Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
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.
Since feldspar melts so well, it is common to find glazes that contain high percentages, even up to 70%. High feldspar glaze can cause alot of misery in ceramics and pottery. Anything above around 40% is usually trouble.
1) High feldspar glazes settle in the bucket. Why? Almost all glazes need significant Al2O3 (for durability and to thicken the melt). Typically it is sourced mainly from clay, especially kaolin, and secondarily from feldspar. But when feldspar percentages are high kaolin must be reduced, or Al2O3 is oversupplied. That accounts for the poor application and slurry properties (e.g. settling, dusting, drips and running). These situations can be fixed using glaze chemistry to source Al2O3 more from kaolin or ball clay and less from feldspar (the Na2O/K2O can come from a much lower alumina material, like a frit (e.g. Ferro Frit 3110).
2) High feldspar glazes have high amounts of Na2O and K2O. Yes, these are good melters (fluxes) but they have by far the highest thermal expansions of any oxide. This means crazing. High feldspar glazes almost always craze. Glaze chemistry is again needed. The solution is to trade some of the K2O and Na2O (KNaO) for lower expansion fluxes (preferably MgO, but also CaO, SrO, Li2O; any other flux because they all have much lower expansion that KNaO). When feldspar is reduced in the recipe Al2O3 and SiO2 are lost but these can be easily made up by kaolin and silica.
High feldspar glazes are often the product of a line or triaxial blending project. But the problem with this approach is that glazes are selected based too much on the visual appeal of a fired sample. When the chemistry is not considered the out-of-balance recipe gets into production and later slips into the online trade in undocumented unsuitable glaze recipes. Thousands of recipes too-high-in-feldspar are in common use. Not only is crazing an issue, but their tendency to have an unbalanced chemistry impacts their leachability and durability.
There is often discussion online about substituting one feldspar for another in specific recipes. People sometimes claim that a recipe requires a specific brand of feldspar do produce the visual effect, but they most often do not know why. Glazes of this type (where the mechanism is unknown) are generally too "touchy" to produce consistent ware in production. Substituting feldspars is not generally "rocket science". If two feldspars have about equal KNaO (total K2O+Na2O) then they are likely interchangable. Of course, some feldspars have lower iron content and thus produce whiter glazes. The difference between a soda feldspar and a potash feldspar may not be a much as the names indicate. Soda feldspars are named such if they have more Na2O than K2O (meaning they have significant K2O, e.g. 5% K2O, 6% Na2O) whereas potash feldspars generally predominate in K2O (e.g. 9% K2O, 3% Na2O). If you calculate the chemistry of the glaze recipe, then it is practical to simply view feldspars as contributors of K2O, Na2O, Al2O3 and SiO2. In this way it is easy to juggle a recipe to restore any chemistry differences introduced by switching a feldspar.
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.
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). Feldspars have high KNaO, that means they have high thermal expansions. That is why high-feldspar glazes often craze.
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.
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.
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.
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 in this GLFL test. The Montana materials on the right are not commercially available, they were being evaluated for viability.
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