Aluminium Oxide - Al2O3

Amphoteric
Stabiliser
Unit 6 Topic 11
Oxide Qualities

Alumina - necessary in most glazes - increases viscosity - refractory
.


Action

  • Refractory. Has a high melting point.
  • Intermediate oxide second only to silica in importance linking flux and silica in glass.
  • Most glazes contain 5-15%. More than this (25%) promotes matt textures and opacity.
  • Excessive amount may cause crawling, pin-holing and greyish colours with poor opacity.
  • Increases viscosity and extends the melt range.
  • When introduced as clay alumina helps keep glaze materials in suspension and acts as an adhesive binding unfired gaze materials on the surface of the form.
  • Increasing alumina raises the melting point of a glaze (twice as effective as silica).

Sources

  • Feldspars - together with flux oxides and SiO2
  • Cryolite
  • Clay - the preferable source of alumina not provided by feldspars or other sources of flux.
    Ball clays or china clays are preferable as they have minimum iron or manganese impurities.
  • Alumina trihydrate - less desirable but useful where clays are unable to provide the required ratio with SiO2.
  • Calcined clays - useful for avoiding cracking of glaze surface after application where clay content of the glaze is high.

Colour Responses

  • Orange / pink with copper + high alumina and low silica.
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