Quite simply, feldspar glazes are high in feldspar. Feldspar by itself melts well at high temperatures, however to be a balanced glaze (durable, well fitted to the body, non-leachable, etc) it needs additions of other fluxes and silica. It is very educational to work through the process of comparing the chemistry of a feldspar to a target formula for a typical medium or high temperature glaze. Instructors commonly show students how to add materials to a feldspar to bring the chemistry into line with the type of glaze being sought.
Melt flow tester used to compare feldspars
Fired to cone 10 oxidation. Although feldspar is a key melter in high and medium temperature glazes, by itself it does not melt as much as one might expect.
Feldspars, the primary high temperature flux, melt less than you think.
A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.
Feldspar melts by itself to be a glaze? Hold on!
Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K2O and Na2O (KNaO), these two oxides have the highest thermal expansion of any other oxide. Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
A soda feldspar cone 4-7
Pure soda feldspar (Minspar 200) fired like-a-glaze at cone 4, 5, 6 and 7 on porcelainous stoneware samples. The bottom samples are balls that have melted down at cone 7 and 8. Notice there is no melting at all at cone 4. Also, serious crazing is highlighted on the cone 6 sample (it is also happening at cone 5 and 7).
Do your functional glazes do this? Fix them. Now.
These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.
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