All Glossary

Ceramic Oxide

In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.

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Details

An oxide is a compound of oxygen with another element. Only a relatively small group of oxides needs to be understood for most ceramic glazes. Examples are CaO (calcium oxide or calcia), SiO₂ (silica) and Al₂O₃ (alumina). These can be supplied by many different ceramic materials. CaO, for example, can come from calcium carbonate, wollastonite, dolomite, frits and other materials.

In glaze calculation, these oxide formulas are used as chemical bookkeeping units. A recipe is typically converted into the amounts by moles of oxide equivalents such as SiO₂, Al₂O₃, CaO, MgO, Na₂O, K₂O and B₂O₃ that its materials contribute. This does not mean that the fired glass consists of separate molecules of those oxides. During firing, ceramic materials undergo many reactions. Some decompose and release gases, some react with neighboring particles, and many dissolve progressively into the developing melt. The original crystal structures of the raw materials are destroyed or altered, and their constituent elements become incorporated into the melt. On cooling, these usually form a complex interconnected glass structure, sometimes together with crystalline phases.

Glaze chemistry is therefore a way of looking past the raw-material recipe and describing the fired composition in terms of oxide equivalents. Volatile components such as H₂O and CO2 are excluded because they escape during firing. The remaining oxides can then be compared using their weights or, more importantly for many glaze calculations, their relative numbers of moles.

There are strong relationships between oxide chemistry and fired glaze properties, but individual oxides do not impose fixed properties by themselves. Their effects depend on concentration, temperature and interaction with the other oxides. Thus, glaze chemistry is most useful for comparing related compositions and understanding how changes in oxide proportions are likely to alter melting, thermal expansion, durability, crystallization, surface character and color response.

The same fired oxide composition can be expressed in several ways. An oxide weight-percent analysis states what percentage of the fired oxide total is contributed by the weight of each oxide. Mole percent expresses the relative number of moles of each oxide equivalent. A unity molecular formula (UMF) also expresses relative molar proportions, but normalizes a selected group of oxides, traditionally the fluxes, so that their total is 1.0.

Despite its traditional name, a unity molecular formula should not be interpreted as a description of actual molecules present in the glass. It is a normalized molar representation of the oxide composition. Traditional Seger notation organizes oxides into groups such as R2O/RO, R2O₃ and RO2. These categories are useful for comparing glaze formulas, but terms such as “flux,” “intermediate” and “glass former” should not be treated as fixed descriptions of what an oxide always does. Oxide behavior depends on temperature, concentration and interaction with the complete chemistry. B₂O₃ is a good example: it is a glass former, yet it also strongly promotes melting in many low- and middle-temperature glaze systems.

When relationships between oxide chemistry and fired behavior are understood, molar formulas become powerful tools for adjusting recipes. Increasing or decreasing the molar proportion of an oxide, or changing the balance among oxides, can influence melt fluidity, gloss, crystallization, thermal expansion, durability, color response and other properties. But chemistry is only part of the picture: the raw materials supplying those oxides, their particle size and mineralogy, gas evolution, firing schedule, atmosphere and degree of dissolution can also strongly affect the fired result.

“Target formulas” or “limit formulas” describe molar oxide ranges commonly associated with successful glaze families. They are useful guides, not rules. A glaze outside published limits may work very well, while one inside them may still fail to produce the desired surface or firing behavior.

Lower-temperature glazes generally require oxide combinations that produce a fluid melt more readily. In a UMF this often appears as lower molar proportions of SiO₂ and Al₂O₃ relative to the flux group and, commonly, the presence of effective low-temperature melters such as B₂O₃. Some chemistry-property relationships can be estimated numerically. Calculated thermal expansion is an important example. Empirical expansion factors are assigned to oxide equivalents and combined according to their proportions in the formula. Na₂O and K₂O, for example, almost always increase calculated expansion, while SiO₂ generally lowers it. Such calculations are especially useful for comparing related glazes, but they estimate the behavior of the fired glass rather than measuring it directly. Other relationships are more empirical. A high SiO₂:Al₂O₃ molar ratio often favors a glossy surface, whereas a lower ratio can favor matte behavior. Significant MgO can promote matte surfaces in many glaze systems. Lower Al₂O₃ commonly permits a more fluid melt, while increasing Al₂O₃ generally stiffens it.

