Introduction

Formulating and adjusting ceramic glazes can be approached on different levels: The pure oxide, the oxide in the presence of others with which to interact, the mineral, the material, the recipe and the process. To understand what you are doing you first separate the problem into aspects that relate to each of these levels. Working on the right level, or mix of levels, is the the key to understanding and solving many problems.

Some oxides exist as physical powders we can add to a recipe, but oxides in the context of glaze chemistry is more of a theoretical concept. In this model a fired glaze matrix is viewed as a construct of interacting oxides and the properties it displays are rationalized in terms of the oxide proportions and what we know about what each contributes and how they interact. Materials are viewed as warehouses that supply these oxides to the glaze melt. Oxides within this model are not the same as those outside of it. For example, pure CaO powder is among the most refractory of all physical oxides. Yet within a glaze melt, in the presence of other oxides with which to interact, it really knows how to party: it is a very strong flux! Sometimes textbooks will provide data on the properties of individual oxides. For example, melting behaviour might be quoted as a reason for firing in a certain way. However, it is important to realize that oxides are "packaged" within materials that we use in ceramics, those materials are normally sourcing other oxides along with them. While an individual oxide might melt at a certain temperature, the individual particles of that material may not release the oxide to participate in the chemistry until a much higher temperature. Or, they might release it even lower. Additionally, interactions between it and particles of other materials will affect the manner of melting and dissolution.

Oxide chemistry is not normally factored into understanding what bodies do when fired, it is difficult to draw relationships between the chemistry of vitreous bodies and their physical fired properties. This is because bodies are not melted during firing as are glazes, normally firing creates conditions of crystal growth in the body. Thus bodies of similar chemistry can develop completely different crystalline matrixes and therefore different physical properties (depending on firing and mineralogy of ingredients).

At the oxide level we study the relationships between the fired properties of glazes and their chemistry (assuming all oxides have gone into solution in the melt). Understanding what each of the oxides is bringing to the chemistry is one of the most important aspects to controlling the fired properties of glazes (on this level there are complicating factors of oxide interaction, issues relating to the sourcing of the same oxide from different materials, non-linear property changes with changes in proportion).

Chemistry needs to be put in context with the other levels. For example, consider a glaze that is crazing: If it contains a lot of Na₂O, then it has a high thermal expansion, that is almost certainly the cause. Glaze chemistry is needed to reduce the amount of sodium and substitute another flux of lower thermal expansion (which of course is also sympathetic to the glaze color, surface character, melting temperature, etc.). Nepheline syenite, for example, contributes Na₂O. Glaze chemistry software can be employed to reduce the amount of nepheline (therefore the Na₂O) and source the other oxides it was contributing from other materials. Doing this actually helps alleviate a mineral level issue also: the mineral nepheline is slightly soluble over time and excessive amounts can flocculate glazes if they are stored (thus they require more water and then begin to settle out in the bucket).

Many ceramic oxides have mineral and even material equivalents, others do not. Some have forms we can buy and touch (like silica), others are available but in an impractical form (expensive, soluble, unstable, difficult-to-get). The mineral quartz, for example, is almost pure SiO₂. We usually just refer to the oxide SiO₂ as silica (although it is more correctly termed silicon dioxide). The powdered material is the silica that we use in glaze and body recipes is ground quartz, thus it is also SiO₂. If the silica powder is being put into a glaze and completely dissolves and reacts then it will impose a lowering effect of the thermal expansion. Documentation about this belongs at the oxide area of this website because it is the SiO₂ molecules of which the material is composed that are effecting the change. However, discussion about the use of silica in bodies belongs in the materials area since it is seen, not as an addition of chemistry, but as an aggregate, a filler and a source of glass. Theoretical information about the mineral quartz (e.g. its thermal expansion, hardness) belongs on the mineral level (since there are other pure mineral sources of SiO₂). Then, of course, there are issues related to particle size (which determines how well the quartz grains can dissolve in a glaze melt), cost and supply, these belong on the material level. This site has a flexible linking system between all areas and records (which we regard as the most value aspect of the database) and we are constantly working on improving the links.

Links

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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