ARCHAEOLOGICAL WOOD

Wooden Bowl Of Bronze Age
Image Credits: Wessex Archaeology, Flickr

Author: Anushka Singh

Abstract

Archaeology is not only the handmaid of history but also the conservator of art. Echoing maturities in other pertinent fields, archaeology’s focus has swung from hermeneutics, the decrypting of the gist and sense of things, to refocusing on the substantial things. Amidst this shift, dichotomies that motivate the structure of archaeology are being probed. Amid all the artefacts excavated, those composed from organic materials prove to be severely difficult within all aspects of archaeology, the chief of it being conservation. Of these organic relics, wood is perchance the most significant.

Theory

The chemical composition of wood is contributed by carbon (50%), oxygen (44%), hydrogen (6%); minor amounts of other chemical compounds are also present, particularly calcium, potassium, nitrogen, and magnesium compounds and traces of several pigments. These elements combine in different proportions together to give mainly cellulose, hemicellulose and lignin depending on the type of three. The species of the wood is important since it influences the nature and distribution of the chemical components, which consequently changes the rate and nature of the decomposition process at a microscopic level. Lignin in the cell wall ensures increased resistance to compression whilst cellulose has high tensile strength. The compactness of the wood is proportional to the speed of growth and deterioration of the cells. This lays the basis of the division of woods—hard and tender; poplar, pine, fir and linden are tender woods while walnut, oak, pear, boxwood, and cherry come under hardwoods. The types of wood for ornamental purposes lie in the midst strength-wise, their properties accordingly modified to provide resistance to animals and decrease sensitivity to temperature changes.

Archaeological wood may be charred, waterlogged, mineralized, or dried. The deterioration of inorganic materials in a given deposition environment can be understood with the assistance of simple chemical redox equilibria. The decay of organic materials is calculated by applying the same thermodynamic principles with the additional intervention of living organisms. As a result of this, the decay process takes into account the chemistry of the deposition environment. Before advancing any further, it is necessary to study archaeological wood to understand the conditions of its preservation. Sectioning (waterlogged, desiccated wood) and fracturing (charcoal, mineralized wood) are two commonly used procedures. Organic artefacts are only preserved in two explicit types of deposition environments: dry and within an anoxic condition. For most apparent reasons it is important that the environment surrounding organic material transits into an anoxic state to be preserved. With an elongated diffusion pathway for oxygen entering the deposition environment, most of the biological activity concludes. Nevertheless, the biological attack recommences whenever the environment is permissible. A lower amount of organic nitrogen degrades seasoned wood from a reliable food source. However, it can be eaten by bacteria and fungi. In the marine environment gribbles, and shipworms are the principal agents of attack. On land, various insects are their counterparts.

The artefacts also undergo chemical and physical decay. Chemical hydrolysis causes loss of polyose and cellulose, though the damage induced by this route does not do well to match that havocked due to biological processes. The fastest occurring chemical process is solvent swelling, wherein the cellulose molecules are encased in hydrogen-bonded water molecules. True to the propensity surrounding minerals, chemical changes are also brought by mineral inclusion. Sea salt goes hand in hand with marine environments, but active chelation of some ions by tannates and cellulose leads to the formation of high concentration corroded products. Abrasion and compression are other important physical processes that lead the analysis astray.

Past the preliminary obstacles, we come across chemical sources of conservation problems. Upright, the modification of artefact components owing to protracted exposure to the burial environment. This leads us to the next cause—the contamination of the artefact by minerals lingering in this environment.

Analytical Instrumental Techniques to Study Degradation

FTIR spectroscopy is a useful technique for analysing changes concerning biological and chemical decay in wood. The characteristic lignin components and the content of lignin can assist in identifying softwood and hardwood, whereas, from the relative amounts of cellulose, hemicellulose and lignin, whose characteristic infrared absorption is displayed in “fingerprint” wavenumber regions from 800-1700 cm-1, the degree of deterioration can be anticipated.

Nuclear magnetic resonance spectroscopy (NMR) is a fairly critical analytical technique for the characterization of organic materials. However, its impact in archaeology has been limited due to its sensitivity and resolution. With the recent dynamical approach, NMR has been promoted to decipher the interests of cultural heritage. Moreover, with unilateral NMR, which is a portable and non-invasive technique, it is possible to make in-situ measurements.

