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An oxide is a combination of oxygen and another element. There are only about ten common oxides that we need to learn about (most glazes have half that number). CaO (a flux), SiO2 (a glass former) and Al2O3 (an intermediate) are examples of oxides. CaO (calcium oxide or calcia), for example, is contributed by calcium carbonate, wollastonite and dolomite.

The heat in the kiln "decomposes" materials into their oxides. This decomposition breaks the bonds that held the oxides together in their materials creating a liquid "melt" (or mixture) of unbonded oxide molecules (ready to form a new compound, normally a glass, when the kiln cools). This phenomena occurs because the molecular bonds that hold oxides together are stronger than those that hold oxides one-to-another.

There are direct links between the oxide chemistry of a glaze and its fired presence, each oxide imposes specific properties (according to its concentration) on the fired glass. Its effects can be modified by interactions. Glaze chemistry is about looking at mixtures of raw material powders as if they were already the fired glaze. It does an inventory of all the oxides being contributed by all the materials, discards the ones that will gas off during firing (e.g. CO2, SO4, H2O) and tries to visualize what glass will be like.

A glaze "chemical formula" expresses relative oxide molecule numbers in the fired glass (a "chemical analyses" expresses the concentration of each oxide, by weight of molecules, in a material). Oxides are divided into three categories that recognize their functions. There is a correlation between the amount of oxygen in each class and the contribution that class of oxide makes. Fluxes are designated RO, intermediates R2O3 and glass formers RO2. When the links between how a glaze fires and its oxide formula are well understood, the glaze formula can be manipulated to produce the desired changes in the fired properties.

"Target Formulas" (or limit formulas) characterize the balances of oxides in a type of glaze (e.g. low temperature, crystalline, matte). However limit formulas are general guides only, they express what is likely to melt into a usable glaze.

Glazes for lower temperatures have more flux. We see that in unity formulas as lower SiO2 and Al2O3. Some of the properties that oxides contribute can be quantified. Thermal expansion is an example. Na2O, for example, has a very high expansion. SiO2 is low. We know the thermal expansion of each oxide, thus it is fairly simple to tabulate their expansions, according to percentage, to predict the thermal expansion of the glaze (high expansion glazes craze). When the SiO2:Al2O3 ratio is high (e.g. 12:1) the glaze will likely be glossy (matte when low). If MgO is 0.3 or higher (in a unity formula) the glaze will likely be matte. If Al2O3 is low the glaze melt will be runny. If SiO2 is also low it will melt at a low temperature.

When both mineralogy and chemistry are shown on a data sheet

When both mineralogy and chemistry are shown on a data sheet

Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.

Ceramic Oxide Periodic Table

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).

A limit or target glaze formula. What does this mean?

A limit or target glaze formula. What does this mean?

Recipes show us the materials in a glaze but formulas list oxide molecules and their comparative quantities. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get the oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. You can view glazes as recipes-of-materials or as formulas-of-oxides. Why use formulas? Because there is a direct relationship between the properties a fired glaze has (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and the oxides it contains (links between firing and recipe are much less direct). There are 8-10 oxides to know about (vs. hundreds of materials). From the formula viewpoint materials are sources-of-oxides. While there are other factors besides pure chemistry that determine how a glaze fires, none is as important. Insight-live automatically shows you the formulas of your recipes and enables comparing them side-by-side. Click the "Target Formula" link (on this post at digitalfire.com) to see what each oxide does.

Out Bound Links

  • (Oxides) KNaO - Potassium/Sodium Oxides
  • (Oxides) Al2O3 - Aluminum Oxide, Alumina
  • (Oxides) CaO - Calcium Oxide, Calcia
  • (Oxides) SiO2 - Silicon Dioxide, Silica
  • (Oxides) B2O3 - Boric Oxide
  • (Oxides) K2O - Potassium Oxide
  • (Oxides) MgO - Magnesium Oxide, Magnesia
  • (Oxides) Na2O - Sodium Oxide, Soda

In Bound Links

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  • (Glossary) Formula

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  • (Videos) Lesson 1A - Comparing a Theoretical and a Real-World Feldspar

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  • (Glossary) Mineralogy

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  • (Glossary) Target Formula, Limit Formula

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  • (Glossary) Analysis

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  • (Glossary) Decomposition

    Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view). "Elements" are one kind of atom which cannot be broken down any further (except b...

  • (Articles) Where Do I Start?

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  • (Glossary) Glaze Chemistry

    Glaze chemistry is learning what each oxide does in a fired glaze and the relative advantages and disadvantages of each material supplying it. The chemistry of a glaze is expressed in a manner similar to its recipe, except that the items are oxides and the amounts can be by weight (an analysis) or n...

By Tony Hansen

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