collect lithic artifacts to bring back to the lab for proper documentation. You either have to photograph them in the field (usually with less-than-satisfactory resolution of artifact details), or hand-draw the flake scars (a slow and tedious process, and often highly inaccurate). This portable RTI system makes it possible to thoroughly document lithic artifacts on-site.
This system has a few more tricks up its sleeve. Full analysis of a lithic artifact may require microscopic analysis of edgewear to determine how it was used.
A simple reconfiguration of the system (Figure 3) allows high-magnification RTI imaging of lithic artifacts, using either the USB microscope (as pictured), or a DSLR equipped with a macro lens that has a working distance of 6″ or more (roughly 90-100mm focal length). A micrometer stage allows for accurate positioning of the artifact under the microscope.
You can also reconfigure the stand to mount the dome vertically for imaging larger artifacts. While I plan to use it in this mode to image the surface of Southwestern pottery, Figure 4 shows the system in vertical mode being used to image an oil painting.
The normals may be off a bit because of the increased spacing between dome and painting, but you can still get useful results, like the specular mode image shown in Figure 5.
Total parts cost of this portable RTI dome, including the Canon camera, was well under $800. Scaling the dome up to a higher size would increase the expenditure by only the extra cost of the dome plus additional LEDs if desired (e.g. 64 instead of 48). For example, a one-meter dome with 64 LEDs would add approximately $400 to the total cost. In the near future, I hope to post information/instructions online that would allow anyone to build a system of their own. If I can build a system without instructions, I’m sure many others could easily build such a system with instructions.
In an upcoming post, I’ll present some of my lithics RTI imaging results from both of my Dome-RTI systems.
Filed under: Commentary, Guest Blogger, On Location | Tags: AIST, Archaeological Research, Archaeology, argon, Digital Preservation, guest blogger, Lake Jackson, Reflectance Transformation Imaging, RTI, virtual archaeology
This is a Guest Blog by Photographer Joseph Gamble.
As an affiliate with the University of South Florida’s Alliance for Integrated Spatial Technologies, I traveled with a team of archaeologists doing imaging research and 3D laser scanning of artifacts to Tallahassee last year to work in the Florida State Bureau of Archaeological Research (BAR) and experiment with RTI on a number of Native American artifacts from Lake Jackson, Florida. AIST Directors, Drs. Travis Doering and Lori Collins along with AIST archaeologist Dr. Jeff DuVernay, helped me to manage a challenging RTI of a Native American copper breastplate as well as other copper and metal objects from Lake Jackson and several other Florida sites.
The artifacts were from the ancient Lake Jackson settlement, a civic-ceremonial center of a Mississippian chiefdom that flourished across parts of northern Florida between c. 900-1500 A.D. The breastplate (23 X 54 cm) was cold-hammered from a sheet of native copper and contains extensive iconographic and symbolic that today are faint and difficult to discern. In the 1970s, the piece was encased in a clear Plexiglas, cube-like chamber that had been infused with argon gas as a conservation measure to halt corrosion of the artifact. The reflective polymer barrier that enclosed and protected breastplate seemed to pose an insurmountable obstacle for its accurate high resolution documentation. To stabilize the breastplate it had also been pressed into a plaster base to prevent further fragmentation and distortion leaving the piece with a cracked or crenelated surface texture. This condition was an additional for the documentation because of the shadowing that further limited the usability of the image set.
To acquire an inclusive data set that would contain sufficient usable images to build an RTI, we placed the case on black velvet, mounted the black balls and commenced to shoot. The total image count came to 156 raw files of which 57 were used to build the RTI file and, much to our delight, it worked well.
You can learn more about the Alliance For Integrated Spatial Technologies at: http://aist.usf.edu/