Investigating the processes necessary for satisfactory freeze-drying of waterlogged archaeological wood

Preliminary studies into methods for improving and safeguarding the preservation of waterlogged archaeological wooden artefacts are presented. Mannitol, sorbitol and trehalose have been tested as replacements for polyethylene glycol (PEG) as a treatment prior to freeze-drying of wooden archaeological objects. The highest anti-shrink efficiency achieved in this short experiment was (70 ± 15) % using a 20% weight per volume solution of sorbitol after soaking for one week. The collapse or eutectic temperature of each solution used was determined using a freeze-drying microscope. Importantly, it was noted that the collapse temperature of the solutions was reduced by several degrees after use for treatment of the wood samples for one month. A freeze-drying microscope with a cold light source has been used to observe the real-time freeze-drying behaviour of various treatments inside a thin section of wood, confirming that this novel technique is indeed possible and useful. Lastly, the effects of magnesium phytate as an iron chelator have been studied. It was found that the treatment lowers the collapse temperature of a PEG 600/2000 solution by roughly 3 °C meaning that objects must be freeze-dried at a correspondingly lower temperature. Iron K-edge X-ray absorption near-edge spectroscopy analysis showed that the treatment extracted approximately half of the iron(III) ions present in the timbers.     

Material evidence for the use of Attic white-ground lekythoi ceramics in cremation burials

White-ground lekythoi ceramics offer important evidence for funeral practices in Ancient Greece (5th century BC). The images painted-on these oil containers provide the best visual narrative for the events surrounding death, including indications that the vessels themselves were used as part of the funerary rites. However, until now, their specific function and treatment within the funeral ceremony was not well understood. We present here material evidence that the vessels were ritually burned, together with the body of the deceased, during cremation, as evidenced by a diffuse purplish-red discolouration found on many white-ground lekythoi. Through EPMA and μXAS studies, we show that: (1) this characteristic purplish-red discolouration is due to the presence of metallic copper nano-particles embedded in a glassy layer; (2) this metal-glass matrix formed as the result of a high temperature reaction between painted-on Cu-based pigments (e.g., Egyptian blue) and the white-ground ceramic slip; and (3) the reaction occurred under a reducing environment. Given the mortuary context for these vessels, we propose the reduction firing to which the vessels were exposed was that associated with the cremation of the body. The observation of discontinuous formation of the purplish-red discolouration along adjoining fragments supports the hypothesis that the vessels were broken prior to being burned. The majority of lekythoi in museum collections lack information on their original archaeological context, and our data suggest the presence of this purplish-red discolouration may serve as a visual marker for cremation. As such, it is expected our findings will provide a new basis for interpreting how this important class of ceramic, and associated iconographic imagery, relates to Athenian funerary practices and the ancient Greek notion of death.     

Sulfur and iron analyses of marine archaeological wood in shipwrecks from the Baltic Sea and Scandinavian waters

Analyses of marine archaeological wood from shipwrecks in the Baltic Sea area, Kronan, Riksnyckeln, Tattran, the Puck Bay Boat and the Ghost wreck, and at the Scandinavian West coast, the Göta wreck, Stora Sofia and the Viking shipwrecks of Skuldelev, show accumulation of sulfur compounds. The penetration profiles of sulfur and iron into the wood and the speciation of characteristic sulfur groups were evaluated by combining X-ray spectroscopic analyses, in particular S K-edge XANES (X-ray absorption near edge structure) and X-ray fluorescence, with ESCA and elemental analyses. The combined analyses support the hypothesis that hydrogen sulfide produced by sulfate-reducing bacteria reacts and accumulates at low iron concentration mainly as organically bound sulfur, which as in previous studies was found by X-ray spectro-microscopy to accumulate in lignin-rich parts of the wood cell walls. The presence of iron(II) ions from corroding iron promotes formation of pyrite and other iron(II) sulfides, which easily oxidise in aerobic conditions with high humidity. No significant differences in sulfur and iron accumulation were found in wood from shipwrecks in the east coast brackish water and the west coast seawater. Sediments from three wreck sites, the Göta wreck, Stora Sofia and Kronan, were analyzed to a depth of a few decimeters and showed especially at the Stora Sofia high sulfur concentrations, exceeding 3 mass%. S K-edge XANES analyses of the sediments showed mainly reduced forms of sulfur, in particular pyrite and iron(II) sulfides together with elemental sulfur.     

A Study Of The Firing Technique Of Pottery From Kaman‐Kalehöyük,Turkey, by Synchrotron Radiation‐Induced Fluorescence X‐Ray Absorption Near‐Edge Structure (Xanes) Analysis*

Fe K‐XANES spectra of a series of pottery sherds excavated from the archaeological site of Kaman‐Kalehöyük, Turkey, were measured to reveal the firing technique of the pottery. The analysis disclosed relationships among the chemical form of the iron, the firing conditions and the colour of the pottery. The pottery referred to as ‘Grey Ware’ found at this site was classified into two groups according to the chemical form of the iron. Also, it was revealed that the Iron Age pottery referred to as ‘Cream Ware’ was produced first under reducing conditions, and then fired under oxidizing conditions.