Glowing in the dark: multispectral imaging and Egyptian blue

There is something I’ve mentioned before on this blog, but never actually shown, and that is the ability to “see” Egyptian blue on objects using multispectral imaging. On many objects Egyptian blue is very well-preserved, so there is no need for special examination techniques in order to spot it. But there are cases in which being able to accurately identify this pigment is important. Sometimes Egyptian blue deteriorates either by changing color (to green or black) or by becoming lost altogether, making it difficult to know which areas may have originally been blue, or if blue was used at all.

And then there are objects like this one:

Front view of the shabti box in normal lighting conditions

Front view of the shabti box in normal lighting conditions

You’ve seen it before, it’s our painted wooden shabti box. I have been working on the treatment of this box for awhile now, mostly to stabilize the flaking paint and varnish. And this thick, orange-yellow varnish, which we believe is original, and is pistacia resin, makes it difficult to see the painted surface, both the details and the colors. While I could see that there is some green and possibly blue paint on this box, between deterioration of the paint and/or pigment, and the thick application of pistacia resin, I couldn’t say for sure which areas may have originally been painted blue…until now…

Taking advantage of the fact that Egyptian blue has luminescent properties when illuminated with visible light and captured in infrared, we can detect where Egyptian blue was applied. And wow, look at these results:

Visible-induced IR luminescence image of the shabti box. Light source: SPEX Mini Crimescope with 600nm band-pass filter. Captured with a Nikon D5200 modified camera with an IR 87C filter.

Visible-induced IR luminescence image of the shabti box. Light source: SPEX Mini Crimescope with 600nm band pass filter. Captured with a Nikon D5200 modified camera with an IR 87C filter.

This is the same surface of the shabti box seen in the first photo, but zoomed in a bit, and taken under different lighting conditions and captured with a different camera. The areas that appear white are where Egyptian blue was applied. Because everything else pretty much disappears on the box in this image, to better visualize where the Egyptian blue is in relation to other details, we created a false-color image in Photoshop:

False color image of the shabti box. The areas painted with Egyptian blue appear red.

False color image of the shabti box. The areas painted with Egyptian blue appear red.

In this false color image, the areas that appear red are where the Egyptian blue was applied. It’s not perfect (you can see that the bands in the hair of the figure on the right don’t really show up) but we could play around with the photographs a bit to improve this.

We did this imaging on all surfaces of the box, and on the box lids. Here is a regular photo, a visible-induced IR luminescence photo, and a false color image of one of the box lids, also showing lots of Egyptian blue:

Shabti box lid, normal light

Shabti box lid, normal light

Visible-induced IR luminescence photograph

Visible-induced IR luminescence photograph (areas in white = Egyptian blue)

False color image (areas in red = Egyptian blue)

False color image (areas in red = Egyptian blue)

You can use any regular/visible light source to produce the luminescence, but in this case, we used our fancy-schmancy new Mini Crimescope, which was developed for forensic work, but is useful to us because it allows us to examine objects under specific wavelengths of UV and visible light. We found that using a peak emission 600nm light source worked best for the excitation of the Egyptian blue.

In order to “see” the luminescence, we have to capture images using a modified digital camera, with an 87C IR filter.

In summary, we’re having lots of fun with our new equipment, and finding that these Egyptian objects are perfect subjects for learning how to use the Crimescope and the modified camera, because they produce such great, dramatic images.

 

Reimagining an ancient Egyptian material

Have you checked out our In the News section of this blog? Periodically, we try to update this page with some interesting articles related to our Egyptian collection, stories about projects and discoveries in Egypt, and even our own lab highlighted in the press.

One of the more recent stories that we’ve posted is about a new discovery related to Egyptian blue, one of the world’s first synthetic pigments. The ancient Egyptians made it by heating together copper, silica (sand), lime (calcium oxide) and an alkali such as natron (sodium sesquicarbonate) and it is found on objects from as early as the 4th Dynasty through to the Roman Period. We see this pigment on artifacts here in the lab, including Tawahibre’s coffin (and for more details read our blogpost on how we know this.)

