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Black Underglaze Turning Blue After Firing

Blue-ish blacks after firing

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Black Underglaze Turning Blue After Kiln Firing

Causes

  • Clear glaze is too thick - over application of clear glaze causes darker colours to become milky in appearance
  • Over-packing kiln (poor air flow)
  • Over application of underglaze colour
  • Kiln temperature:  not hot enough or too hot as underfired wares will not mature and over-fired colours will burn out
  • Kiln soak:  too long or too short
  • Organics in the bisque

Remedy and Prevention

  • Look at your kiln programme and ensure:
    • You have a of soak (circa 10-20 minutes)
    • The top temperature is correct and not too high or too low
  • Look at your kiln loading style and consider:
    • Increasing the space between each ceramic item (3-6cm between each item)
    • Position of elements:  remember to allow at least one element between each layer of a loaded kiln shelf
    • Position of wares:  taller  and larger items are placed in the centre of the kiln shelf and away from the elements (to allow the heat to circulate more easily from the kiln elements).
  • Glazing and dipping process
    • Go for a thinner glaze viscosity
    • Ensure the glaze is mixed well and is in suspension
  • Try adding a little yellow to the black underglaze bottle.
  • Keep a log.  Sometimes there is a common denominator.