Magnesium Oxide - MgO

Alkaline Earth Flux Unit 6 Topic 5
Oxide Qualities

High temperature flux - low expansion - highly viscous melts.


  • Acts as a flux above C.4 and tends to produce a late fluidity.
  • Similar action to calcium, barium, zinc and strontium oxides but is not interchangeable for any one of them.
  • A refractory and opacifier at low temperatures.
  • One of the lowest coefficients of expansion - useful in reducing crazing.
  • A very high surface tension - can give viscous melts and soft, rounded edges but can cause crawling faults.
  • Can act as an opacifier and matting agent in sufficient quantities.


  • Magnesium carbonate MgCO3.
  • Dolomite CaMg(CO3)2 - also brings CaO (contains about 44% MgCO3).
  • Talc and steatite 3MgO.4SiO2.H2O - also bring silica SiO2

Colour Responses

  • Will effect colour development - with cobalt tends to give magentas and violets.
  • Reduction encourages more iron oxide from the body to enter the glaze.
  • Late fluidity is useful in underglazes which will boil through late in firing (e.g. oilspot)
  • High surface tension useful in creating lizard skin in a high magnesium overglaze.
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