Examination of interactions between ferrous metals and the archaeological burial environment at a seventeenth-century plantation site

Mathias, Cathy (1998) Examination of interactions between ferrous metals and the archaeological burial environment at a seventeenth-century plantation site. Masters thesis, Memorial University of Newfoundland.

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Abstract

Few data exist on the burial environment condition and its affect on archaeological artifacts. Dowman (1970) introduced the need for archaeologists to incorporate the conservation of artifacts and analysis of soils into their field work. Fortunately the need for field conservation has been accepted by most institutes supporting archaeological research. Unfortunately extensive soil analysis has not been generally accepted as a means to understand better what has occurred to the buried artifact over time. The past few decades have seen Canadian and international archaeological sites excavated without soil data to aid the conservator in the stabilization of artifacts. During the years since Dowman's publication both archaeology and conservation methods have changed as technology advances and our understanding of past material culture grows. However our understanding of the burial environment has not grown at the same pace. Because of this void in data pertaining to the burial environment it is important that a concise survey of the techniques used for soil analyses be assessed. This thesis, in part, provides a guide to methods and techniques which can be used for assessment of burial conditions. -- This thesis focuses on a seventeenth-century plantation site located at Ferryland, Newfoundland. This investigation centres on gaining a better understanding of the interaction between ferrous metals and the archaeological burial environment, whether or not predictions of iron condition can be made based on soil analyses alone and evaluating the methods and techniques used to characterize the soils and iron. -- Analyses of soil samples involved chemical analysis by XRF, ICP-MS, soil solution ion activity by pH and conductivity meters. Corrosion rates were measured using a potentiostat, identification of mineralogy was performed using XRD, particle sizes were estimated by sieving, organic content was measured by weight loss after digestion and soil colour was evaluated using a Munsell colour chart. Iron preservation was determined qualitatively based on magnetic attraction and physical appearance. Analyses of individual iron nail samples, representing strata of varying depth and horizontal distribution, were performed using reflected light, transmitted light and electron microscopy to describe the metal, radiographic techniques to measure the metal loss and XRD for mineral identification of the exterior corrosion layer (corrosion halo). Colour of corrosion halos was measured using a Munsell colour chart. Chemical analysis of slag samples was performed using XRF. -- The results show that variations in iron preservation is linked to soil porosity, pH, conductivity, corrosion rate, Cl, SiO₂, and P₂O₅, concentrations. Variation in soil colour, particle size distribution and element composition are linked to both the natural environment and the presence of the seventeenth-century colonists. The best preserved iron was excavated from soil with the following conditions; 43% gravel, 53% sand, 3% clay, pH of 4.9, conductivity of 24.9 micromhos, corrosion rate of 0.09 mmpy, organic wt% of 12.7%, 54.8 wt % of SiO₂, 2.5 wt% of P₂O₅, Cl concentration of 3,858 ppb for soil solutions and a Cl concentration of 417 ppm for soil samples. This soil was removed from events representing seventeenth-century occupation and subsequent destruction. Thus the bulk of the iron from the Ferryland site is found in soil which offers good conditions for preservation. -- An extensive survey of iron and soil can therefore provide general information about the corrosive nature of the soil and identify areas of occupation and industry. Examining the soil environment, however, involves lengthy chemical processing and instrumental analysis. It was hoped that one analytical technique, in this case the corrosion rate analysis, might provide the basic information necessary for predicting iron condition. However, this was not realized. Given the heterogeneous nature of the archaeological burial environment several techniques must be used to describe the soil effectively. Of the methods and techniques used, those which show the greatest promise for future work include: element mapping using an electron microprobe and the elemental and chemical analysis using XRF and ICP-MS techniques. These methods of analysis examine the elemental compositions of iron and soil. Also the method for radiographic analysis used in this thesis, if adopted by other conservation laboratories, could facilitate a standardization of iron condition terminology for conservators. Thus this thesis provides a framework for future research in the field of archaeological conservation.

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