Ceramic Raw Materials
This chapter is mostly about pottery. Pottery is only one of the large number of products known as ceramics. Other ceramics are covered briefly. Although pottery is composed of predominantly crystalline phases, glass (including glaze) is not. Glasses (inc
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Ceramic Raw Materials
8.1 Introduction This chapter is mostly about pottery. Pottery is only one of the large number of products known as ceramics. Other ceramics are covered briefly. Although pottery is composed of predominantly crystalline phases, glass (including glaze) is not. Glasses (including obsidian) are formed from the solidification of molten silicate – solidification that took place too rapidly for minerals to crystallize. Terracottas are thick, coarse, porous wares normally fired well below 900°C. Porcelain is a ternary mixture of clay, quartz, and feldspar. The latter acts as a flux that aids in the development of a glassy phase in the fired product. Ceramic production has always been an empirical art. This is especially true of pottery making. The lack of any scientific basis in ancient pottery construction within a society led to a slow, trial-and-error process that often lasted for at least decades, if not much longer. Choosing raw materials was one of the most important aspects. The preparation of these materials before firing also had significant effects on the final product. However, at the heart of ceramic production is a pyrotechnology – how the raw materials change during firing. The raw materials for pottery making can be divided into three categories: (1) clays, the dominant material, (2) temper (additives) used to control physical properties of the ceramic, and (3) minerals to impart a glaze, a color, or other desirable property. Low-grade clays, those that do not make good pottery without refining or mixing, are available almost everywhere, so the manufacture of building brick and tile did not require much consideration of raw materials. Clays have two important functions in the production of ceramics. First, their plasticity is basic to shaping the material in its green (unfired) state. Second, clays fuse over a temperature range without the body losing its shape. The modern ceramics industry has benefited from a solid scientific foundation, but the production of ancient ceramics discussed in this book was an empirical art, practiced with a limited array of raw materials.
G. Rapp, Archaeomineralogy, 2nd ed., Natural Science in Archaeology, DOI 10.1007/978-3-540-78594-1_8, © Springer-Verlag Berlin Heidelberg 2009
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Ceramic Raw Materials
8.2 Clays The term clay has three meanings: (1) a rock term describing a natural, earthy, fine-grained material that develops plasticity when mixed with a limited amount of water, (2) a particle-size term for the smallest particles (less than 2 µm), and (3) the name of a group of sheetlike silicate minerals. Some clays have little or no plasticity, e.g., the so-called “flint clay,” but, for the purpose of this book, the plasticity criteria will hold. Clay minerals form as a product of weathering, as a product of pedogenesis, and as a deposited sediment. With rare exceptions, such as some kaolinite deposits, natural clay is not a single mineral but rather an aggregate of minerals and colloidal substances. All natural clays contain both non-c
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