Conceptually we consider fired ceramic glazes as being composed of 'oxides' (materials contribute these). 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 (provided all the materials in the glaze dissolve in the melt). That means we can predict what will happen during firing and we can propose changes that have a high likelihood of fixing a problem or moving a glaze property in a certain direction. The chemistry of glazes are normally expressed as formulas (sometimes people refer to "glaze formulas" when what they actually mean is "glaze recipes").
An example of a raw formula:
Notice in the above that oxides are grouped into three columns: the bases, acids, and amphoterics (or simply as the RO, R2O3 , and RO2 oxides; where "R" is the element combining with oxygen). Actually, the ratio of R to O is significant. The right column has the greatest oxygen component, the left has the least. Simplistically, we can view these three groups as the silica:alumina:fluxes system. This latter convention is not really correct because there are more glass builders than SiO2 , other intermediates besides Al2O3, and the RO's do more than just flux. But because this method evokes immediate recognition, let's use it anyway. Ancient potters referred to these three as the blood, flesh, and bones of a glaze (not a bad way to think of it).
Any formula has a formula weight, that is, the total calculated weight for that mix of molecules. Atomic weights are published in any ceramic text so it is easy to calculate the weight of each oxide. Calculating the weight of the whole is just a matter of simple addition and then increasing that weight to account to LOI.
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.
Ceramic glaze oxides periodic table
All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
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.
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
In Bound Links