The feldspar group includes the alkali feldspar group, which is composed of chalcopyrite, silicic acid, and alkali, and the alkali earth feldspar group, which is composed of earth, silicic acid, and alkali earths. In the ceramic industry, the term “feldspar” is used to refer to orthofeldspar (potash feldspar). Soda feldspar (soda feldspar) and ash feldspar dissolve in all proportions to form solid solutions. Feldspar and feldsparite rocks come in considerable variety, and both are major raw materials for ceramics and porcelain. The most important factors in feldspar as a ceramic raw material, especially for ceramics, are not the alkali content or mineral structure, but rather impurities such as quartz, mica, boxwood, talc, amphibole, and titanium ore. When quartz is mixed in at a rate of 10% or less, the melting temperature decreases, and the viscosity of the molten liquid, in particular, decreases dramatically. The most important feldspar for porcelain is orthofeldspar, but a part of this alkali is often replaced by soda, calcium, or iron. The most commonly used alkali is potassium oxide (K2O), which is around 13%. At a firing temperature of about 120 degrees Celsius, the glass becomes a white, polybubbly glass from which the porphyrite is separated. The gas present in the bubbles is mostly water vapor, which is mostly released at 800-900 degrees Celsius.
Acidic gases are released at temperatures between 600 and 900 degrees Celsius, beginning at 500 degrees Celsius. The more potassium oxide in feldspar, the higher the melting temperature and the greater the viscosity of the molten glass. Lime significantly lowers the melting temperature of feldspar. The effects on the properties of feldspar and soda feldspar in the porcelain base are as follows: the shrinkage and translucency increase as the amount of soda in the feldspar increases, and an opposite effect occurs when equal amounts of both are used. In terms of translucency, the order of magnitude is: feldspar, feldspar + soda feldspar, and soda feldspar, and the opposite is true for calcination shrinkage, with the high-soda feldspar base having the lowest shrinkage. Mechanical strength is affected by heat treatment and molding method rather than feldspar type. The thermal expansion coefficient is greatest for the soda feldspar base. The action of feldspar in the glaze is similar to that in the base. In this case, it must be finely ground. Both feldspar and soda feldspar are used in the same way, but feldspar dissolves silica better and the resulting glaze is more durable.
