Special Photography for Larger Objects: Photogrammetry

By Christy Ching

Conservation Technician Christy Ching photographing the underside of an Egyptian coffin 2017-20-1.3 for photogrammetry.

One project I have really enjoyed working on as a pre-program conservation technician is documenting larger objects for a process called photogrammetry. Photogrammetry is a technology that gathers spatial and color information of an object from multiple photographs to form a geometrically corrected, highly detailed, stitched image called an orthomosaic. Essentially, photogrammetry creates a distortion-free, three-dimensional model of an object based on two-dimensional photos of every surface photographed in sections. 

Left: Four photographs of an ancient Egyptian coffin lid L-55-16B at various angles, which were used to create a 3-D model. Right: 3-D model draft of L-55-16B.

*L-55-16B (21-46-9) is a loan object from the Philadelphia Museum of Art (PMA)

This can be done for objects of any size. However, we are mostly reserving this technique for larger objects, specifically larger textiles and Egyptian coffins. This is because photographing the coffins and textiles normally with a single shot requires a greater distance between the object and the camera in order to fit the entirety of the object into the frame, and doing so reduces the image quality. Not only that, but the camera distortion that is inherent in all photographs will become more obvious. The resulting image will not be an accurate representation of the coffin or textile, which is not ideal for documentation purposes. 

The image on the left is a single-shot photograph of L-55-16B while the image on the right depicts the same coffin lid created by photogrammetry. When comparing the two images, the camera distortion in the single-shot photograph can be seen especially in the feet and head of the coffin lid.

With photogrammetry, we can take parts of the 3-D model and use them as high resolution, distortion-free, 2-D images of the object instead.

Six views of L-55-16B depicting the top, interior, and the four sides of the coffin lid generated using photogrammetry.

So far, a little less than ten coffins, a few textiles, a pithos fragment, and a giant granite relief have been documented using photogrammetry. The models and orthomosaic images are all generated by Jason Herrmann from CAAM, and we are very grateful that he is doing this for us! To learn a little bit more about the photogrammetry process, view this Digital Daily Dig here.

This project was made possible in part by the Institute of Museum and Library Services.

Progress update on the stola coffin treatment

For the past few weeks, it has been full steam ahead on the treatment of the stola coffin lid. The lid is made of smaller pieces of wood joined together, then covered generously in areas with a thick layer of coarse mud plaster, followed by a thin layer of a finer mud plaster, followed by paint and a varnish. There are also raised details that were built up with gesso before painting.

This large piece of painted mud plaster (detached from the foot of the coffin, seeing on the left) is 11 cm thick.

This large piece of painted mud plaster (detached from the foot of the coffin, seen on the left) is 11 cm thick.

This area of damage clearly shows the wood substrate (green arrow), coarse mud plaster (blue arrow), and finer mud plaster (red arrow).

This area of damage clearly shows the wood substrate (green arrow), coarse mud plaster (blue arrow), and finer mud plaster (red arrow).

The two major condition problems on the coffin are found in the mud plaster layers: the coarse mud plaster has lost cohesion and in many places has separated from the wood below, and the finer mud plaster has also lost cohesion, so much so that it has deteriorated to a fine powder in places. I have spent over 150 hours so far readhering detached plaster, consolidating the powdery plaster, and realigning and stabilizing loose fragments on the coffin. Today I’m posting a few before and after treatment details to show the progress.

Here are before and after details of the top of the head showing an area where I had to readhere some large fragments of painted plaster:

Top of the head before (left), during (center), and after (right) reattaching painted plaster fragments

Top of the head before (left), during (center), and after (right) reattaching painted plaster fragments

Here are before and after details of the left eye showing the consolidation of exposed powdery mud plaster:

Detail of losses near the left eye before, showing powdery mud plaster (left) and after cleaning and consolidation of the mud plaster in the losses

Detail of losses near the left eye before, showing powdery mud plaster (left) and after cleaning and consolidation of the mud plaster in the losses (right)

And here is an area on the side of the head where I found that some fragments were previously attached in incorrect places. They were repaired long ago (with no documentation) with an adhesive that is soluble in warm water. I reversed the old repairs and found the correct locations for the fragments. I’ve outlined the fragments in their incorrect locations in the before treatment image on the right, below:

Detail of treatment on the side of the head before, with misplaced fragments outlined in pink (left), and after respositioning (right)

Detail of treatment on the side of the head before, with misplaced fragments outlined in pink (left), and after respositioning (right)

I still have lots of work to do before the treatment is complete, but I’m making good progress! I hope to be finished with the treatment early in the new year.

