Glaze Terms

Glaze Science Terms

  • Alkali—the opposite of acid, another term for flux. The stronger alkalis are lithia, soda, and potash; the weaker are called alkaline earths, and include magnesia, calcia, strontia, baria, zinc and lead oxide.
  • Amphoteric—an intermediate oxide able to act as either an acid or alkali, which acts as a stablizing link between fluxes. Alumina is the most important; others common ones are boric oxide, red iron oxide, the opacifiesr titania, zirconia, and stannic oxide.
  • Eutectic—the lowest point at which a mixture of 2 or more substances will melt, which is lower than the melting point of any one of the substances alone. The eutectic varies with each combination of materials.
  • Flux—a glaze material that lowers the melting point and promotes fusion of different materials in a glaze.
  • Frit—a combination of glaze materials that have been fired and ground down to powder again in order to
    • render soluble materials insoluble;
    • lower the fusion point;
    • lower the risk of coming into contact with caustic or toxic materials.
  • Glaze—a combination of oxides fused by heat to form a glass—like coating on ceramics. It usually is a combination of Silica, Flux, and Alumina. (see The Glaze Buddies Al-X-Si)
  • Opacifier—a mineral added to a glaze that makes it opaque, either by creating a structure of tiny crystals that deflects light, or remaining suspended and causing light interference, or causing minute bubbles that interfere with light passage.
  • Oxide—a chemical combination (of atoms) of an element with oxygen.
  • Reticulation—a glaze surface created during firing when one glaze bubbles up through the glaze over it to create rings of color, or a crater-like texture.

Glaze Attributes

  • Alkaline Glazes—are characterized by bright turquoise blue from copper, pink from cobalt, and blue from iron, typical of early Egyptian and Persian earthenware pottery. The glaze is dominated by fluxes such as lithia, soda, potash, and baria, but low in amphoteric fluxes. Boric oxide is usually added to lower the rate of crazing of these brittle an unstable glazes that are not appropriate for food containers.
  • Crystalline Glaze—large snowflake—like crystals can be encouraged to form by extremely careful control of the firing and cooling cycle. Reduce the alumina conten, use zinc oxide, Borax, sodium, potassium, rutile, or rion. Glazes run, so pieces should be fired on supports over saucers.
  • Glossy —hard, reflective surface which may be opaque to clear.
  • Mat / Matte—non-glossy finish due to the glaze composition. Barium (toxic) or alumina plus slow cooling help form mattes.
  • Opaque—light can not pass through.
  • Refractory—the quality of being resistant to the effects of high temperatures.
  • Translucent—almost clear.
  • Vitreous—the quality of being hard, glossy, and non-absorbent.

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