Feldspars Used in Clay and Glazes Mimi Obstler

Instructions

Feldspars are important ingredients in clay bodies and glazes. In both applications, their primary function is to supply fluxes to the formulations, but they also provide additional alumina (Al₂O₃) and silica (SiO₂). Feldspars are naturally occurring minerals and are generally classified as either potash (potassium) or soda (sodium) feldspars based upon the predominant alkali metal element (the flux) that is present. The minerals commonly referred to as lithium feldspars are not true feldspars, but they are aluminosilicates like feldspars and contain the fluxing element lithium, and are used for the same purposes as the feldspars.

The following table presents typical chemical analyses (in weight percent) provided by the suppliers, for a number of common feldspar products and related materials. Most of the names are trade names, with the exceptions of lepidolite, petalite, and spodumene, which are true mineral names. Nepheline syenite is actually a rock composed of potash and soda feldspars plus the mineral nepheline (a sodium aluminum silicate). In the table, the trade names for the feldspars are grouped according to the actual type of feldspar that they contain. Distinguishing between the different types of feldspars based upon the fluxes that they provide is important because of the different characteristics that each of the fluxes contributes to a wide variety of properties such as melting point, thermal expansion, glaze color and hardness. For each analysis, the remaining percentage needed to bring the total of all the elements to 100% is the ignition loss (not shown in the table).

The analyses will allow you to compare the compositions of different raw materials when it is desirable to make substitutions in clay body and glaze recipes. The weight percent values will be useful in the conversion of glaze recipes from Seger molecular formulas to weight percent recipes when using these raw materials. The theoretical formulas and molecular weights for the different types of minerals present in the products are as follows:

However, all of these naturally occurring mineral products often contain additional minerals or elements as minor impurities, as can be seen in the table, and as a result, their calculated molecular weights will differ somewhat from the theoretical values. The final column in the table shows molecular weights calculated for each of the materials, based on the specific analysis shown (all the elements including the impurities). These specific molecular weights can be used, instead of the theoretical molecular weights given above, in the conversion of glaze recipes from weight percent to Seger molecular formulas. It should be kept in mind, however, that as the actual composition of the minerals varies with time and source, these calculated molecular weights will also change.