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Conceptually we consider fired glazes as being composed of 'oxides'. Materials supply those oxides to the melt. The ten major oxides likely make up 99% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires.

Thus we can view materials as simply suppliers of the oxides needed to get the desired fired result. Well, actually, it is not quite so simple. While frits have a wide range of chemistries and melt nicely to yield their oxides to the melt, it is not the same story with many raw materials. Some of them do not melt nearly so well. Wollastonite, for example, is not quite as ready to dissolve in the melt as calcium carbonate. Some materials melt poorly by themselves but are active in the presence of others. Some materials are extremely refractory and very difficult to dissolve into the melt (e.g. calcined alumina). Knowledge of material specific melting is needed to rationalize how to apply chemistry, especially when recipes being compared draw on different sets of materials to source the chemistry.

An analysis (or percentage analysis) is typically used to express the oxide content of materials. An analysis is expressed in percent and each number represents the amount by weight of the oxide (in the raw material, not the fired material). Thus, as appropriate, an analysis will often show LOI (loss on ignition) as one of the items, this represents the volatile portion of the material that is lost during firing. Sometimes the volatiles are specified (e.g. H2O, CO, CO2, SO2).

An analysis can be converted to a formula by dividing each oxide amount by the molecular weight of that oxide. The formula can optionally be unified after that.

An "analysis" compares oxides by the weights of their molecules (not the numbers of molecules as does a formula or mole% formula). Thus an analysis comparison of the oxides in a material can look quite different from a mole% comparison, this is because oxide molecule weights differ greatly. The analysis format is best suited to showing how much of each individual oxide is in a mix. For example, feldspars are used as a source of flux, although they also provide SiO2 and Al2O3. A percentage analysis enables a buyer to compare how much flux each brand has.

As noted, material analyses include organics, water, and additives which are burned away during firing. For example, if a material loses 10% by weight on firing, we can just say the LOI (Loss on Ignition) is 10%. Or we could say it loses 5% H2O and 5% SO2 (10% LOI can only be expressed in a formula by increasing its formula weight, but this is not obvious to the reader unless they know what the formula weight would be if there was no LOI). As noted, the formula is ideal to express the mix of oxides in a fired ceramic where there is no LOI. But the analysis has become a standard used to express the make-up of raw clay materials on manufacturers data sheets. If you wanted to express the makeup of a premixed raw glaze powder, it would be appropriate to do so as an analysis also (unless it had no LOI).

Here is how we would convert a unity formula into an analysis (each amount is being divided by the formula weight of that oxide, then by the total):

Oxides Formula Weights                  Percent
K2O 0.27 x 94.2 = 25.43 / 353.78 = 7.19%
CaO 0.59 x 56.1 = 33.10 / 353.78 = 9.36%
MgO 0.09 x 40.3 = 3.63 / 353.78 = 1.03%
ZnO 0.05 x 81.4 = 4.07 / 353.78 = 1.15%
Al2O3 0.41 x 101.8 = 41.74 / 353.78 = 11.80%
SiO2 4.09 x 60.1 = 245.81 / 353.78 = 69.48%
Formula Weight 353.78

Example of a whole rock chemical analysis lab report

Powdered samples were sent to the lab. The numbers shown on this report are in percentage-by-weight. That means, for example, that 15.21% of the weight of the dry powder of Alberta Slip is Al2O3. Insight-live knows material chemistries in this way (whereas desktop Insight needs them as formulas). Some non-oxide elements are quantified as parts-per-million (these amounts are not normally high enough to take into account for traditional ceramic purposes). The LOI column shows how much mechanically and chemically bound water are gassed off during firing of the sample. The total is not exactly 100 because of inherent error in the method and compounds not included in the report.

Desktop INSIGHT showing formula and analysis side-by-side

Desktop Insight was the first to enable users to compare two recipes and their formulas side-by-side and interactively update when recipe changes were made. It also enabled users to show formulas and analyses side-by-side.

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.

A Limitation of the Seger Unity Formula

Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide a different system with its own relative chemistry is entered. An extreme example of this would be to source Al2O3 to a glaze using calcined alumina instead of kaolin. Although the formula may be exactly the same, the fired result would be completely different because very little of the alumina would dissolve into the glaze melt. At the opposite extreme, a different frit could be used to supply a set of oxides (while maintaining the overall chemistry of the glaze) and the fired result would be much more chemically predictable. Why? Because the readily and release their oxides the the melt.

Whole rock analysis show some as percent, some as PPM

The ppm items are not oxides, they are elements. Ba for example, is shown as 4276 ppm. We do not know the form. It could be barium sulphate, barium carbonate, barium nitrate, barium chloride. But altogether they supply this amount of Ba. The same is true of chrome, strontium, nickel and vanadium.

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.

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By Tony Hansen

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