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Black Underglaze Bleeding Running Smudging

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Black Underglaze Glaze Running Smuding Bleeding on Pottery Ceramics

Sometimes black underglaze can run, smudge or bleed on ceramic pottery wares.  This seems to happen more on vertical surfaces

Causes

  • Over-firing kiln
  • Over-crowded kiln
  • Over application of underglaze colour
  • Over application of clear glaze
  • Firing too fast (insufficient time to allow trapped gases to expel). 

Remedy and Prevention

  • Look at your kiln programme and ensure:
    • Your firing cycle is not too short (consider lengthening the firing time)
    • You have a long enough soak (circa 20 minutes)
    • The top temperature is not too high
  • Look at your kiln loading style and consider:
    • Increasing the space between each ceramic item (3-6cm between each item)
    • Heat circulation:  remember to allow at least one element between each layer of a loaded kiln shelf
  • Glazing and dipping process
    • Go for a thinner glaze viscosity
    • Ensure the clear glaze is mixed well.
    • Ensure the clear glaze is in suspension (this can be a common issue if you are glazing a large batch)
    • Don't shake the item too vigorously during dip glazing.
    • Don't keep the shape in the same position whilst the clear glaze is wet - move it slowly and sway in different directions. 
  • On important pieces consider a pre-fire, then dip in clear glaze and fire again.
  • Ensure your bisque ware is room temperature or warmer so that the clear glaze dries quicker (especially if you can see that the black runs during the glazing process - sometimes it is only apparent on fired wares).
  • Keep a log.  Sometimes there is a common denominator.