Wet chemical methods for wood analysis, established in the pulp and paper industry, is used to calculate the concentration of wood components.

Evolved gas analysis-mass spectrometry (EGA-MS) had been employed to study archaeological wood, to ascertain its chemical degradation. This technique singlehandedly also provides information on the thermochemistry of archaeological wood and its compositional data. Here, the wood degradation can be determined by translating the differences between lignin content and carbohydrates.

X-ray Analysis: Put simply, in X-ray analysis an object is placed before a high energy X-ray source and scanned, then the transmitted X-rays yield an image based on the relative density. This image is a direct projection of the internal structure, thereby sporadically laying out the topography of superficial density. Apposite calibration of the X-ray image gives a quantitative value of density merely on the basis of image colour, though factors such as the object thickness and moisture content are not to be neglected.

Ultrasonic testing: For a long, ultrasound has been used in the forestry industry to obtain a detailed profile of wood density. The base of the method lies within the fact that sound waves travel much faster in healthy wood vis-à-vis decayed timber. Perse, the signal from reflected waves varies congruously with the state of degradation of the object. Ultrasound accommodates the effect of water which makes it ideal for the study of waterlogged wood.

Microscopic analysis: Various realms marred on physical levels like the presence of inorganic salts, loss of wood substance, and collapse of the structural integrity of cell walls can readily be learnt using an array of microscopic methods. Microscopy provides constructive information such as the likes of the causes and mechanisms of deterioration, which is much harder to explicate with chemical and physical analysis. Every so often, the juxtaposition of microscopy techniques and other analytical methods allows making assumptions regarding the state of wood preservation. Spatial analysis is yet another advantage of microscopy over other techniques that provide only an average synopsis of the condition of the object.

Light Microscopy: Thin-section microscopy assists in identifying the species of the mounted archaeological sample. Microscopic techniques are often applied to quantify the nature as well as to weigh in the extent of decay—in thin-section, distinctive decay patterns, say, erosion or tunnelling, identify the origin of a biological attack. Additionally, biological stains can enunciate biological activity by underlining fungal hyphae and bacterial colonies.

Scanning Electron Microscope (SEM) gives a highly magnified image of a sample by superficial examination with an electron beam. SEM gives insight into many fields of interest owing to the high resolution, high magnification, and three-dimensional nature of the images. Furthermore, in cases where thin-sectioning is impossible (wood has dried out making it brittle), SEM is an exceptional method for studying the morphology of the exterior of a wood sample.

Conclusion

On the ebb of the postprocessual wave and experimental archaeology, archaeology finds itself amid the transformation. Archaeologically, the world is being reconstructed as a symbiotic collective, comprising of, and moulded by, several actors—attributed by organisms, with humans among them, impassive things, and processes of alteration, episodic and cyclical. Analytical techniques can clear the air surrounding the deterioration of archaeological wood. With the minuscule details still being lost to restrictions imposed due to biological factors and consequences of human activities, we are being robbed of a significant chunk of history.

REFERENCES

Fink, J. (2017). Chemicals and methods for conservation and restoration. Beverly, Mass: Scrivener Publishing.

Fabbri, B. (2012). Science and conservation for museum collections. Fireze: Nardini Editore.

Kaye, B. (1995). Conservation of waterlogged archaeological wood. Chemical Society Reviews, 24(1), 35. doi: 10.1039/cs9952400035.

High, Kirsty & Penkman, Kirsty. (2020). Analytical Methods for Assessing Preservation in Waterlogged Archaeological Wood: Their Importance for Site Management Decisions.

Fors, Yvonne & Jalilehvand, Farideh & Sandström, Magnus. (2011). Analytical Aspects of Waterlogged Wood in Historical Shipwrecks. Analytical sciences: the international journal of the Japan Society for Analytical Chemistry. 27. 785-92. 10.2116/analsci.27.785.

About the Author:

Anushka Singh, a graduate student of the University School of Chemical Technology, GGSIP University, Delhi. She is pursuing her graduation in chemical engineering.