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Feldspar

Oxide Analysis Formula
K2O 16.92% 1.00
Al2O3 18.32% 1.00
SiO2 64.76% 6.00
Oxide Weight 556.74
Formula Weight 556.74

Notes

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.

Alert
'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.

Related Information

Closeup of feldspar deposit in the MGK Minerals quarry in India

Augite Feldspar

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 with Hornblende

Laboradite Feldspar

Labradorite Gem Feldspar

Corundum On Feldspar

Beryl Feldspar Mica

Which common American/Canadian feldspars can substitute for each other?

After comparing the chemistries of an original feldspar and a tentative substitute, these melt flow tests are an excellent way to confirm physical similarity also. These were done at cone 6 (2200F). Each feldspar is mixed with 15% Ferro Frit 3195. Some things to note: Nepheline Syenite is the champion melter. Mahavir is very similar to G200. Kingman and Custer are very similar. Our Minspar substitute is very similar to Minspar itself.

Pure Custer Feldspar and Nepheline Syenite on cone 10R porcelain bodies

These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved epsom salts). Notice the crazing (feldspars always craze because they are high in K2O and Na2O, these oxides have by far the highest thermal expansions).

Tourmaline On Feldspar

Orthoclase Feldspar Rock

Labradorite Feldspar

Feldspar melts by itself to be a glaze, but crazes badly

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.

Stockpile of crude feldspar from MGK Minerals (India) deposit

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). Feldspars have high KNaO, that means they have high thermal expansions. That is why high-feldspar glazes often craze.

Aquamarine On Feldspar

Links

Glossary Feldspar Glazes
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.
Glossary Crackle glaze
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 Kona F-4 Feldspar
Materials Minspar 200
Materials Pegmatite
Materials Kaolin A
Materials Feldspath ICE 10
Materials Ferro Frit 3110
Materials Nepheline Syenite Norwegian
Materials Calcium Feldspar
Materials Potash Feldspar
Materials Soda Feldspar
Materials Custer Feldspar
Materials Spodumene
Materials Nepheline Syenite
Media A Broken Glaze Meets Insight-Live and a Magic Material
Use Insight-Live.com to do major surgery on a feldspar saturated cone 10R glaze recipe with multiple issues: blistering, pinholing, crazing, settling, dusting and possibly leaching!
Media Desktop Insight 1A - Compare Theoretical and Real-World Feldspars
While comparing a real-world and theoretical feldspar learn to enter, edit, save, normalize recipes and the materials dialog. Glaze chemistry concepts.
Media Mica and Feldspar Mine of MGK Minerals
URLs http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
Article about cristobalite in clay bodies
URLs https://www.youtube.com/watch?v=FVUuhjnjAoA
German potter Cornelius Breymann investigates limit formulas, eutectics
On this Youtube video Cornelius will take you on a slow and deliberate journey. If you stick with him you will discover how, by industrious blending of feldspar, calcium carbonate and silica we can see what ratios of CaO, SiO2 and Al2O3 (and the materials sourcing them) produce a well-melted high temperature glaze. You will see how the process demonstrates where feldspar comes up short as a glaze by itself and what it needs to be one. And you will see the CaO:SiO2:Al2O3 eutectic demonstrated.
URLs http://en.wikipedia.org/wiki/Feldspar
Feldspar at Wikipedia
URLs http://en.wikipedia.org/wiki/Feldspar
Wikipedia Feldspar Page
Hazards Feldspar
Articles 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.
Articles 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.
Articles 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.
Articles 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.
Articles 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.
Articles 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.
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
Troubles Glaze Crazing
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.
Troubles 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.
Typecodes Feldspar
The most common source of fluxes for high and medium temperature glazes and bodies.
Typecodes Generic Material
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.
Typecodes Feldspar
The most common source of fluxes for high and medium temperature glazes and bodies.

Mechanisms

Body MaturityFeldspar 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.

By Tony Hansen


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