Analysis of the shabti box varnish

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).

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:

L-55-23A_GCMSlabeled2Dr. 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!

 

Putting the finishing touches on the shabti box

I have put a lot of work into our troubled shabti box, including investigating and analyzing the varnish (more on the analysis in an upcoming post), doing some pretty cool imaging, and consolidating all flaking and unstable varnish and paint with methylcellulose. After consolidation of the surfaces, the box does not look much different than it did when I started the treatment (and this is a good thing). As a reminder, here is an image of the front of the box before treatment:

shabti box frontAt this point, I could call the treatment done, or take it a step further, by filling in some of the losses of the painted surface, which appear bright white since those losses expose the gesso below. After consulting with Dr. Jen Wegner in the Egyptian Section and with Lynn Grant, the head of our department, I decided to fill in some of the larger losses which really make it difficult to appreciate the object and “read” the designs. I have even heard some visitors refer to the box as “that badly damaged piece of wood”, and that is not what we want people to be thinking when they eventually see this on display. While I know I can never return the box to its original condition, I can reduce the appearance of some of the damage. But how to fill the losses on such a fragile surface, in a way that will be reversible/retreatable?

After some hemming and hawing and some failed tests, I ultimately decided to fill the losses by first placing a small piece of Japanese tissue paper into the loss, then applying a tinted fill mixture over the paper. I did this by doing the following:

1. I took a quick snapshot of the surface I was about to work on. I then downloaded the image and copied it into a Word document. Using the scale in Word, I was able to resize the photograph in order to print it approximately true to size, and then I printed the image in black and white. This took no more than 5 minutes.

2. I placed a piece of Mylar over the B&W print-out and traced the losses I wanted to fill with a black marker.

L-55-23A_template2

B&W image with Mylar template moved off to the right side

3. After trimming the Mylar around one of the tracings, I taped it to a piece of Japanese tissue paper with a small piece of blue tape.

L-55-23A_template34. I cut out the Japanese tissue paper and adhered it into the loss on the shabti box with a small amount of 5% methylcellulose.

5. I then applied a fill mixture over the Japanese tissue paper. The fill mixture is made of 5% methylcellulose, glass microballoons, and powdered pigment.

Fill mixture (in the jar and on the spatula)

Fill mixture (in the jar and on the spatula)

This may sound tedious, but the whole process works very smoothly and relatively quickly. It also minimizes the amount of time I need to spend touching the object and therefore minimizes damage that might be caused by touching the very fragile surface.

I’m not finished, but so far I’m pretty happy with how the front of the box is looking:

L-55-23A_dt01_compressed

Front view, during filling

It’s subtle, but to see the difference that filling makes, here are views before and after, side-by-side:

Picture1

Before treatment (left) and during treatment (right)

The only problem is, I feel like I’ve opened a can of worms. There are so many losses and I am not going to fill them all, but as soon as the larger losses are filled, I start seeing all of the small ones! I think it’s looking better though and I will get some feedback from my colleagues before proceeding further.