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Lesson Plans


Re: Glaze Question for Numo...or anyone

From: Robert Alexander Fromme (rfromme)
Date: Tue Jan 25 2000 - 17:45:18 PST

  • Next message: L Skeen: "Re: kiln temp question"

    Eugenia Hess wrote:

    " Someone wrote to this list that one glaze applied over another made a nice
     color. Should I experiment with this first? Or can I make gray by mixing
     white and black glaze?"

    Well, the subject points your question to Numo ...or anyone, so I am an
    anyone.....

    Most glazes have a variety of fluxing oxides and when two glazes are mixed,
    one often gets unpredictable results. If an eutectic is formed (two
    ingredients melting at a lower temperature then either by themselves), the
    glaze may become very fluid. This may or may not be what you need. Shelves
    are expensive and are hard to clean. In some cases, the mix eats through
    the kiln wash and deep into the surface. In other cases, the new varied
    surface and fluid effects on the pot or sculpture may be just the thing you
    are looking for. Test your results on a sherd with another sherd placed
    between it and the kiln shelf in case ther glaze runs. Keep lin mind that
    the increased chemical activity which often results from the mix of one
    glaze over another often creates additional gas blisters in the extra runny
    mix.

    Now, the question about gray glaze. Use a grey mason stain or something
    similar.

     Black is usually created by a mixture of three concentrated metal oxides
    added to the base glaze. Cobalt, iron, chrome, manganese are frequently
    used. In the case of cobalt in the black, the addtion of white to the black
    yields a blue. In the case of manganese in the black, it give a brown.
    Chrome usually gives a dry green, brown or grey with white and may not melt
    in the more refractory white mix.( Whites are usually created by adding
    very refractory zirconium siliicates to the base glaze and they are usually
    quite stiff. Chrome is also quite refractory ) Iron may go toward red,
    brown, blue or green iin the white mix. At any rate, the effect of metal
    oxides in glaze or glass is not the same as subrtactive color theory applied
    to paint.

    hope this helps,

    Bob, the old potter turned H.S. Elect. Media teacher. 

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