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 high temperature 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, for use in glazes, we can view feldspars as 'warehouses of oxides' (e.g. because they supply K2O, Na2O, Al2O3 and SiO2 to the glaze melt). For this reason other materials like cornwall stone and nepheline syenite, which have similar chemistry, can be viewed as feldspars.
'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.
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.
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.
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.
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. The Montana materials on the right are not commercially available, they were being evaluated for viability.
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.
Crackle glazes are used on decorative ceramic ware. They have a crack pattern that is a product of thermal expansion mismatch between body and glaze.
|Materials||Feldspath ICE 10|
|Materials||Ferro Frit 3110|
|Materials||Nepheline Syenite Norwegian|
|Materials||Kona F-4 Feldspar|
|Oxides||KNaO - Potassium/Sodium Oxides|
|Oxides||SiO2 - Silicon Dioxide, Silica|
|Oxides||Li2O - Lithium Oxide, Lithia|
|Oxides||Na2O - Sodium Oxide, Soda|
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
Powdering, Cracking and Settling Glazes
Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type of clay.
|Media||Mica and Feldspar Mine of MGK Minerals|
|Media||Desktop Insight 1A - Compare Theoretical and Real-World Feldspars|
|Media||A Broken Glaze Meets Insight-Live and a Magic Material|
|Suppliers||A/S Norsk Feldspat Company|
Article about cristobalite in clay bodies
Feldspar at Wikipedia
Wikipedia Feldspar Page
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
MgO is the secret weapon of craze control. If your application can tolerate it you can create a cone 6 base glaze of very low thermal expansion that is very resistant to crazing.
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.
Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
A Low Cost Tester of Glaze Melt Fluidity
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Are Your Glazes Food Safe or are They Leachable?
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
The most common source of fluxes for high and medium temperature glazes and bodies.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
The most common source of fluxes for high and medium temperature glazes and bodies.
|Body Maturity||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.|