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Milky White Hazy Glaze Ceramic Pottery Firing

Foggy White Appearance With Clear Glaze


Opaque Clear Glaze After Kiln Firing

A clear glaze should fire clear if it is fired to the correct maturing temperature and at the correct consistency/viscosity.

It us not uncommon to have a milky/foggy appearance in areas where the clear glaze puddles e.g. at the base of a vessel like a vase or mug.   Also, this glazing defect can also be seen more clearly when then is a darker underglaze base colour. 

A milk, foggy, or opaque appearance is caused by tiny bubbles trapped in the clear glaze and this can be down to two things - see below:

  1. The glaze/underglaze has reached  maturity during the firing process - the temperature was not hot enough and the clear glaze has under-fired.
  2. There has been an over application of underglaze or clear glaze or a combination of both.

Tips:

  • Ensure the glaze viscosity is correct.  Did the foggy colour appear on lots of pieces or just one piece? 
    • If it was on lots of pieces then it is likely that the viscosity of your clear glaze is too thick or you are over applying the glaze (brush glazing) or you are dipping too long (dip glazing).
    • If the glaze defect was apparent on just a few pieces, it is likely that too much underglaze was applied. 
  • Check that the kiln is firing to the correct temperatures.  Muted/milky/foggy fired wares can also be a result of the kiln not reaching temperature (the glazes have not reach maturity).  If this is the cause then there will be more than a few pieces in the kiln firing with the same problem.
  • Ensure that you finger-sand the clear glaze (after application once dry and before firing) in problem areas where clear glaze can accumulate e.g. the bottom of mugs or vessels, the centre of bowls, around detail or dimensional designs etc.

Rectifying the glaze defect:

  • You can try re-firing the piece (making sure the ramp is longer for a slower firing to prevent the bisque cracking).  By re-firing you are allowing more time for the bubbles to work their way up through the glaze.