Analysis of the shabti box varnish

Posted on December 20, 2014 by mollygleeson

This is a long overdue post about the varnish on our beloved shabti box (in my last post I referred to the box as troubled…I’ve developed a tiny bit of a love-hate relationship with it, which I’m only now admitting).

$A detail of the shabti box before treatment, showing the actively flaking and fractured orange-yellow varnish$

A detail of the shabti box before treatment, showing the actively flaking and fractured orange-yellow varnish

Anyway, I’ve briefly mentioned that we believe that the varnish on our shabti box is a pistacia resin, but how did we come to this conclusion? I started out by doing some research into similar objects, and into painted wood from the New Kingdom in general. As I mentioned in a previous post, we know that some painted wooden objects were varnished with pistacia resin during this time period, and these varnishes often look like the coating we see on our shabti box. But there were some things about the coating, including the fact that it was actively flaking, and the fact that there are areas on the box where the paint is lost and where the coating extends over the loss onto the gesso below, which is strange.

In order to start characterizing the coating, I looked at the box under different light sources and did a microchemical spot test, all described here. All roads were leading toward the conclusion that the coating is pistacia resin, but since we had so many available samples (i.e. detached pieces of the varnish) I wanted to investigate further.

First, we turned to a resource that we have in-house: Fourier transform infrared spectroscopy, or FT-IR. FT-IR is a method of infrared spectroscopy, where IR radiation is passed through a sample, and some of the radiation is absorbed and some of it is passed through or transmitted. A spectrum is produced that represents the molecular absorption and transmission, which is unique to that material. I collected samples of detached varnish from the shabti box and from one of the shabti figures, and passed them along to Tessa de Alarcon, a conservator in our department, and consulting scholar Dr. Gretchen Hall. Here is what the spectra look like for each:

FT-IR spectra for samples of varnish from the shabti (top) and the shabti box (bottom). The characteristic peaks are labeled on the top spectrum.

They look virtually identical, which confirms that the varnish on the box is the same as the varnish on the shabtis.

Dr. Hall then compared the spectrum for the shabti box sample to spectra for mastic (Pistacia lenticus) and terebinth (Pistacia terebinthus), both pistacia resins.

Spectra for (from top to bottom): the shabti box sample, a sample of terebinth, a sample of mastic from Chios purchased in Athens, and a sample of mastic from Kremer Pigments (the Kremer Pigment mastic sample spectrum was found in the IRUG database). IRUG = Infrared and Raman Users Group

Spectra for (from top to bottom): the shabti box sample, a sample of terebinth collected from the Uluburun shipwreck, a sample of mastic from Chios purchased in Athens, and a sample of mastic from Kremer Pigments Inc. (the comparative spectra were found in the IRUG database, IRUG = Infrared and Raman Users Group)

They all look very similar, with characteristic resinous acid peaks that occur between 1700 & 1720 cm-1 (carbonyl stretching) & the acid OH stretching that occurs ~1460 cm-1.

In order to see if we could classify the shabti box resin even further, Dr. Hall took a sample to Dr. Chris Petersen, Affiliated Associate Professor in the Winterthur/University of Delaware Program in Art Conservation (WUDPAC), where they analyzed it using Gas Chromatography-Mass Spectrometry (GC-MS). GC-MS is a technique that combines 2 methods of analysis, and in conservation we use it to analyze organic compounds.

Dr. Hall and Dr. Petersen ran the sample and here is what the GC-MS chromatogram looks like:

Dr. Petersen labeled the peaks and included their structures. The structures are consistent with pistacia resin, either mastic or terebinth. They did identify a peak for 28-norolean-17-en-3-one (#3 above), characteristic of heated pistacia resin, which could indicate that the resin was heated before application (which would have turned it from clear to a yellowed varnish). We cannot be certain what color the varnish was when it was first applied, but the analysis does confirm the fact that the shabti box and the shabtis all have aged pistacia resin coatings.

We are grateful to both Dr. Hall and Dr. Petersen for their work on this analysis!

Investigation of a mummy bead “coating”

Posted on July 3, 2013 by mollygleeson

While we continue to work on the conservation of PUM I‘s remains, we also have been taking this opportunity to carry out some analysis on the residues and substances preserved on his wrappings and on the beads that once made up his beaded burial shroud.

Since the last time we wrote about these beads, we have recovered even more in the conservation process; we now have a total of 35 beads – all either tubular or circular in shape. As we wrote about in a previous post, all of the beads are covered with concretions, mostly a brown, waxy material. Here is an image of one of the beads before cleaning, and after partial exploratory cleaning, revealing the beautiful blue color of the bead:

A tubular bead before (left) and after (right) exploratory cleaning to remove the residue on the surface ( 10X magnification)

This material does not appear to be dirt or accumulated debris from the mummy. But, it can be removed rather easily from the beads, especially with the help of some mineral spirits, which suggested to me that it is some sort of wax.

Based on this information, I was suspecting that either this material was related to a substance applied to the beads to help the beaded shroud stay in place at the time of burial (but we have yet to locating any research supporting this theory – it was more common to sew or tie these beaded shrouds in place) or that it is related to a substance applied to the shroud at the time of discovery, to assist with the removal of the shroud.

In conservation, when it comes to investigating unknown, likely organic substances, there are several analytical techniques that can be helpful. One of these techniques is Fourier-transform infrared (FTIR) spectroscopy. FTIR works by exposing a sample to infrared radiation, which causes the sample to selectively absorb radiation, depending on the molecules present. The individual peaks in the resulting absorption spectrum can be analyzed or the spectrum can be compared to reference spectra to help characterize or identify a material.

