Fluxes

A summary of the actions and uses of the major ceramic glaze fluxes. This paper leaves out less common fluxes and the coloring oxides or which many have fluxing characteristics.

Alkali Metals: Sodium, Pottasium and Lithium

• Sodium and Pottasium have so many characteristics in common that they are often left grouped together in calculations. As the Soda and Pottash in a glaze increase the glaze tends to craze more and after a certain percentage glazes with soda and pottash become more and more soluble. Sodium silicate a glass melted from soda and silica is very soluble in water. Potassia makes a similarly soluble glass. Soda tends to melt glazes more quickly than pottasia and Soda tends to cause a bit more crazing. Soda and potash dominated glazes tend to be very glossy and fluid. Major sources: Feldspars, Frits, and soda ash. Also Borax (Na,B)
• Lithia is an even stronger flux than soda or pottasia on a weight basis. Per gram it is one of the strongest fluxes. That is becasuue the individual atoms are light and there are more per gram that there are of the other fluxes. Even on an atom per atom basis Lithia is a strong flux.
• Lithia in small quantities promotes glossiness. Although in large quantities it leeds to matts. It tends to work against crazing and is often combined with high Soda glazes to counter soda glazes tendancies to craze. Major Sources:Lithium carbonate, Spodumene, Lepidolite, Petalite.
• Glazes with lots of Lithia like the other alkali metals tend to be soluble in foods. This group of metals tends to cause copper to color blue rather than green in oxidation.

Alkaline Earth Metals: Calcium, Magnesium, Barium, Strontium

• Of this group Calcium is the most widely used. At stoneware temperatures it can produce a reasonable stable glaze in combination with kaolin and silica. Most TAMUCC stoneware glazes are fluxed predominantly with calcia. In higher quantities it tends to become matt but can be made to remain glossy with the right amounts of silica and alumina. Magnesium is similar to Calcium except that it does not melt as strongly and cannot be used alone as a stoneware flux. It tends to produce more viscous melts and tends towards matt glazes much earlier than Calcium. It lowers expansion more than calcium so is more effective at preventing crazing. Calcia and magnesia generally help produce durable glazes, but glazes too high in either are likely to not be durable.
• Barium and Strontium are often added to glazes in small quantities to promote melting and brightness of a gloss. But in larger quantities both produce matts. Copper pigmented matt glazes based on Sr and Ba are interesting. Barium is however poisonous and glazes with this colorant can be poisonous enough to cause immediate harm to humans and other animals. The unfired glazes are also hazardous. Strontium is similar enough in action that for small amounts (less than 6 percent or so) it has become normal to subsitute  strontium carbonate for barium carbonate. The normal substitution is .75 grams of strontium for every gram of barium carbonate. The substitution is not always effective and the new glaze should be tested.
• Major sources: Calcium ,Magnesium, Barium, and Strontium Carbonates, Dolomite(Mg,Ca), Talc(MgSi),Wollastonite(CaSi), Frits.

Zinc Oxide. Zinc Oxide is an active mid range flux. It is not very active at lowfire temperature (below cone 1) and it appears to not remain in glazes above cone 7 or 8. It does however seem to have an effect on some of the glazes perhaps by stirring them as it evaporates.. Cone 6 zinc based glazes are known as Bristol glazes and are very viscous and tend to be matt. Sources: Zinc Oxide, frits, Zinc Borate (have not seen this stuff since the 1980's

Boron: Boron is in some sense not a flux. By itself it melts to a glass. Boron based glasses or glasses where boron is dominant tend to be soluble in mild bases such as soaps. Glazes need silica as well to be durable. High boron glasses are soluble in alkali solutions. Boron does melt into a glass at a low temperature however and can be used to greatly depress the melting temperature of an otherwise high fire glaze. Even small amounts however can effect the color and surface of a glaze. Boron based glazes tend to be glossy. Major sources: Gertsley Borate, CadyCal, frits, Borax.

Lead: Lead can be posionous to the ceramist as well as the user of lead glazed pottery. The kiln fumes, raw glaze and at time fired glaze can be hazardous. That said lead produces easy to use versatile glazes in a wide variety of surfaces and colors. Major Sources: Lead Frits, Lead Bisilicate, Lead Monosilicate, Lead Carbonate, and red lead. Lead compounds are hazardous and misuse can cause death, brain damage and other severe injuries. It is extremely hazardous to fetuses and young children.