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Understanding Ceramic Oxides
Section: Glazes, Subsection: Chemistry
Fired glazes are composed of oxide building blocks. Each of the oxides contributes different properties to the fired glaze and interacts with others in different ways. Understanding these gives you control.
The ancient Chinese thought of glazes as being made of bones, flesh, and blood. They were very perceptive! The silica content of a glaze acts like a framework, the alumina acts to give it body, and the fluxes melt it and impart character acting as the lifeblood.
In understanding fired properties, it is helpful to view materials as sources of oxides. However, remember that although it is chemically possible to supply a given oxide from many materials, often other factors make one of them preferable. For example, kaolin is an ideal source of alumina since it also imparts suspension and hardening properties to the glaze slurry.
Following is a list of the major oxides. There are a multitude of textbooks with more information on this subject. However, the data is widely scattered and thus difficult to study. The best source of information is the oxides area on this website.Not only is each oxide described in detail but properties are assigned so that it can be searched by category. For example, if you need to produce purple you can look up the 'purple' property area and see a list of all the chemistries that produce it.
- SiO2 - Silicon dioxide is supplied by silica, feldspar, and kaolin. It is the principle glass forming oxide and normally comprises more than 60% of most glazes and clays. It has a low expansion and high melting temperature.
- Al2O3 - Aluminum oxide is supplied by kaolin and feldspar. It combines well with silica and basic oxides to give body, durability, and stability to glazes. It has a low expansion and high melting temperature.
- B2O3 - Boric oxide is supplied by borax frits, gerstley borate, (or colemanite). It is a low temperature low expansion equivalent of silica.
- CaO - Calcium oxide is supplied by whiting, wollastonite, feldspar, colemanite, or dolomite. It is the principle flux in medium and high fire glazes. In higher amounts it can give matte qualities through the formation of calcium silicate crystals.
- K2O - Potassium oxide is supplied by potash feldspar and cornwall stone. It is an important auxiliary flux in high temperature glazes. It has a high expansion.
- Na2O - Sodium oxide is supplied by feldspar, nepheline syenite and sodium frits. A slightly more powerful flux than potassium for high temperature glazes. It has a high expansion.
- MgO - Magnesium oxide is supplied by talc or dolomite. At lower temperatures, it is a matting agent and opacifier; at higher temperatures, it is an active alkaline flux.
- Fe2O3 , FeO, Fe3O4 - Ferrous-ferric oxide is supplied by iron oxide and stained clays. It is the most popular colorant. In a reducing atmosphere, it can act as a flux in both bodies and glazes at high temperatures.
- ZnO - Zinc oxide is available in a pure state. It is a low expansion secondary flux, in moderate to higher amounts it acts to produce mattes and crystalline surfaces.
- TiO2 - Titanium dioxide is available pure or in rutile. It is a complex material because of its opacifying, crystallizing, and multitude of color responses.
- ZrO2 - Zirconium oxide is available in a pure state and is supplied by zircon opacifiers. It is used to opacity glazes. Zirconium has a very low thermal expansion.
- BaO - Barium oxide is supplied by barium carbonate. It is a flux which encourages the growth of micro-crystals to produce attractive satin-matte surfaces. It also has unique color responses.
- Li2O - Lithium oxide is supplied by lithium carbonate or lithium feldspar (i.e. spodumene). It is a powerful auxiliary alkaline flux. Its expansion is lower than soda or potash.
- PbO - Lead oxide is supplied by lead frits. It reacts easily with silica to form low melting lead silicates of high gloss and deep character.
- SrO - Strontium oxide is supplied by strontium carbonate. It has matting and crystallizing properties similar to barium although it produces brighter and more fusible glazes with fewer surface defects.
Ceramic Oxide Periodic Table
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
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By Tony Hansen
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