We provided a small sample of our “bead coating” to Gretchen Hall, a consulting scholar in the Biomolecular Archaeology lab here at the museum. She ran the sample for us and provided the resulting spectrum and interpretation. Here is the spectrum produced by our sample:

This spectrum shows that the sample is mostly organic as evidenced by the dominant peaks in the 2900 cm-1 region which are characteristic of C-H bond stretches. In addition, there were many peaks in the “fingerprint” 1800-1000 cm-1 region where various organic molecules absorb. The absorption around 1730 cm-1 (due to C-O double bond stretches) suggests organic acids are present, possibly from resins or beeswax. Both of these families of compounds would also have bands around 1470 (a O-H bending absorption) which are seen in our sample. Importantly, the sample also shows a strong band around 720-730 cm-1 (due to the C-H in long hydrocarbon chains) which is only present in beeswax.

For comparison, here is our bead coating sample spectrum displayed just below the spectrum for a standard beeswax:

Based on this analysis, our “bead coating” sample likely contains some beeswax, which is consistent with our observations of the solubility and consistency of the material as well. It is known that beeswax was used in ancient Egypt – as an adhesive, a sealant, a binding medium, and in the mummification process. Bees were considered by the Egyptians to be precious insects with magical and economic prestige, and these values would have extended to their wax (Ikram and Dodson 1998).

For a more definite identification of our sample, the next step would be to analyze the material using gas chromatography–mass spectrometry (GC-MS).

Special thanks to the Biomolecular Archaeology Lab and Dr. Gretchen Hall for running this sample and providing the analysis.

A technical study of a child sarcophagus

Posted on March 6, 2013 by mollygleeson

Casey analyzing the surface of the child sarcophagus using a portable XRF analyzer. Photo by Vanessa Muros

I have a special treat for our readers today – I recently had an interesting discussion with Casey Mallinckrodt, a current graduate student in the UCLA/Getty Program in the Conservation of Archaeological and Ethnographic Materials (which is also my alma mater!).

As part of her graduate studies, Casey has been working on a rare example of an Egyptian child sarcophagus, and instead of recounting the conversation for everyone, I asked her if she’d be willing to answer a few questions for our blog. Read on to hear more about Casey and this fascinating project.

1. First, tell our readers a little more about yourself.

I am a second year MA student in the UCLA/Getty Program in the Conservation of Archaeological and Ethnographic Materials. This is a career shift for me having received an MFA in sculpture in 1988 and having had a career in fine arts and education. I did pre-program internships in conservation at the Museum of Modern Art and the American Museum of Natural History in New York before applying to this program.

I am working on this project with Dr. David A. Scott and Prof. Ellen Pearlstein, both of whom are faculty in our program. Marie Svoboda, a conservator of antiquities at the Getty Villa, and Dr. Kara Cooney, an Egyptologist at UCLA are advisors.

A view of the upper half of the sarcophagus

2. Tell us more about this child sarcophagus that you’re working on and why it is so unusual (that’s assuming that it is!!)

The sarcophagus (or coffin) lid is on loan for this project from the San Diego Museum of Man. It was given to the museum in 2001 and it dates to the Ptolemaic period, 305 – 30 BCE although the exact date of this object is not known. It is unusual because sarcophagi were costly and rarely made for children. This is supposed to be one of only seven from this period known to exist. It seems to have been for a girl, and scholars I consulted suggest she may have been a child wife, which would justify the expense. Many people have asked about the mummy and the base of the coffin but these are unknowns.

It is made of a carved out section of a tree trunk with pieces added to create the chest, face and foot. Wood dowels were used to secure the pieces in place. The exterior is completely covered with a polychrome layer made up of a base of a brown granular paste, then a white “gesso” layer and the paint applied onto that. The interior is unpainted.

A side view of the sarcophagus in the UCLA/Getty labs

3. What is your ultimate goal for this project?

The project is a technical analysis of the structure and materials, and development of a treatment plan. The treatment goal is to stabilize the structure and fragile paint layer, and reduce or removal modern fills that may be damaging the original materials. If the museum approves I will carry out treatment in selective areas to establish a protocol so the museum can continue the work after the coffin returns to San Diego in June.

4. Since I get asked this question almost every day, I’m going to turn around and ask you – what is the most interesting and/or unexpected thing that you’ve found so far in your work on the sarcophagus?

It is a fascinating project in so many ways, but two things come to mind. One involves the manufacture, and the other a mysterious surface condition.

The ancient Egyptians frequently reused parts of coffins and I have found evidence of reuse here. X-rays reveal empty dowel holes that indicate changes in the structure, and there are modern metal screws holding the foot block in place.

X-radiographs of the head (left) and foot (right) of the sarcophagus. Note the modern screws holding the foot together.

Scholars I have consulted find the carving of the face more consistent with earlier periods. I am examining the types of wood and indications of changes in the construction, and analyzing the pigments, gesso, fill materials, and coatings to identify differences that might point to different sources for the different parts.

The most unexpected development was the emergence of small waxy exudates on the lower front of the “torso”. I am doing chemical tests on samples, and FTIR (Fourier transform infrared spectroscopy) is being carried out by Getty Conservation Institute Scientist Herant Khanjian to identify the substance and determine whether it poses a threat to the original materials. This is probably the result of a coating or consolidant that was put on the object, but there is no record of its treatment before SDMM acquired it.

Thanks Casey! This is a terrific project, and a great example of how conservation is often a very collaborative process, involving conservators, scientists, archaeologists, and other specialists.

I will be keeping up with Casey as her work progresses. For more information on the UCLA/Getty Conservation Program and other student projects, follow the links I’ve included in the text above.

In the Artifact Lab

Tag Archives: FTIR

Analysis of the shabti box varnish

Investigation of a mummy bead “coating”

A technical study of a child sarcophagus