A closer look at our stola coffin lid

In my last post about this late 21st/early 22nd stola coffin lid, I referred to it as eye candy.

Overall before treatment image of the stola coffin lid, L-55-16B

Overall before treatment image of the stola coffin lid, L-55-16B.

This coffin is beautiful, and we could discuss how nice it is to look at all day. But in this post, I’m going to focus on some of the more subtle, somewhat less-attractive (but perhaps even more interesting) features that I’ve discovered about this coffin lid as I’ve begun to work on it. In fact, on Friday I spent awhile discussing some of these details with a group of conservators and interns from the Philadelphia Museum of Art (PMA). As I mentioned in my last post, although this coffin has been on long-term loan to us since the 1930s, it still technically belongs to the PMA, so I am carrying out my treatment in close consultation with one of their objects conservators.

Penn Museum conservator Alexis North (far right) discusses a cat mummy with the PMA conservators and interns.

Penn Museum conservator Alexis North (far right) discusses a cat mummy with the PMA conservators and interns.

I always appreciate the opportunity for professional exchange, and I was happy to be able to discuss my plan, including materials and approaches with them. This discussion energized me to get started with the treatment, and that is exactly what I did, almost as soon as they left.

One thing that I noticed in my initial examination is that this coffin lid has been worked on before. There are residues of old adhesives in areas, including an adhesive mixed with a sawdust-like material, especially around the foot of the coffin.

Adhesive residues on an area of exposed wood.

Adhesive residues on an area of exposed wood near the foot.

I have seen these same types of repairs on at least one other coffin in our collection, and while there is no documentation of them, I am suspicious that this treatment was carried out in our museum soon after we acquired this coffin, but long before our conservation lab was established 49 years ago.

The foot of the coffin has another type of repair as well – there are 2 brackets that are held on with screws on the underside of the foot, apparently to secure some of the wood components. This whole area moves when handled, so the brackets and screws will have to come off so that I can better assess what is going on.

Detail of the foot of the coffin, showing the brackets used in an old repair.

Detail of the foot of the coffin, showing the brackets used in an old repair.

There is some very powdery material associated with areas of damage, especially on the face, which appears to be partially due to deterioration of the mud plaster, but also may be due to insect activity. I’ll have to investigate this further over the course of the treatment.

Detail of the powdery material near the left eye.

Detail of the powdery material near the left eye.

And then there are some materials included in the mud plaster, including some course linen threads and even a leaf, which I am documenting as I work to clean the surface.

Detail of a detached piece of mud plaster with linen thread inclusions.

Detail of a detached piece of mud plaster with coarse linen thread inclusions.

L-55-16B_detail7

Detail of an area of damage, which exposes a small corner of what appears to be a leaf caught in the mud plaster.

The next time I post images of this coffin lid, it should be a lot cleaner, as I plan to complete the initial surface cleaning within the next week. More soon about this, and other observations and developments in the treatment.

 

X-raying fragments of a painted wooden coffin

I recently completed the treatment of these coffin board fragments.

E12617A-C, boards from a painted wooden coffin, before treatment

E12617A-C, boards from a painted wooden coffin, before treatment

In addition to the cleaning, which I blogged about before, the treatment involved stabilization of loose and powdery gesso and paint, filling losses where needed for structural support, and x-radiography, multispectral imaging, and portable X-ray fluorescence (pXRF) analysis. It has also enabled a translation of the text. I’m going to write a few shorter posts to highlight the different components of this project, starting with the x-radiography.

During my initial examination of the boards, I could see that the boards represent just a portion of the front, head end of the coffin. The rest of this side of the coffin would have continued much further to the left, but at some point these pieces were cut down and finished off on the left side to a smooth edge. This is most evident when you look at the hieroglyphic text, which obviously should continue to the left.

