Except for clay and silica, feldspar is the most common raw material in ceramics. It is also the most common mineral on the face of the earth—making up more than half the earth’s crust. Most feldspar has an almost perfect ratio of flux, alumina, and silica to make a glass at high-fire temperatures.

Defining the Terms

Feldspar—Any of a group of natural crystalline aluminum silicate minerals containing sodium, potassium, calcium or barium. Alkali feldspars (those containing sodium and potassium) are used most in ceramics.

Albite—Pure sodium feldspar with the chemical formula Na2O·Al2O3· 6SiO2. Very rare in nature.

Orthoclase and Microcline—The two crystalline forms of pure potassium feldspar, both with the chemical formula K2O·Al2O3·6SiO2. Very rare in nature.

Frit—A synthetic source of glaze flux and frequently of alumina and silica, manufactured by melting the ingredients together, cooling the resulting glass, and grinding it to a fine powder. 


A Natural Frit 

As a crystalline mineral precipitated from molten rock over geologic time, feldspar is definitely not a designer material. Feldspar is sometimes called a natural frit and is composed entirely of crystals, but a commercial frit is made up of a finely ground glass manufactured with a specific composition. More energy is needed to melt crystals than glass, so to give it time to melt, feldspar requires a somewhat slower firing, most often to higher temperatures. While a frit can be manufactured with any desired ratio of flux, alumina, and silica, with feldspar what you mine is what you get. Thus feldspar is a sort of good-news bad-news story. The good news is that the natural laws controlling how silicon, aluminum, and oxygen link to form the feldspar crystal ensure that the ratio of silica and alumina in pure feldspar is fixed.* More good news is that the flux elements exist in a fixed ratio to the alumina and silica. Part of the bad news, however, is that nature permits sodium and potassium to occupy that flux amount in infinitely variable proportions to one another. The amount of either in a given feldspar depends entirely on what was handy when the feldspar precipitated from the molten rock in the earth’s crust. Virtually every alkali feldspar deposit on earth has at least some difference in analysis. In scientific terms, albite and microcline/orthoclase can form a solid solution. That is, an alkali feldspar can theoretically vary from 100% sodium to 100% potassium as its flux constituent. Soda feldspars actually tend to have at least 30% of their flux as potassium, while potash feldspars usually have at least 15% of their flux as sodium. The rest of the bad news is that feldspar most commonly occurs as a rock, usually along with mica, quartz, and other minerals. In a feldspar mine, the rock is ground to a powder and sophisticated techniques are used to separate the minerals. How well and how consistently mining companies clean and concentrate the feldspar that artists use has virtually nothing to do with artists and focuses on the folks who buy 100-ton rail-car loads of feldspar to make literally millions of tons of glass per year. Quality control good enough to make beer bottles may not be as good as we would like in the studio, but who is ultimately the bigger end user of feldspar—studio artists or folks molding beer bottles? Feldspar is ultimately an industrial mineral and we have to accept that its quality is controlled by what’s good enough for industry. * (Note the difference in the ratio of silica and alumina between feldpars, spodumene, and nepheline syenite. There is less silica in the latter two. The crystal structure explains this. This also explains the differences between potash feldspars to nepheline syenite and spodumene.

Making Adjustments in the Glaze Lab

Commercial frits have generally consistent analyses. Naturally occurring feldspars are less consistent and subject to change over time. While all raw materials should be tested before use, this needs to be a requirement before using each new batch of feldspar in the studio. When feldspar is added to a clay body, it helps to melt very fine quartz into a glass phase that provides strength in the fired body. The amount of feldspar needed in a stoneware body depends entirely upon the flux level of the clays composing the body. For a fixed recipe of clays, various amounts of feldspar are tested to achieve a body with the desired level of vitrification from a given firing cycle. The difference in silica content between Custer and G-200HP feldspars (see graph on previous page) is enough to change glaze fit. While these two potash feldspars can generally be substituted one-for-one, if one wants precise control of glaze chemistry, then a more accurate substitute for Custer is G-200HP plus 3% silica. When an existing feldspar disappears or a new one enters the market, some substitution such as this is likely to be necessary to achieve consistent results. Time is also a factor. The landscape varies and as industry excavates from one mine to another the composition of feldspar changes along with it. The feldspar you were using five or ten years ago is most likely not exactly the same as what you are using today, even if it is the same brand name. Fusion button tests of the new and old material will guide you in whether and how to substitute other materials to accommodate the new feldspar’s chemistry. To start: 1 Get a full chemical analysis of the new and old feldspars, if they are available.2 Fire fusion buttons (a few grams of feldspar pressed into a small mold such as a crucible) of both materials side by side to get a visual indication of the differences in the two materials. Note color changes, melting temperatures, opacity, and surface effects. 3 Adjust recipes as these differences indicate and fire recipe tests to confirm that the adjustments are correct. Some ceramic artists use chemistry to adjust clay and glaze recipes before testing. Others rely entirely on testing. The method chosen may say something about an artist’s working style, but not the results, both methods work equally well

This article was written by Dave Finkelnburg and pulled from the December 2011 issue of Ceramics Monthly.