Mystery fiber update

Posted on February 23, 2013 by mollygleeson

A quick update on our mystery fiber (see my previous post for details):

Today I decided to do a chemical spot-test to see if I could determine if the fiber was cellulose or protein-based. Chemical spot tests are inexpensive, generally simple procedures that conservators may use to characterize materials on artifacts. These spot tests are often carried out on small samples removed from artifacts using chemical reagents. In the case of my mystery fiber, I cut a small piece off of one of the fiber samples I previously examined under the microscope-this small piece was enough for a spot test, and there was no need to remove more material from the coffin in order to do this.

The first test I chose to carry out was the Biuret test for protein (according to instructions in Material Characterization Tests for Objects of Art and Archaeology), using copper(II) sulfate. After placing a drop of copper (II) sulfate solution on the sample, I waited for a few minutes, then soaked up the excess solution and added a drop of sodium hydroxide solution to the sample. It immediately turned purple (see below), which indicates the presence of protein (and just in case it’s not clear on your screen, believe me, it is purple!).

Magnified image of the sample used for the protein spot test. The purple hue indicates a positive reaction for presence of protein. 50X magnification

What this means is still unclear, but it’s another clue. It is possible that my earlier comparison of this fiber to sinew was not a bad suggestion! But it’s also possible that this fiber was coated in a protein-based glue before it was incorporated into the gesso (or something like this).

This calls for further investigation!

Mystery fiber

Posted on February 23, 2013 by mollygleeson

In a recent blog post I mentioned that I am working on the painted coffin of Tawahibre, which has fibers mixed into the ground layer (gesso). In my examination prior to starting treatment, I had noted these fibers, and observed that they are present all over the coffin lid, mixed into the ground layer just below the painted surface. They are exposed in many places where there are losses-here is an image of one area where the surface of the coffin is badly damaged, revealing these fibers:

Fibers visible in the ground layer of the painted coffin lid

There are quite a lot of these fibers in some areas (as seen in the photo above), and then in others, there are very few. They are found in areas where the ground is thick and also where it is applied very thinly. They are not arranged in any particular way-they appear to have been mixed haphazardly into the ground. The fibers are light brown in color, and while most of them are very stiff, they react almost immediately to moisture, becoming very flexible when wet. I had initially assumed that these were plant fibers-possibly flax-but they always seemed a bit odd, and to be honest, these fibers remind me a little bit of sinew (animal tendon).

As I have been working on the coffin, several of these fibers “presented themselves to me” for sampling-meaning, as I’ve been working to stabilize some of the areas with these fibers, a couple became detached, allowing me to investigate them further using PLM. So far I have looked at 2 samples, and both look the same. I prepared the fibers by mounting each one on a glass slide with water. When looking at them in plane-polarized light, they look like this:

Two different fibers from the coffin ground layer viewed at 50X magnification

I didn’t really know what I was seeing-it was difficult to pick out any really distinguishable features, so I then viewed both fibers under crossed polars. This is what I saw:

Same two fibers viewed under crossed polars at 50X magnification

What the heck is that? I’ve never seen anything like this before. When I showed this to a few other people, the first reaction has been-it looks like a worm! And it totally does. This very regular banding pattern has got to be characteristic of something-I just don’t know what.

I thought I had a lead last week-I found this image in a book, showing a bundle of sisal fibers with a commonly-seen spiral element:

Sisal sample showing a characteristic spiral element. Image from “Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators” by Nicholas Petraco and Thomas Kubic, p. 94.

However, just last week I obtained a sisal sample from one of the Winterthur art conservation graduate students and I’m pretty sure that’s not what I have. The sisal sample looks distinctly different to me-here it is in both plane polarized light and under crossed polars:

Sisal reference viewed at 400X magnification in plane polarized light (left) and crossed polars (right). Also, note the difference in magnification between these fibers, viewed at 400X, as opposed to the coffin mystery fibers above, viewed at 50X.