I could see that these 3 boards were originally joined with wooden dowels, because there are wooden dowels protruding from the join edges. I also noted some large cracks in the painted surface of the largest (center) piece in the image above, which led me to realize that this central piece was made of more than 1 piece of wood. I turned to x-radiography to get a better understanding of what is going on below the surface.

Here is a composite image showing the radiographs of the 3 boards:

E12617A-C x-ray image

E12617A-C x-ray image

The dowels joining the 3 pieces together are very clear in the x-ray image above. There are some darker (almost black) areas, which represent the holes that were drilled out for inserting the dowels. The denser (whiter) areas within those voids are the wooden dowels themselves. I’ve outlined these areas in green in the image below.

E12617xraymapped-1The x-ray image also helps clarify how the center piece is constructed, with 3 pieces of wood, which I’ve outlined in red above. Where those 3 pieces of wood join correspond directly with the cracks observed in the painted surface on the exterior.

Also visible in the x-ray image are two small nails driven into the lower edge of the bottom board. These nails are historic additions, likely added at the time when the boards were cut down and modified, although their purpose is unknown.

In my next post, I’ll focus on what cleaning, pXRF, and multispectral imaging has revealed about the painted surface, and I’ll include some after-treatment images.

 

Spring cleaning?

It may be a little early for spring cleaning, but no matter what time of the year, there is not much that I find more satisfying than a good, deep clean (on a grimy artifact). Last week, Tom Stanley (the museum’s Public Relations/Social Media Coordinator) posted this image on the museum’s Facebook page, which shows some cleaning in progress on an Egyptian painted wooden coffin here in the Artifact Lab:

coffin board cleaningHe also posted this on our Instagram page.

Here is a before treatment image of the coffin board (which is in 3 separate fragments):

E12617A-C, boards from a painted wooden coffin

E12617A-C, boards from a painted wooden coffin

While Tom was in the lab taking photos, I promised him that I’d put some additional information about this project on the blog. I thought this would be a great opportunity to take another video with our binocular microscope, kind of like the video I captured of the paint consolidation on the shabti figures I worked on awhile ago.

To see the process of how we go from

————————–this————————–to————————–this———————-

corner before after

click on the link below.

Cleaning an Egyptian painted wooden coffin from Molly Gleeson on Vimeo.

In the video, you’ll see (at 7.5X magnification) that I first used a soft-bristled brush to remove loose sediment and dust from the surface, by brushing directly into the nozzle of a variable suction HEPA-filtered vacuum. Then I used a cosmetic sponge to further, gently, lift away grime from the surface. Finally, I used a kneaded rubber eraser to remove the grime that is more embedded in the painted surface.

Okay, so I’ll admit that this may not be as cool as the video of Conservator Tessa de Alarcon laser cleaning a stone table from Ur (this one is hard to top), but it’s pretty gratifying nonetheless.

I’m currently trying to learn more about this object too, by checking into our museum records. I’ll keep you posted.

Views inside a painted wooden coffin

Update – this post contains blurred images of human remains. To read more about this decision, follow this link. 

It is impossible not to see this object when you enter the Artifact Lab, as it’s front and center, and immediately impressive, due to its well-preserved painted details:

This is the lower-half of a Third Intermediate Period (21st Dynasty?) painted wooden coffin, that recently came up to the Artifact Lab. Our visitors are always commenting on how vibrant the colors are, and that is mostly based on what they can see from the exterior. But the interior of this coffin is fully decorated, and arguably even more impressive, and I promised some people this week that I’d post photos of the interior soon. Here they are:

Overall view of the interior of the coffin from above
Overall view of the interior of the coffin from above
coffin right side
View of the right side
coffin left side
View of the left side
View of the interior, top of the coffin
View of the interior, top of the coffin

The only areas on the coffin that are not decorated are the exterior of the back, and both sides of the foot/base.

All in all, this coffin is in great condition, but it needs some treatment, including surface cleaning and stabilization of the wood and paint in some areas.

There are also a few mysterious things about it, in particular, these drilled holes in the back – what the heck are these all about?

There are 8 rows of holes drilled through the back of the coffin
There are 8 rows of holes drilled through the back of the coffin

Stay tuned as we investigate further.