Wellman, H.: May 1996

Introduction

In March of 1995, archaeologists and conservators from the Universitat Autonoma de Barcelona, the Consel Insular de Menorca, the University of Reading, and the Institute of Archaeology, University College London (UCL), excavated a unique cache of organic artefacts from Cova d'es Carritx, Menorca, a sealed cave system that was used as a mortuary during the Late Bronze/Early Iron Age Talaiotic Period, in the late second, early first millennia BC (Figure 1) (Garcia 1972).

Map

Figure 1: Map of Menorca and Cova d'es Carritx

This deposit, found in a side cave separate from the larger mortuary site, consisted of a number of artefacts including ceramic and wooden vessels, wooden implements, copper alloy items, and a set of unique conical tubes made of wood and an unidentified material, stuffed with unidentified fibres, and capped with wooden or bone lids (Figure 2). The cache was dated to circa 900 BC by ceramic vessel typology, but radiocarbon dates on a wooden tube were 1016-852 cal. BC and dates on fibres from the tube were 760-406 cal. BC. This range of dates seems to suggest curation and reuse of 'ritual' objects before their final deposition about the time of the cave's sealing, circa 500 BC (Whitehouse 1996).

Site photo

Figure 2: The cache in situ

After limited in situ conservation (consisting of minimal cleaning and consolidation) and lifting by M. Mertzani, the four tubes of unknown material were deemed to be in particular need of examination and conservation. This was in part due to their delicate state, but mostly due to their absolute uniqueness in the archaeology of Menorca. Objects of this material, style, or appearance have never been excavated before. With the permission of the Consel Insular de Menorca they were brought to the Institute of Archaeology by R. Whitehouse, and delivered to the Department of Conservation and Museum Studies for investigation and treatment.

Investigation and Assessment

The four tubes were in different physical states: one was essentially complete (Figure 3), two were in two or more pieces, and one was completely crushed. The complete objects were about 4-6 cm long, and ranged from 3-6 cm in diameter. They were very light weight, friable, rigid, and brittle. Mertzani's report indicated that when excavated, they were soft and damp (but not water-logged). A summer's storage in a standard refrigerator, even in sealed polythene bags, had allowed them to dry out to ambient conditions to their present state. The fibres were also brittle, and the overall surface was covered in a fine cream-white coherent powder.

Artefact photo

Figure 3: Cova d'es Carritx object no. 15 after investigative cleaning

Initial examination and investigative cleaning suggested that these objects were tubes of horn sheath, stuffed with hair or other fibres and capped with wooden disks (which had remained in Menorca for separate treatment). This assessment was based on the laminate structure and striated surface visible with the naked eye (O'Connor 1987). Horn is a hard animal tissue related to hair, with a rigid structure due to the sulphur cross-linkages in the protein keratin (Mills & White 1994).

More intensive investigative techniques were needed to confirm this assessment. Comparison with other animal hard tissues under the scanning electron microscope (SEM) showed that bovine horn was the closest match (Figure 4). This was confirmed by Fourier transform infrared spectroscopy, performed by B. Pretzel and D. Ford at the Victoria and Albert Museum (Pretzel 1994) (Figure 5), which clearly distinguished between horn and bone/antler samples.

Scanning electron photomicrographs

Figure 4: Scanning electron photomicrographs of ancient cow horn, modern deer antler, modern goat horn, and a sample from CC-15

Fourier transform infrared spectrographs

Figure 5: Fourier transform infrared spectrographs of modern bone, modern deer antler, modern sheep horn, and a sample from CC-15

A chemical test that should have identified horn by its sulphur content (Feigel & Anger 1966; Daniels & Ward 1982) failed, probably since it was too sensitive for the rough-and-ready bench-top conditions in the laboratory. The fibres were identified as human hair, goat hair, and an as-yet unidentified plant fibre by K. Thomas of the Institute of Archaeology, UCL. Other materials found in association with the tubes included a copper-tin alloy half-round rod with incised lines (Figure 6) which was initially detected during x-radiography of the artefacts (Figure 7). The x-radiograph also revealed the presence of a wooden cap, with the mounting pegs still in situ (Figure 8).

Artefact photo

Figure 6: The copper alloy half-round rod from CC-10a

X-radiograph

Figure 7: X-radiograph of CC-10a, showing the rod and the wooden end cap

Artefact photo

Figure 8: Wooden cap from CC-10a, showing mounting pegs in situ

The adherent powder covering the surface was analysed by X-ray diffraction spectroscopy, courtesy of the Ancient Monuments Laboratory. The powder proved to be a mixture of gypsum (calcium sulphate) with some silica (Figure 9). The lack of calcium carbonate from the cave soil suggests that this powder was either an intentional covering applied to the objects, or it is a decay product of the objects. Since the powder was observed erupting through the surface, and was found inside the tubes, it was assumed that the gypsum was the result of a reaction between calcium in the soil, and sulphur released by the decaying horn. A similar reaction is observed on limestone buildings, where calcium from the limestone reacts with air- or water-borne sulphur compounds to form gypsum crusts.

