Secondary clay

Montmorillonite, also one of fhe mosf common clay minerals, occurs as a soft rock after it is powdered and mixed with water, it acquires plastic properties. It is the main constituent of fuller s earth, used since antiquity for fulling (cleansing textile cloth). 

Sepiolite, also known as meerschaum or sea foam, is a white or off-white soft clay of low density. It is composed of hydrated magnesium silicate and occurs in consolidated masses that are easily carved and whose surfaces can be smoothed and endowed with a durable polish. It is for this reason that meerschaum has been used, since antiquity, for making ornaments and decorative carvings. For many centuries meerschaum has been intensively mined in Greece and Turkey. 

Secondary clay, also known as sedimentary clay, transported clay, or potter s clay, is a mixture of clay and nonclay particles. Affer it is formed, much clay is 

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Clays occur naturally either in a relatively pure condition or mixed with other materials and they are therefore classified into one of two large groups primary and secondary clays. Primary clays are quite pure, uncontaminated by other materials, and have a rather uniform composition. Secondary clays are mixtures of clay with other minerals such as quartz, talc, mica, iron oxides, and even organic matter (the latter derived from the decay of dead plants and animals) the particles of most of the contaminating materials are generally of similar size to those of the clay (Kingery et al. 1976). 

Common ancient ceramic materials often found in archaeological excavations, such as fired brick and pottery, were made mostly from a mixture of a secondary clay and fillers. The nature, composition, and properties of clay have been already discussed the nature of the fillers, the changes undergone by the clay as well as by the fillers during their conversion to ceramics, and the unique properties of ceramic materials, are reviewed in the following pages. Attention is drawn also to studies that provide information on the composition and characteristics of ancient ceramic materials. 

Primary clay, for example kaolin, is colorless, and when such clay is heated to a high temperature it produces white ceramic materials. Most pottery, however, is colored its color is due to the fact that most of it was, and still is, made not from primary but from secondary clay. Secondary clay contains minerals other than clay, and colored metal ions in them endow the pottery with their color. Iron ions (in iron oxides), for example, tend to make pottery yellow, brown, or red, and manganese ions (in pyrolusite, a mineral composed of manganese oxide) make it either dark or black. 

Reverse weathering Chemical reactions that are theorized to occur in the sediments. In these reactions, seawater is thought to react with clay minerals and bicarbonate producing secondary clays and consuming alkalinity and some cations. This process is approximately the reverse of chemical weathering on land that produces clay minerals. 

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