Pottery Pottery Basics

Fluxes for Ceramics and Glaze

By
Beth Peterson
Beth Peterson
Beth Peterson is a pottery expert, sharing her knowledge about producing pottery on the wheel, as well as handbuilt ceramic sculpture, and the Raku firing technique of pottery making. She also teaches mask making to children and adults. Beth has over 30 years of experience crafting with clay.
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Updated on 06/13/19
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Although a potter does not absolutely need to know about the various fluxes, a base knowledge of the types of fluxes is helpful in making sense of your experiences as you work with glazes.

One of the easiest ways to categorize fluxes is by their major component. It must also be remembered that these are mined materials that have been minimally processed (usually simply being pulverized). Variations do occur at times, and mines do play out.

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    Wood Ashes

    Wood ash is the exception to the above categorization. Chemical components in ash vary wildly and are also complex. Ashes can be considered to be naturally occurring.

    Other sources of ash that can be used are reeds, grasses, straw, leaves, and so on. The high alkaline content, which acts as the flux in the glaze, is caustic, so some people wash the ashes before use. Washed ash does not flux as well as unwashed ash, and will require more ash content in the glaze.

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    Sodium Fluxes

    Sodium fluxes are generally useful at mid- to high- fire ranges. The sodium fluxes include:

    • Soda feldspar, such as the commercially available Kona F-4.
    • Nepheline syenite: A high soda feldspar that included some potassium, has a lower melting temperature than soda feldspar and is useful in the mid-range temperatures. Often abbreviated to "neph sye" by many potters.
    • Sodium carbonate: aka soda ash.
    • Sodium chloride: aka table salt. Used in salt firing and vapor glazing in saggers.
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    Potassium Fluxes

    Potassium-fluxed glazes have greater durability than soda-fluxed glazes. Potassium is preferred for high-fire glazes. The potassium fluxes include:

    • Potash feldspars, such as Custer and G-200.
    • Cornwall stone: aka Cornish stone. Contains mainly potassium, but also has sodium and calcium.
    • Volcanic ash: Generally richest in potassium, but the chemical composition can vary widely. Always do a few test pieces when using a new bag of volcanic ash when mixing your own glazes.
    • Potassium carbonate: aka pearl ash. Used mainly as a color modifier.
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    Lithium Fluxes

    Lithium is used both for fluxing and for encouraging crystal growth in crystalline glazes. The lithium fluxes include:

    • Lithium feldspars, such as spodumene and petalite.
    • Lithium carbonate: The preferred source of lithium for crystal growth.
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    Boron Fluxes

    Boron is the most commonly used low-fire flux, other than lead. Boron fluxes include:

    • Gerstley Borate: No longer mined, but some limited amounts are still available. Synthetic replacements are available from many suppliers.
    • Colemanite
    • Borax: Often used in raku glazes and to smooth out higher firing glazes.
    • Boron-containing frits such as Ferro 3110, 3124, and 3134.
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    Calcium Fluxes

    Calcium fluxes are not used quite as often as the feldspathic fluxes. They include:

    • Whiting: aka calcium carbonate and lime. Used in high-fire glazes.
    • Dolomite: A calcium-magnesium carbonate used in high-fire glazes.
    • Wollastonite: A calcium silicate used in both clay bodies and glazes. Promotes strength and reduces shrinkage.
    • Bone ash: aka calcium phosphate. Used to produce opacity and opalescence in glazes, as well as being a flux.
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    Magnesium Fluxes

    Magnesium fluxes include:

    • Magnesium carbonate: flux for high-fire range, increases glaze adhesion and viscosity. Used for matte glazes.
    • Talc: Used as a flux in low-temperature clay bodies, and as a flux in both low- and high-fire glazes.
    • Dolomite: A calcium-magnesium carbonate flux used in the high-fire range when both elements are desired.
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    Barium Carbonate

    Barium carbonate is used as a flux at high temperatures. Can produce satin-matte, matte, and stony matte glazes.

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    Strontium Carbonate

    Strontium carbonate can be used very much like calcium carbonate (whiting) but also increases a glaze's resistance to crazing and scratches.

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    Lead Fluxes

    Lead historically has been the most commonly used flux. It produces very nice glazes at low temperatures and enhances colors. It is also highly toxic and is rarely used by potters. Even when fired, lead will leach out of the glaze into food or beverages, especially those which are acidic. Lead glazes of any form should never be used for functional pottery.

    • Red lead and white lead: These are two forms of raw lead and are extremely hazardous in large amounts. Lead remains in the body indefinitely, resulting in cumulative amounts increasing with each exposure.
    • Lead silicate and other lead frits: Fritted lead compounds are less dangerous than raw leads. They are still hazardous, however, and are best avoided.
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    Zinc Oxide

    Zinc oxide acts as a flux at high temperatures. However, it will vaporize in a reduction atmosphere, resulting in highly dangerous fumes. Zinc oxide also can lend opacity and in large amounts can encourage crystal growth.

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    Iron Fluxes

    Iron is generally known as a colorant, but it also can act as a powerful flux, especially in reduction atmospheres. Ferric oxide is used as a colorant, whereas 5% or more of ferrous oxide provides a powerful fluxing action.