For the moment, I’m stumped. But I’m continuing to investigate this and to get input from colleagues, and I’m open to suggestions/ideas! I’ll also certainly provide more information when I know more. To be continued…

Polarized Light Microscopy

Posted on December 12, 2012 by mollygleeson

Our Conservation Department recently purchased a Zeiss polarized light microscope-”the best microscope on campus” according to the specialist who set it up for us, and who is knowledgeable about the other scopes in use at Penn. Having the nicest equipment around isn’t familiar territory for conservation labs, so we’re enjoying having this status, but more importantly, having such a nice piece of equipment to use.

Our new microscope installed in the Artifact Lab

Polarized light microscopy (PLM) is used for examination of specimens in many types of laboratories, including biology and geology labs. In conservation, we use PLM for identification of minute fragments from objects-anything from pigment particles to wood fragments to textile fibers. We also use this technique to examine corrosion products, salts, and other materials found on artifacts-all of this work helps us better understand what the objects are made of, their condition, and ultimately provides important information for making conservation treatment decisions.

For example, our Conservation Fellow Tessa de Alarcon, who is conducting a year-long condition survey of Penn Museum artifacts from Kourion, Cyprus, has been using PLM to examine salts present in ceramic vessels from this collection. Tessa is desalinating the ceramics to remove the salts, which likely accumulated in the ceramics in the burial environment and will cause damage if not removed. To confirm which salts are present, she removed samples of the salts and examined them under the microscope. Here is an image of one of the salt samples, which shows that there are 2 different types of salts present-nitrates and sulfates.

Magnified image of 2 types of salts present on a ceramic vessel from Kourion (400X magnification).

You can read more about Tessa’s work with the Kourion collection (and view a cool video clip!) here on the Penn Museum blog.

In the Artifact Lab, one of the first ways that I’ve used our new microscope is to examine fibers from a thread that detached from the fabric wrappings of the falcon mummy I described in a previous blogpost. Fortunately for me (but unfortunately for the poor falcon mummy!) there are lots of detached threads that were available to sample for examination under the microscope. Here is a magnified image of one of these threads:

A small detached thread from the falcon mummy’s wrappings (40X magnification). I noted that the thread has an “S” twist and the fibers are shiny.

Using our binocular microscope, I put a drop of water on the thread and teased out several individual fibers from the thread on a glass slide, and then covered the fibers with a cover slip.

This image shows all of the tiny fibers from the larger thread-it is important to examine these fibers individually in order to identify what type of textile the falcon mummy is wrapped in (40X magnification).

Once the slide was prepared, I mounted it on the polarized light microscope and examined it at 50, 100 and 200X magnification.

Fiber from falcon mummy textile wrappings (200X magnification)

Under such powerful magnification, it is possible to see features such as a very small lumen (central cavity) and nodes along the length of the fiber. These features are characteristic of flax fibers, and comparing my sample with known references (including in this great Fiber Reference Image Library), it was immediately clear that this is what it is. Flax is used to make linen, and since the majority of ancient Egyptian textiles are linen, I already had a good idea that this is what was used to make the falcon mummy-but this proves it!

You can see from this work that PLM is a very useful technique, but it also is important to have an idea about what the possibilities are for what your sample-background research and close examination before microscopy is essential.

pXRF follow-up

Posted on November 9, 2012 by mollygleeson

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.

The Outer Coffin of Ahanakht – part 2

Posted on October 28, 2012 by artifactlab

One of the boards from the inner coffin of Ahanakht, before treatment.

Previously I began to tell you about this multi-part artifact. Then, I was just starting to get acquainted with it. When conservators first look at any artifact, the first thing we think about is not where it’s from, not how old it is, not even what culture made it. The first and most important fact for conservators is what it’s made of. The material tells us what kind of problems it might have and what kind of treatments we can use or not use – it’s the starting point of everything we do.

