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7 March 2012

Conservation of K1536: gold boss with black residue inside

One of the hoard objects that has recently been conserved is K1536, a gold boss with a domed head, beaded filigree decoration, and a blunt-ended gold pin (Figure 1).

A gold boss with beaded filigree decoration.
Figure 1. K1536, a gold boss with beaded filigree decoration. The diameter of the boss is 13mm.

This boss could have been used to secure sword hilt plates together as seen in a different hoard object, K563 (Figure 2).

 

 A pair of hilt plates riveted together.

Figure 2. K563, a pair of hilt plates riveted together. See circled area on right side for similar boss with pin (though this boss is has sustained greater damage than K1536).

When performing a condition assessment of the boss it was noted that there was unidentified black material inside the boss around the base of the pin (Figure 3).

Black material inside K1536.

Figure 3. Black material inside K1536. Magnification 50x.

Because this object is going out on loan, which will involve additional handling for packing, transportation, and condition checking on installation and de-installation, there is a potential risk that this delicate residue may become detached. As such, the decision was taken to remove the larger, more vulnerable portion of the residue to ensure its safety and to allow scientists to analyse it. Some of the residue remains inside the boss, but now we can move forward with analysis while the object is away on loan.

The standard procedure for dealing with unidentified residues is to photograph and document them and to set aside the object for analysis, which could take it out of circulation temporarily while the work is carried out. However, this is not always practical or possible, in which case an alternative approach is taken: sampling the residue.

In conjunction with the British Museum, the hoard conservation team has created a set of sampling procedures that allow the team to gather samples for analysis while allowing the object to remain available for display or research. The areas with residue are considered carefully, then, working under magnification, a sample is removed. The sample is stored in a small glass tube marked with the accession number (Figure 4) and packed securely into a storage box.

Sample tube containing the residue from K1536.

Figure 4. Sample tube containing the residue from K1536.

Information about the sample is then entered into a spreadsheet that lists all samples taken to date. The sampling procedure and location are noted on the object’s condition report, and photographs of the object before and after sampling are taken and annotated to point out the location from which the sample was taken (Figures 5 and 6).

 K1536 before and after removal of the sample.

Figures 5 and 6. K1536 before (left) and after (right) removal of the sample. The sampled area is marked with an arrow.

To get a better look at the residue after removal, I examined it using our 3D digital microscope (Figure 7).

Examining the residue using a 3D digital microscope.

Figure 7. Examining the residue using a Keyence VHX-1000 3D digital microscope. Magnifying range: 20-200x.

The material has a somewhat rough, glossy, resinous appearance with an apparently light-coloured core (see Figures 8 and 9).

K1536 residue.

Figures 8 and 9. K1536 residue. Magnification 50-100x.

At this point we do not know what the residue is made of or why it is there. It might be some kind of filler or adhesive to help support the gold pin, which is soldered to the inside surface of the boss. Determining the composition of the residue might help us understand its purpose.

We have a partnership with the British Museum for support with conservation and scientific research, allowing us to draw on the BM scientists’ and conservators’ valuable expertise and experience in analysing and treating similar archaeological material. The BM also has analytical equipment not available at Birmingham Museum such as x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and gas chromatography/mass spectroscopy (GC/MS). It is hoped that analysis of residues such as the one found in K1536 may further our understanding of the Anglo-Saxon craftsman’s materials and techniques.

In March the British Museum’s metals conservation department will be hosting the hoard conservation team. The hoard team will get an insight into the different types of analytical processes that are applicable to different objects and work on a one-to-one basis with the metals conservators who have worked on such projects as Sutton Hoo. News about this visit will be posted, so check back again soon.

Cymbeline Storey

Staffordshire Hoard Conservator

Birmingham Museum & Art Gallery

I’m willing to bet that the residue is what modern craftsmen refer to as “cutler’s resin”, typically a mixture of boiled pine resin and binders such as brick dust or charcoal powder, which saturates the otherwise soft resin. Used as a primitive “hot glue”, it is very useful in securing small items, filling voids to prevent unwanted flexing, and anchoring complex constructions where one might use epoxy or some other glue today.

A very small sample placed on a hot plate would release the characteristic pine resin smell, so the human nose may be all that’s needed to come up with an initial assessment… though it appears you’d need to melt nearly all of the sample you have to do that. I look forward to seeing the results of the laboratory analysis.

The black residue may be the remains of carbonized organic gum used in the possible eutectic soldering of the pin to the inside of the boss, it would be interesting to see if there are any traces of copper compounds present in the residue

Either as above suggestion or a resin flux used for a quick repair job, similar to that used on modern solder.
Type of flux would depend upon the quality of the “gold”
and the matals melting temperature.
These range from lead solder resin flux to silver solder borax.