X-ray diffraction spectrograph

Figure 9: X-ray diffraction spectrograph of the adherent white powder

Treatment Considerations

The Consel Insular de Menorca instructed that these objects be prepared for display in the Museum of Ciutadella in Menorca. This presupposed a degree of strengthening (to ensure safe handling and display) that would conflict with archaeological aims of preserving the artefacts essentially unchanged for future study. This conflict of owner versus conservator versus research interests is a common problem with archaeological materials.

One aspect of treatment would be the removal of obscuring materials. Because the adherent white powder was deemed to be a decay product, rather than foreign dirt, it was decided not to remove any more than had been cleaned off during lifting and investigation. Part of the underlying surface was already visible due to the in situ cleaning prior to lifting.

The other major concern was with the strength and durability of the tubes. They were not strong enough to withstand movements and shocks that might come of being displayed, handled, and studied. The condition of the four objects helped to solve this potential conflict, since one artefact was in good condition, two were in varying states of decay, and one was a pile of fragments. After testing various consolidants, it was decided to treat the two most complete artefacts by localised consolidation with 5% Paraloid B-72 (ethylmethacrylate methylmethacrylate copolymer) in toluene, applied to the laminated edges, and taken up by capillary action. The other two were left untreated (and uncontaminated).

Display-ready mounts of moulded Perspex (polymethyl methacrylate sheet) were made for the two consolidated tubes, which would help cushion them during transport, and allow them to be handled without direct contact (Figure 10). The other two were packaged in form-fitting Plastazote (polyethylene foam) foam and acid-free tissue to minimise the effects of handling and long-term storage. The artefacts were also documented thoroughly during the conservation process, and all materials removed or fallen loose were preserved. Thus, some of these extremely rare artefacts are available for display to the general public and handling by scholars, while others are prepared for safe long-term storage, and only occasional inspection by researchers.

Illustration of mount

Figure 10: Schematic of the moulded Perspex mount

Future Considerations

All these objects are still very sensitive to changes in environmental conditions. Decayed horn is very sensitive to changes in relative humidity (RH), and decays in moist environments. It was recommended that the objects be stored and displayed under strictly controlled conditions, with RH maintained at 55% (though it is more important to maintain a steady RH than that precise value) at ambient temperatures (Erhardt & Mecklenberg 1994). The objects must also be protected from excess light, which will cause further degradation due to photo-oxidation. Light levels should be kept below 50 lux, with ultraviolet levels below 75%.

Acknowledgements

I would like to thank the Consel Insular de Menorca and the Ciutadella Museum, Bob Chapman of Reading University, and Ruth Whitehouse of the Institute of Archaeology, University College London for their support and co-operation, and the opportunity to work on these objects.

I would like to thank Boris Pretzel and David Ford from the Victoria and Albert Museum for performing the Fourier transform infrared analysis. Malcolm Ward of English Heritage performed the X-ray diffraction analysis. Jacqui Watson of English Heritage and Ken Thomas and Louise Martin at the Institute of Archaeology shared their specialist knowledge. For their technical expertise and advice, I would like to thank Deb Glandfield, Sandra Bond, Kevin Reeves, Naomi Mott, and Stuart Laidlaw of the Institute of Archaeology.


References and Bibliography

Daniels, V. & Ward, S. 1982. "A rapid test for the detection of substances which will tarnish silver." Studies in Conservation. 27, 58-60.

Erhardt, D. & Mecklenberg, M. 1994. "Relative humidity re-examined." In: Roy, A. & Smith, P. (eds.) Preventive Conservation: Practice, Theory, and Research, Preprints of the Contributions to the Ottawa Congress, 12-16 September 1994. London: IIC, 32-38.

Feigel, F. & Anger, V. 1966. Organic Spot Tests. London: Elsevier.

Pericot Garcia, L. 1972. The Balearic Islands. (Trans. M. Brown). London: Thames and Hudson.

Mills, J. S. & White, R. 1994. The Organic Chemistry of Museum Objects. London: Butterworth-Heineman.

O'Connor, S. 1987. "The identification of osseous and keratinaceous materials at York." In: Starling, K. & Watkinson, D. Archaeological Bone, Antler, Horn; UKIC Occasional Papers, No. 5. London: UKIC, 9-21.

Pretzel, B. 1994. "The inaugural meeting of the Infrared Users Group." V&A Conservation Journal. 12/July 1994, 11.

Whitehouse, R. 1996. Personal communication.