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Feldspars play a critical role in mid-range (cone 6, 2232°F) and high-temperature (cone 9, 2300°F) clay body and glaze formulas. Acting as a flux, they help bring other materials into a vitreous melt. As with other ceramic raw materials, the appropriate ratio of feldspar to clay body or glaze material is a critical factor in a successful result.
Potters use three basic types of feldspar: sodium-, potassium- or lithium-dominated minerals combined with the additions of alumina, silica and other trace materials. For example, F-4 contains potassium but in lesser quantities than sodium; therefore, it is classified as a sodium feldspar. The same system is used when potassium and lithium feldspars are classified by their predominant levels, excluding their alumina and silica.
Feldspar Characteristics in a Clay Body
In clay body formulas, feldspars bring into a melt clays, flint, talc, pyrophyllite and other refractory materials. Some high-temperature, vitreous, white firing porcelain clay body formulas contain as much as 25% feldspar, along with 25% flint and 50% kaolin. Too much feldspar in high-temperature clay bodies can cause excessive shrinkage, warping and cracking due to the extreme formation of glass in the clay body that results in thermal shock upon cooling. Too little feldspar can cause an immature clay body, resulting in less durable, porous ware. In low-temperature clay body formulas below cone 04 (1945°F), feldspars do not go into an active melt and are frequently used as a filler along with clays and other ceramic raw materials, depending on the fired color, handling characteristics and fired temperature required in the clay body.
Feldspar Characteristics in a Glaze
In glazes, feldspars act as a flux above cone 6 and bring other glaze materials into a melt. By themselves, they form a white, semi-opaque glass containing many bubbles suspended in the glaze layer. Feldspar glazes also exhibit high surface tension when vitreous, much like water beading up on a glass tabletop. This property can cause glaze crawling, exposing the underlying fired clay body.
Too much feldspar added to a high-temperature glaze can result in crazing a fine network of lines in the glaze due to its high coefficient of expansion and shrinkage when cooling. Excessive feldspar can result in a gloss glaze and glaze dripping on vertical surfaces or pooling on horizontal surfaces. Too little feldspar can cause a dry, opaque, pitted surface due to the lack of fluxing action in the glaze. In low-temperature glazes below cone 04, feldspars are not highly effective fluxes and must rely on lower fluxing materials for a mature glaze melt. Feldspars in low-temperature glazes frequently contribute to glaze opacity and dry surface texture.
Other Potassium Feldspars
Two popular potassium feldspars, Custer and G-200, are currently used by potters in relatively small amounts and ceramic suppliers that use large quantities in their prepared clays. However, as of January 2009, G-200 feldspar mined by Imerys North America Ceramics has been discontinued. While there are still stocks in potters studios and ceramics supply bins, it will eventually go the way of Albany Slip, Gerstley borate, Kingman feldspar, Oxford feldspar and other materials that have gone out of production. G-200’s demise is based on economic considerations that potters do not control. Originally, G-200 as potters know it was mixed at the processing plant by using 70% Minspar 200 and 30% HP G-200. The blended feldspars then became G-200 and were shipped throughout the U.S.
The increasing costs of shipping Minspar 200 from Spruce Pine, N.C., and G-200 HP mined in Siloam, Ga., to the processing plant in Monticello, Ga., a trip of over 200 miles, made this situation uneconomical. Potters are now faced with either using Custer feldspar in place of G-200 or finding a new potassium-based feldspar. Potters can also separately purchase Minspar 200 and HP G-200 and mix them in the 70/30 ratio to achieve the original G-200 feldspar, a time-consuming procedure. While Custer feldspar is slightly stiffer or more refractory when compared to G-200 at cone 9 (2300°F) in 100% amounts, it does offer a workable substitution in clay body and glaze formulas in the majority of situations.
The demise of a feldspar or any other ceramic material is not new and, in fact, the problem has been going on since economic factors began to dictate the availability of any material. Potters should acquire the knowledge and skill to insure a replacement for any raw material used in their clay body and glaze formulas.