Related Information

When both mineralogy and chemistry are shown on a data sheet

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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.

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

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Recipes show us the materials in a the glaze powder (or slurry). Formulas enumerate the oxide molecules and their comparative quantities in the fired glass. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get their oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. There is a direct relationship between fired glaze properties (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and its oxide formula. There are 8-10 oxides to know about (vs. hundreds of materials). From the formula-viewpoint materials are thus "sources-of-oxides". While there are other factors besides pure chemistry that determine how a glaze fires, none is as important. Insight-live can calculate and show the formula of a recipe, this enables comparing it side-by-side and with a target formula (or another recipe known to work as needed). Target formulas are opened using the advanced recipe search, choosing the limits batch and clicking/tapping the search button (search 'target recipe' in Insight-live help for more info).

Oxide effects on frit physical properties

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Ionic Forces Chart

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Cations having high charges and small radiuses (and thus high field forces like boron and silicon), are network formers. Network modifiers have small charges, large radiuses and a big coordination number for oxygen ions. While considering ions as rigid spheres is an over-simplified way to describe reality, it has still proven useful to describe characteristics of each. For instance lithium has a ionic radius smaller than sodium and so it can locate into smaller cavities. The ionic field force of lithium is also stronger than sodium and it is essentially non-directional, thus it more easily produces crystals of a separate phase. Alkaline earth elements locate into cavities of the network as well, but they have double charges and thus act like a bridges between two oxygen ions (preventing the three-dimensional network from being fully destroyed). Moreover bonds between alkaline earth ions and oxygen are stronger than alkaline so we observe neither a rapid decrease in viscosity or a significant increase of the thermal expansion coefficient. It is notable that for similar molar percentages, frits containing magnesium crystallize more easily than frits containing calcium.

Aluminum, titanium and zirconium are classified as intermediate glass formers because they have a strong four-way coordination for oxygen ions, like silicon. Thus, for these oxides, we do not observe any interruption of the three-dimensional silicate-based network. For a better understanding consider more details about aluminum, boron, zirconium and titanium.

Aluminum: Usually aluminum shows a four-way coordination when it acts as a glass former, tetrahedrons are linked to four oxygen atoms while the local excess of negative charge is counterbalanced by an alkaline cation placed close to the aluminum ion. Thus additions of aluminum to a glass help to stop alkaline ions from breaking the three dimensional network of the glass. This produces the characteristic lower melt fluidity and tendency to crystallize and also reduces the thermal expansion and thermal stability. One downside to alumina is that it contributes to a higher viscosity of frit batches during melting (in the furnace tank) making homogenization more difficult. Usually the percentage of aluminum oxide is in the range 4 – 12%.

Boron: Boron is a basic component of frits yet its characteristics are so peculiar that it cannot easilly be compared to other elements. Boron, like aluminum, exhibits a four-way coordination when forming a glass network (being in the center of a tetrahedron of oxygen ions). This is possible only when the molar alkaline percentage is less than 30-40% because above this limit boron has three-way coordination, forming triangles.

Another peculiar characteristic is that boron is not just dispersed as tetrahedrons or triangles in the network of silica tetrahedrons. Rather it forms boric groups, containing from 3 to 5 boron atoms and the groups are randomly dispersed in the glassy matrix. However single BO₃ triangles and BO₄ tetrahedrons are always present. For quenched frits the presence of these groups is likely minimal but we can presume they form again when glazes containing the frit are fired (there are experimental evidences demonstrating this).

Boron oxide is an important component of low melting frits because it increases fusibility without a proportional increase in thermal expansion. Moreover boron oxide and sodium borate, due to their low melting point, are useful during smelting of frits because they form a glassy matrix early and act as catalysts in the melting and dissolving of other materials.

Zirconium - Titanium: Their influence on surrounding oxygen ions is very strong and scarcely directional so their solubility in frits is poor. Their solubility in glass and actions as glass formers are proportional to temperature. In quenched frits they remain in the dissolved in the glassy matrix but when we fire them again (within a glaze), these oxides easily precipitate crystal compounds.

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Ceramic Oxide Periodic Table

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