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When feldspar or soapstone (steatite) is added to the clay and exposed to a temperature of 2,000 to 2,650 °F (1,100 to 1,450 °C), the product becomes translucent and is known as porcelain.

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Clay, the basic material of pottery, has two distinctive characteristics: it is plastic (i.e., it can be molded and will retain the shape imposed upon it); and it hardens on firing to form a brittle but otherwise virtually indestructible material that is not attacked by any of the agents that corrode metals or organic materials.

Firing also protects the clay body against the effects of water.

If it is mixed with a substance that will vitrify at a lower temperature (about 2,200 °F, or 1,200 °C) and the mixture is subjected to heat of this order, the clay will hold the object in shape while the other substance vitrifies.

This forms a nonporous opaque body known as stoneware.

If a sun-dried clay vessel is filled with water, it will eventually collapse, but, if it is heated, chemical changes that begin to take place at about 900 °F (500 °C) preclude a return to the plastic state no matter how much water is later in contact with it.

Clay is a refractory substance; it will vitrify only at temperatures of about 2,900 °F (1,600 °C).Porcelain made in this way resembles that of the Chinese only superficially and is always termed soft, or artificial, porcelain.The date and place of the first attempt to make soft porcelain are debatable, but some Middle Eastern pottery of the 12th century was made from glaze material mixed with clay and is occasionally translucent (France.The body can be covered or decorated with slip (a mixture of clay and water in a creamlike consistency, used for adhesive and casting as well as for decoration), with a clear glaze, or with an opaque tin glaze.Tin-glazed earthenware is usually called majolica, faience, or Kamakura period).By coincidence they apply also to the physical properties of the two substances: for example, soft porcelain can be cut with a file, whereas hard porcelain cannot.

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