Zinc Oxide - ZnO

Metal Flux Unit 6 Topic 7
Oxide Qualities

Wide range flux - low expansion - viscous melts - crystals.
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Action

  • A strong oxidation flux in amounts of 3 - 4%. Excessive amounts render the glaze refractory and very viscous.
  • Is lost in reduction atmospheres.
  • Very low coefficient of expansion - useful for controlling crazing.
  • Increases the strength and durability of a glaze.
  • Can opacify with a slight pinkish tone.
  • Can be used in large quantities to produce big crystals. Alumina levels must be low. Such glazes require special firing cycles involving extended periods of cooling.
  • Softens the matt texture in alumina and barium matt glazes (0.35 mols).
  • When introduced in light (uncalcined) form can cause crawling and cracking.
  • Glazes relying heavily on zinc oxide are fickle and difficult to fire with consistent results.
  • Prone to cracking when drying after application.
  • Common constituent of matt oxidation glazes.
  • Historically important as a replacement for lead oxide (Bristol Glazes).

Sources

  • Zinc oxide.

Colour Responses

  • Affects some colours adversely particularly chrome oxide which turns a dirty brown.
  • Cobalt and copper are enhanced
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