The coffin boards are wood, with some paint applied. Four thousand year-old wood. Right away, that tells us something about what kind of wood it must be, since wood generally doesn’t survive so long in the archaeological record. Because there’s been a lot of research done on Egyptian materials, we can say with some confidence that the wood is cedar of Lebanon (Cedrus libani). Cedar is a prized wood because the trees produce chemicals that make them resistant to insect damage and various forms of rot.

A detail of the board showing construction details.

The first step of any treatment is careful examination. The coffin boards, despite being entombed thousands of years ago in the desert environment of Egypt and then brought to Philadelphia with its humid summers and desiccating winter heating seasons, appear to be in excellent condition for the most part; their most obvious problem being a thick coat of dust from uncovered storage for many decades. I documented the appearance of the board, noting its construction details, such as four wooden pegs and mitered edges. One curious feature was thin metal ribbons running in channels along the long axis of the board. I was unsure whether these were an original feature or something done in modern times to put the coffin back together. It seemed an unusually elaborate repair but the metal was in such good condition that I didn’t think it was 4000 years old. Even under a microscope, I couldn’t tell exactly what the metal was. There were slight traces of green corrosion, which usually means copper or copper alloy, but the metal was mostly dark grey and quite flexible, so it could be lead. I made a note to analyze it using our new portable X-ray fluorescence analyzer which has since happened and to do some research on Egyptian coffin technology. Dr. Joe Wegner, also an Associate Curator in the Egyptian Section, recommended a book about a similar coffin at the Boston Museum of Fine Arts (The Secrets of Tomb 10b) and there I found this information: “the sides have beveled edges fastened together by dowels and copper ribbons“. So it looks like those metal ribbons are original. Perhaps their unusually excellent condition has something to do with the cedar around them.

During treatment, showing dirt partially removed and tools used for cleaning.

Treatment was relatively straightforward. I used a HEPA-filtered vacuum with variable speed control to remove the loose dust from the surface of the board. Conservators choose their cleaning methods based on the type of dirt to be removed and the substrate from which it is to be removed. ‘Dry’ cleaning methods (those not using liquids such as water or other solvents) are generally less likely to damage the artifact and are preferred wherever possible, although care must still be taken to ensure that only the dirt is removed and not any of the original surface. By using a very small vacuum attachment at low speed and monitoring the process closely using a magnifying visor, I was able to clean the surface safely. Not very glamorous but I’ve discovered that this artifact has a pretty important role in the history of archaeological science – see my post on the Museum’s blog for information on that!

pXRF In the Artifact Lab

Posted on October 26, 2012 by mollygleeson

Our Conservation Department is fortunate to have a portable x-ray fluorescence analyzer (pXRF), and today we started putting it to use in the Artifact Lab!

Conservator Nina Owczarek uses the pXRF to analyze pigments on a wall painting fragment

What can we do with a pXRF, you might ask? Well, Nina Owczarek provides a good overview about the use of this instrument in a previous post and also in a presentation which you can watch by following this link.

I’ve used a pXRF before, but it’s been awhile, so today Nina came up and gave us all a refresher. Essentially, x-ray fluorescence is a non-destructive analytical method that uses x-rays to identify elements present in objects or samples. This technique is particularly useful for characterizing pigments and metal alloy components, and that is what we’re using it for in the Artifact Lab.

A view of Nina and I discussing the pXRF from outside the lab

After examining a few artifacts visually, we had some questions about materials and wanted to do some further investigation with the pXRF. For instance, we are interested in these metal ribbons on the Ahanakht coffin boards (see Lynn Grant’s previous post about the boards).

The pXRF positioned in contact with the metal ribbons on one of the smaller coffin boards

After examination of these ribbons under the microscope, it was still difficult to determine what type of metal they are made of. With the pXRF, after a 180-second scan and using special software, a spectrum was produced that showed a large peak for copper and very small peaks for tin, iron, arsenic and lead. We haven’t been able to analyze the data much yet, but this does tell us that these are indeed made of copper.

We will follow this post soon with more information and interpretation of our results.