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Theoretically speaking, the term 'feldspar' refers to a family of minerals with a specific crystalline presence. However actual feldspar powders are made from crushed crystalline rock containing a mixture of aluminum silicates of sodium and potassium (with minor amounts of lithium or calcium). They normally contain 10-15% alkali (K2O, Na2O) and melt well at medium to high temperatures and are an economic source of flux. Commercially available feldspars tend to predominate in one specific mineral kind with lesser amounts of another and traces of others. Manufacturers can be found around the world and they usually do a good job of delivering uniform and clean products when one considers that feldspar deposits vary widely in composition and contain many impurities. Large quantities of feldspar are used in non-ceramic industry (e.g. cement, agri).
In many countries feldspar companies draw upon the same mine year and year, marketing a brand name product that gains wide acceptance. In other countries (e.g. India) large feldspar suppliers do not even have mines, they buy raw product according to the chemistry and blend from different sources to achieve a specific product. Over time they change suppliers but through careful quality control their ceramic manufacturing customers often do no realize their feldspar is coming from different sources with different shipments. Increasingly customers are connecting with suppliers on other continents, not infrequently this has led to misunderstandings and delivery of bad or variable product.
Generally high potash feldspars are employed in bodies and promote vitrification by forming a glassy phase that 'cements' more refractory particles together and triggers the formation of mullite from clay mineral. It is typical to see about 25% feldspar in cone 10 vitreous bodies (1300C) and 35% at cone 6 (1200C), although porcelains may have a little more. Much below 1200C feldspar will not produce a vitreous body. Manufacturers often employ feldspar percentages inappropriate to their situations and other materials (like silica sand, alumina, extra quartz) are added to compensate. It is important to test the porosity and fired shrinkage of your body at temperatures above and below your production firing (e.g. -30, -60, -90, +30, +60, +90). This will tell you where you area on the porosity and shrinkage curves (it is not good to be near or on the down-slope of the firing shrinkage or up-slope of the porosity). You can organize this testing in your account at insight-live.com.
In glazes feldspar promotes melting at medium and high temperatures (feldspars are the primary ingredient in most high temperature raw glazes). Sodium feldspars are most common and used mainly as a source of alkalis. Feldspars are mineral compounds of silica, alumina and fluxes and are among the relatively few insoluble sources of K2O, Na2O and Li2O. No other raw material is closer to being a complete stoneware glaze on its own than feldspar. Since feldspars contain a complex mix of oxides, ceramic chemistry calculations are needed to 'juggle' a recipe to achieve the desired balance of fluxing oxides with alumina and silica and to control the high thermal expansion that they impart.
Many feldspars begin to melt around 2100F (1150C) and make good glaze bases because they contribute alumina and silica in forms that participate well in the melt. Feldspars tend to work well in fast fire glazes because they remain relatively inert until the later stages of firing.
Geologists see feldspar as a mineral and classify feldspars as mainly as albite, microcline, orthoclase and anorthite. However, technically minded ceramists tend to view feldspar as a 'warehouse of oxides' rather than a mineral, their priority is on the chemistry (specifically the K2O and Na2O percentages). For this reason other materials like cornwall stone and nepheline syenite, which have similar chemistry, are viewed as feldspars by many technicians.
'Flux-saturated' glazes with high feldspar contents tend to be chemically unbalanced and thus make poor bases for functional ware, especially for glaze containing metallic color oxides (the glazes can be soft, leachable, crazed, oxidizable). High feldspar can contribute to a high surface tension in the glaze melt and may lead to high bubble population (producing milkiness in the fired matrix). Stoneware glazes using large amounts of feldspar as a flux almost always craze because high-expansion sodium and potassium predominate. High feldspar glazes often lack clay content and thus do not suspend well, they settle in a hard layer on the bottom of the container and dry to a powdery surface on ware. In fact, thousands of potters are using feldspar saturated glazes and living with many problems without being aware of the cause.
Any of the aluminum silicate minerals containing alkalis and having very similar chemical structures, especially in their crystal geometry and ratio of alkali metal(s) to aluminum and silicon, and thereby known mineralogically as aluminosilicates. A further classification distinguishes two series of these aluminosilicates based on their continuums (as rock-making minerals) from sodium to potassium and from sodium to calcium, labeling the first series as alkali feldspars and the second series as
As clays (i.e., as hydrous aluminosilicates) feldspars are distinguished by their relative purity of clay + alkali; free, for the most part, of any significant traces of the non-alkali metals or earths so common to other earths .
