Quantitative Porosity Studies of Archaeological Ceramics by Petrographic Image Analysis
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Quantitative Porosity Studies of Archaeological Ceramics by Petrographic Image Analysis Chandra L. Reedy1, Jenifer Anderson1, Terry J. Reedy2 1 2
Center for Historic Architecture & Design, University of Delaware, Newark, DE 19716, USA Independent Statistician, 3 Stage Road, Newark, DE, 19711, USA
ABSTRACT Pores in archaeological ceramics can form in a number of different ways, and reflect both deliberate choices and uncontrollable factors. Characterizing porosity by digital image analysis of thin sections holds a number of advantages as well as limitations. We present the results of experiments aimed at improving this method, focusing on high-resolution scans of entire thin sections. We examine the reproducibility of pore measurements by petrographic image analysis of ceramic thin sections using laboratory-prepared specimens of clay mixed with sand of known amount and size. We outline protocols for measuring Total Optical Porosity, using the Image-Pro Premier software package. We also briefly discuss use of pore size and pore shape (aspect ratio and roundness) in characterizing archaeological ceramics. While discerning reasons for observed amounts, sizes, and shapes of pores is an extremely complex problem, the quantitative analysis of ceramic porosity is one tool for characterizing a ware and comparing a product to others. The methods outlined here are applied to a case study comparing historic bricks from the Read House in New Castle, Delaware; the porosity studies indicate that different construction campaigns used bricks from different sources. INTRODUCTION Porosity has long been recognized as an important feature to characterize in any study of ceramics [1]. Porosity in ceramic materials results from choices in raw materials, clay processing and object fabrication methods, drying and firing regimes, and use, burial, or deterioration factors [2-4]. For example, during clay processing and vessel fabrication, air bubbles can become trapped. Shrinkage during drying and firing can enlarge those pores. Long linear pores with parallel alignment, often wavy with tapering ends, can appear as a result of shrinkage of the clay as excess water is released during firing; this alignment may also emphasize patterns of pressure placed during fabrication. At higher firing temperatures these linear pores may be less interconnected than at lower firing temperatures [5]. As carbonates dissociate and organics burn out or char during firing, additional porosity can be created. If firing temperatures are high enough, porosity can decline if vitrification occurs [1, 6]. Temper additives such as sand or grog can keep porosity higher, as clay tends to shrink away from those particles during drying and firing, creating additional porosity. Round secondary pores can be produced by trapped gases as the clay matrix and silica minerals begin to melt, off-gas, and vitrify [7]. If temperatures become too high, the round pores can become bloated and expand in number, indicating overfiring. Some ceramics are deliberately designed to be porous
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