A detail of Tawahibre's coffin. Based on analysis, this pigment has been determined to be Egyptian blue.

A detail of Tawahibre’s coffin. Based on analysis, this pigment has been determined to be Egyptian blue.

One thing that has been discovered about Egyptian blue is that it has luminescent properties-this luminescence cannot be seen in normal light conditions, but can be detected and recorded using a device that is sensitive to infrared light. This phenomenon is called visible-induced infrared luminescence. Using a regular (visible) light source and a modified digital camera, it is possible to not only positively identify Egyptian blue using a completely non-invasive technique, but it is also possible to discover very small traces of Egyptian blue pigment on surfaces of objects. The British Museum provides a great overview of this phenomenon on their website. It is our hope that we might be able to try this technique to examine some of the artifacts in our collection.

A painted wood uraeus on display in our Upper Egypt gallery. The paint has not been analyzed, but based on appearance the blue is most likely Egyptian blue. Examination with an IR source could confirm this.

A painted wood uraeus on display in our Upper Egypt gallery. The paint has not been analyzed, but based on appearance the blue is most likely Egyptian blue. Examination with an IR light source could confirm this.

Furthermore, it is now understood that this luminescence is produced by the nanostructure of the pigment – scientists have discovered that the calcium copper silicate in Egyptian blue can be broken into nanosheets, which produce infrared radiation similar to beams that communicate between TVs and remote controls and car door locks. It is now being envisioned that these nanosheets could be used for future near-infrared-based medical imaging techniques and security ink formulations!! Talk about a new life for such an ancient material.

You can read and hear more about this by following this link to our In the News section of this blog. Have you read or heard about something recently that you think we should share on our blog? Leave a comment here and we’ll try to incorporate these suggestions whenever possible.

 

pXRF follow-up

A couple weeks ago we brought out our portable X-ray fluorescence (pXRF) analyzer to aid in our study of some of the objects in the Artifact Lab. We provided an overview of this session earlier on our blog-read more by clicking here.

One of the objects that we looked at with the pXRF is this painted wooden coffin. I also wrote a blogpost about this artifact and you can read more about it there.

Painted wooden coffin of Tawahibre

A critical part of the conservation process is examining and documenting objects-their materials, technology, and condition-all of this information is recorded in condition/treatment reports. Beyond saying that this coffin was decorated with red, yellow, white, black, and blue paint, we would like to provide more information in our report about which pigments were used, if possible. Based on knowledge of the painting materials used in ancient Egypt, we had some ideas, and we were hoping that the pXRF could confirm that our ideas were on the right track.

One pigment we were interested in knowing about is the blue. Here is a detail of the blue paint in one area:

Detail of coffin, showing blue paint

Considering that Egyptian blue was the principal blue pigment used in ancient Egypt, this was our first guess. Egyptian blue is a synthetic pigment, one of the first synthetic pigments ever produced, made by heating together copper, silica (sand), lime (calcium oxide) and an alkali such as natron (sodium sesquicarbonate). This pigment is found on objects from as early as the 4th Dynasty through to the Roman Period. The hue varies from dark to light blue, depending on the components and the grinding process (or the final particle size). Dark blue colors tends to have a larger particle size and smaller particles produce a lighter blue.

So how can the pXRF help us understand what blue pigment was used on this coffin? Well, as previously described, XRF is useful for identifying elements present in a sample or targeted area of an object. We simply positioned the pXRF in contact with the area of interest, in this case, a stable area of the blue paint, and took a reading.

The pXRF analyzer positioned in contact with the target area of interest

The reading produced a spectrum with peaks representing the x-ray energies of the elements present.

pXRF spectrum of the blue paint from the coffin

Here we have labeled the peaks of each element detected-you can see that there are very high peaks for Calcium and Copper. This is what we would expect to see for an Egyptian blue pigment!

We’re looking forward to continuing to use this technique for examining other objects in the Artifact Lab-especially those artifacts with only traces of paint left or those objects with surfaces that have darkened and where the original colors are more difficult to interpret. We’ll continue to update the blog with this information as we find out more.