Dating delft blue
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.The Chinese, on the other hand, define porcelain as any ceramic material that will give a ringing tone when tapped.None of these definitions is completely satisfactory; for instance, some thinly potted stonewares are slightly translucent if they have been fired at a high temperature, whereas some heavily potted porcelains are opaque.The only manufactured translucent substance then known was glass, and it was perhaps inevitable that glass made opaque with tin oxide (the German milk glass, for example) should have been used as a substitute for porcelain.The nature of glass, however, made it impossible to shape it by any of the means used by the potter, and a mixture of clay and ground glass was eventually tried.Therefore, the application of the terms is often a matter of personal preference and should be regarded as descriptive, not definitive.Earthenware was the first kind of pottery made, dating back about 9,000 years. The earthenware body varies in colour from buff to dark red and from gray to black.
In this section, is used to denote all pottery substances that are not vitrified and are therefore slightly porous and coarser than vitrified materials.
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).
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).