An essay by John Barrett, Senior Photographer, Bodleian Libraries, about discoveries from the ARCHiOx imaging project, which has been funded by the generous support of the Helen Hamlyn Trust. See also: ARCHiOx: research and development in imaging – The Conveyor
By far the earliest collection of originals to be recorded for the ARCHiOx project originate from the Achaemenid Empire, and date to between 500 and 400BC. The following image shows a clay seal, or letter-bulla, bearing the impression of the seal of Aršāma, a Persian prince and regional governor. It is one of eight seals, which would have accompanied letters sent to the steward of Aršāma’s estates in Egypt. The impression made on this example, and six other bullae from the collection were made using the same cylindrical seal. Lost to time, this incredibly intricately carved tool would have been rolled over the surface of each of these tiny clay seals, which measure little more than four centimetres. The clay which forms these seals is unfired and consequently these small originals are incredibly fragile. In some cases, the seals are held together by the string which would have attached them to the letters they accompanied. Recording such vulnerable originals is of great importance to ensure their preservation.
A one-hundred-megapixel medium format digital camera has been used to photograph the four source images. In place of the custom flash modules, each seal has been illuminated using a studio flash unit. The flash unit is moved to an equidistant position to the original at 90 degrees from the previous location, and the process repeated.
Recording the seals in this way has made it possible to capture them at over six and a half million pixels per square inch, but at this resolution the depth of field is extremely shallow. Focus stacking is a technique whereby multiple images are photographed from a static position with an incremental adjustment made to the focus between exposures. The resulting stacks of images are then combined in software. In this way the depth-of-field is extended and the recording appears absolutely sharp from top to bottom. Perfect alignment of the four focus-stacked source images to enable photometric stereo processing is the most challenging element within the process.
The final recordings are incredibly impressive. Every tiny detail of the impression, historic repair and even the fingerprints of the maker are clearly visible. These features can be explored using a 3D viewer within GIS software. Moving over the surface of the recording is similar to flying over the surface of a desert landscape, where each granular element becomes a geographical feature. This new method of recording represents an important advance in imaging for the purposes of preservation. The recordings of the seals will allow researchers to study originals in a way that has never before been possible.
In the left-hand example below, the shaded representation of the recorded surface has been generated by positioning a virtual light source at 60 degrees from the surface on which the original rests. In addition, other shaders can be applied, as shown in the right-hand example, which uses a spectrum of colour to represent height.
Recording the seals in this way has made it possible to capture them at over six and a half million pixels per square inch, but at this resolution the depth of field is extremely shallow. Focus stacking is a technique whereby multiple images are photographed from a static position with an incremental adjustment made to the focus between exposures. The resulting stacks of images are then combined in software. In this way the depth-of-field is extended and the recording appears absolutely sharp from top to bottom. Perfect alignment of the four focus-stacked source images to enable photometric stereo processing is the most challenging element within the process.
Every tiny detail of the impression, historic repair and even the fingerprints of the maker are clearly visible. These features can be explored using a 3D viewer within GIS software. Moving over the surface of the recording is similar to flying over the surface of a desert landscape, where each granular element becomes a geographical feature. This new method of recording represents an important advance in imaging for the purposes of preservation. The recordings of the seals will allow researchers to study originals in a way that has never before been possible.
The image below shows one of the fourteen parchment letters from the Aršāma collection. The Aramaic text is reasonably well preserved, and has been almost fully transcribed. The letter suggests that Aršāma valued not only horses, two of which feature on his seal, but also three-dimensional artworks. Addressed to Nakhthor, the steward of his estates in Egypt, Aršāma commissions the production of statues to be made by a sculptor believed to be Hinzani.
So it seems fitting that we should carry out Aršāma’s request, albeit two and a half millennia later. Producing a scaled-up three-dimensional facsimile of the fifth seal using the data recorded with ARCHiOx technology. Firstly, the Lucida scanner was used to record the general shape of the seal from each orientation. This volumetric data provided a base, over which the higher resolution, higher frequency data recorded with the Selene could be overlaid.
With the photometric stereo and laser recordings combined, elevated printing was then used to construct the facsimiles at four times the original size. Several variations were made in order to assess which might be most useful for the purposes of study. Firstly, an uncoloured version was made, showing only the volume of the seal. Two coloured versions followed, the first printed with a shaded render in order to enhance the debossed design, and the second printed with the albedo (colour) image recorded from the original seal.
A far greater challenge would be to create a facsimile of the lost cylindrical seal which was used to make the impressions in the seven bullae. Though the fifth, seventh and eighth seals provide much of the design, some elements are clearly incomplete. A collated line drawing from Christopher J. Tuplin and John Ma’s book, Aršāma and his World: The Bodleian Letters in Context reveals two important missing elements from the design. In the drawing, the horse to the left of the soldier holding a spear appears complete. Crucially so too does the inscription above the horse. With the assistance of Professor Tuplin, these additional details were explained. Another seal bearing a partial impression, made using the same cylinder is held in the collections of the Persepolis Fortification Archive in Chicago. A photograph of this seal was used by Eduardo Lopez from Factum Arte in order to incorporate the missing elements into the digital reconstruction.
Prior to producing the facsimile, the 3D recording was inverted so that the embossed design would be capable of creating an impression similar to those from the original bullae. Though limited by the resolution of the 3D printer, the facsimile cylindrical seal is indeed a usable tool and capable of making impressions which look very similar to those which were ordered to be made by Prince Aršāma, two and a half thousand years ago.
