PSI - Issue 10

A. Hein et al. / Procedia Structural Integrity 10 (2018) 219–226 A. Hein and V. Kilikoglou / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Ceramics were among the first materials manufactured and used by people and due to their manifold applications they remained of paramount importance in material culture over millennia even until today. The basic principles of the fabrication process, though, remained effectively the same. A clay paste consisting of fine-grained earthen materials mixed with water was prepared, which could be formed to objects of virtually any shape. After drying, these objects were exposed to heat, which changed irreversibly their material properties primarily through dehydroxylation and decarbonization of clay particles and mineral inclusions. At specific temperatures new mineral phases were formed along with the modification of the ceramics’ micromorphology first by developing vitrified glassy connections among particles and then by merging vitrified particles. Of particular interest appears to be the solidification of the material during this firing process and the generation of a primary resistance of the fired ceramic object against thermo-mechanical loads or impacts. Even though the production process was generally the same, a large variety of ceramic objects were fabricated in order to fulfill different functions in different contexts of use (Sillar and Tite (2000)). In domestic contexts the main ceramic functions concerned storage, food processing and serving. In institutional or commercial contexts apart from storage also transport was of interest. In manufacturing or industrial contexts technical ceramics were of major interest used for example in pyrotechnical processes, such as metal or glass production or in ceramic production itself. Finally, ceramics were always used also as construction and building materials. Different functions, however, required different performance of the ceramic objects. While suitable transport jars for example were distinguished by sufficient mechanical properties, such as elasticity, strength and toughness, but also by impermeability, pyrotechnical ceramics required basic refractoriness, thermal shock resistance and heat transfer properties adapted to their specific application. Production of ceramic types with different functions, thus, had to be customized in order to achieve particular material properties. One group of parameters concerned the shape of the objects, such as wall thickness, curvature or potential loading points. The present study, on the o ther hand, investigates the effect of the ceramics’ microstructure on the material performance under thermo-mechanical loads. The main focus will be on the pore structure of archaeological ceramics either developed randomly during firing or intentionally generated by mixing the clay paste with organic material, which combusted during firing. In contrast to modern ceramics archaeological ceramics were comparably heterogeneous and complex materials, in terms of inclusions as well as in terms of pore structure. Apart from the diverse nature of inclusions, which were present in the ceramic matrix, also the grain size distribution covered commonly a considerably large range. This was the result of insufficient refinement and homogenization of the clay paste or of non-plastic temper materials, which were occasionally added. The inclusions, on the other hand, had also a major impact on the pore size distribution with voids from the sub- μm level up to a level of several 100 μm. The smallest level of pore structure concerned the actual clay minerals, their packing and the development of the micromorphology during their vitrification. This de velopment, however, was subject to a certain variation of firing temperature and duration, which was controlled solely by experience of the craftspeople through kiln design, fuel replenishment and observation of the glow colors. The next larger level of pore structure was related to the non-plastic inclusions in the clay paste, which were not affected by decomposition or vitrification during the firing process. Due to thermal expansion they left eventually voids at the grain boundaries after cooling down and potentially induced cracks in the ceramic matrix. In many cases non-plastic inclusions were added intentionally to the clay paste as temper material assumedly in order to modify the properties of the fired ceramics ( Müller et al. (2015)). Another type of voids emerged in the fabricated object due to incomplete wedging of the clay paste before forming and firing. Thus, air pockets and disjunctive interfaces remained in the clay paste body, which could even increase during firing. Finally, the apparent use of organic materials added in specific cases as temper to the clay paste reveals that ancient craftspeople were apparently aware of the effect of porosity. Organic inclusions mixed with the clay paste combusted during firing leaving voids corresponding to their shape (Fig.1). Particularly the use of organic fibers can be observed which certainly supported additionally the structure and rigidity of the still green and unfired object. 1.1. Pore structure of archaeological ceramics