Feldspathic earths are usually understood to be the rocks made up principally by feldspars and/or feldspathoids (such as gneisses, granites, magmas and pegmatites) available as natural deposits of decomposed rock or industrially packaged rock milled to varieties of grain-size powder. In the broadest sense, given that feldspars constitute more than 50% of the Earths crust, most any earth is feldspathic even if containing only traces; so an earth especially w
ith respect to earths used in making up ceramic clays and glazes is normally considered feldspathic when the feldspar/feldspathoid content is sufficient to make a real difference one way or another in formulations. As pegmatites are the mother-rocks of feldspars, so are the feldspars the mother-rocks of kaolin and its similars.
commonly used forms (as often labeled commercially)
albite sodium feldspar Na2O, Al2O3, 6SiO2
sanidine potassium feldspar K2O, Al2O3, 6SiO2
nepheline sod-pot feldspar K2O, 3Na2O, 4Al2O3, 8SiO2
anorthite calcium feldspar CaO, Al2O3, 2SiO2
labradorite sod-cal feldspar Na2O, CaO, Al2O3, SiO2
pegmatite (*) sod-pot-cal feldspar Na2/K2O/CaO, Al2O3, 6SiO2
spodumene lithium feldspar Li2O, Al2O3, 4SiO2
celsian barium feldspar BaO, Al2O3, 2SiO2
(*) Though pegmatites are mother-racks for feldspars, and not properly feldspars, recipes listing so-called sod-pot-cal feldspars are usually asking for pegmatites, such as Cornwall Stone, for example. As well, feldspathoids are often liberally labeled commercially as feldspars, or, more accurately, as feldspathics.
typical comparative analyses
potassium feldspar K2O 16.89, Al2O3 18.43, SiO2 66.84
sodium feldspar Na2O 11.82, Al2O3 19.56, SiO2 68.82
calcium feldspar CaO 20.14. Al2O3 36.70, SiO2 43.16
pegmatite (generic) K2O 4.79, Na2O 3.92, CaO 1.44, Fe2O3 0.35, Al2O3 15.11, SiO2 74.23
pegmatite (Cornwall) K2O 3.68, Na2O 3.61, CaO 1.68, Fe2O3 0.26, Al2O3 17.37, SiO2 74.55
Feldspars are notoriously short clays, as natural clays go, which is to say that they are on the side of the less elastic clays, and subsequently are rarely used alone but rather in blends with more elastic clays to give them the manageability for a given need. So-called stoneware clays labeled exotically under names such as Rick's Mix, Helen's Biscuit, etcetera, are almost always blends of feldspars (for the benefits of more alkali and silicon) and earthenware clays (for handling
preferences prior to firing and for color after) and kaolin (for strength while firing and for color after). Feldspars usually fire to white, and many so-called white earthenware clays are in fact blends of common buff clays and large portions of feldspars and/or feldspathic clays. White translucent porcelain clays are typically blends of kaolin, feldspar and silica; the kaolin and the feldspar providing the whiteness; the feldspar and silica providing the translucency; and kaolin providing the
post to hold the vitrifying feldspar and silica upright during firing and until cooled.
At temperatures below 1190ºC feldspars are strong refractories requiring strong mid- to low-temperature fluxing agents, such as zinc, lead and/or boron, to bring their silica to a melt. At temperatures over 1190ºC the alkalis inherent to feldspars do the required fluxing, helped when and if needed by non-alkali fluxes and/or other alkalis not present in the particular feldspar(s) being used. Fusion temperatures for the great variety of feldspars are, all else being equal, directly determined by th
e type and proportion of alkalis present in relation to the silica present. Generally, potassium feldspars are the preferred forms to compose body pastes, and soda feldspars to compose glaze coverings; and, for both clays and glazes, the varieties of feldspars provide water-insoluble forms of the otherwise soluble alkalis which define the feldspars.
Magmatic/volcanic feldspars (analcime, etc.), feldspathoids (sodalite, etc.) and feldspathics (pumice, etc.), as Natures fritted feldspars, equally provide water-insoluble forms of their alkalis but also forms which usually fuse at much lower temperatures than their granite-metamorphic complements. see alkali, calcinates, clay, glaze, rock, silicates, soil, etc., as well as feldspars indivi
Feldspar is the most important body flux for cone 2+. Many clays and other body materials contain feldspar. The classic cone 10 porcelain recipe is 25% each of feldspar, ball clay, silica and kaolin.
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.
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.
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.
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.
These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved epsom salts). The nepheline is thicker. Notice the crazing. This is what feldspars do. Why? Because they are high in K2O and Na2O, these oxides have by far the highest thermal expansions. So if a glaze is high in feldspar it should be no surprise that it is going to craze also.
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