Black Underglaze Turning Blue

Odoo • Image and Text

Blue-Black Underglaze

Black underglaze looks black-blue or has a blue hue after glaze firing.  It is not a true black.

Why is this happening?

Causes

  • Over application of clear glaze - clear glaze that is applied too thickly causes darker colours to become milky in appearance.

  • Over application of black underglaze colour - underglaze that is applied too thickly causes darker colours to become milky in appearance.

  • Over application of both clear glaze and underglaze - darker colours can become milky in appearance if there is too much product applied.

  • Over-packing kiln (poor air flow).

  • Kiln temperature.  A kiln that is not hot enough will result in colours that do not reach maturity and the colours will be dull or milky in appearance.

  • Kiln temperature.  A kiln that it too hot for the wares or glazes & underglazes will result in over-fired products (glazes & underglazes) and colour may burn out.

  • Kiln soak.  We recommend a hold of 20 minutes on the glazes and underglazes we sell.  If the top temperature does not penetrate all of the wares then colours may not reach maturity.  If the top temperature is held for too long, colour may burn out.

  • 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.

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