Fluxes are substances, usually
oxides, used in
glasses,
glazes and
ceramic bodies to lower the high
melting point of the main
glass forming constituents, usually
silica and
alumina. A ceramic flux functions by promoting partial or complete
liquefaction. The most commonly used fluxing oxides in a
ceramic glaze
Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a pottery body through firing. Glaze can serve to color, decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding ...
contain
lead,
sodium,
potassium,
lithium,
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
magnesium,
barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
The ...
,
zinc,
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
, and
manganese. These are introduced to the raw glaze as compounds, for example
lead as
lead oxide.
Boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
is considered by many to be a glass former rather than a flux.
Some oxides, such as
calcium oxide
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime''" connotes calcium-containing inorganic ...
, flux significantly only at high temperature.
Lead oxide is the traditional low temperature flux used for
crystal glass
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically al ...
, but it is now avoided because it is toxic even in small quantities. It is being replaced by other substances, especially
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
and
zinc oxides.
In clay bodies a flux creates a limited and controlled amount of glass, which works to cement crystalline phases together. Fluxes play a key role in the
vitrification of clay bodies by lowering the overall
melting point. The most common fluxes used in clay bodies are
potassium oxide
Potassium oxide ( K O) is an ionic compound of potassium and oxygen. It is a base. This pale yellow solid is the simplest oxide of potassium. It is a highly reactive compound that is rarely encountered. Some industrial materials, such as fertili ...
and
sodium oxide
Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as gla ...
which are found in
feldspars. A predominant flux in glazes is
calcium oxide
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime''" connotes calcium-containing inorganic ...
which is usually obtained from
limestone. The two most common
feldspars in the ceramic industry are
potash feldspar (
orthoclase
Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
) and
soda feldspar (
albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. ...
).
Common oxides
List of commonly used ceramic oxides:
See also
*
Loss on ignition Loss on ignition (LOI) is a test used in inorganic analytical chemistry and soil science, particularly in the analysis of minerals and the chemical makeup of soil. It consists of strongly heating ( "igniting") a sample of the material at a specif ...
References
See also
*
Secondary flux
{{DEFAULTSORT:Ceramic Flux
Ceramic materials