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When ceramic materials are "understood" it is possible to control the properties of the bodies and glazes they are used in. Characterizing them is about understanding them. It is about being able to describe what a materials is in terms that will enable a user to determine its suitability (often compared to alternatives rather than in absolute terms). Product data sheets highlight properties of a material that are of interest to its users. It is common for the same material to be sold to different markets and be characterized in a unique way for each one (emphasizing only pertinent properties). All of this being said, we believe that data sheets full of numbers are not an inadequate way to understand a material, especially a clay. These numbers often seem a poor description for a material that can take many years to learn to use. Attempting to compare different materials by these numbers alone can be a frustrating experience. At times it even appears that companies do not really understand a product they themselves are manufacturing for ceramics!

In glazes, the focus is normally on the chemistry of the materials since this has the biggest effect on fired properties. Frits, for example, find their entire merit in their chemistry and switching from one to another is all about how similar that chemistry is (or your ability to do glaze chemistry to juggle recipe ingredients to compensate for a new material that does not have the same chemistry). Feldspars are a similar story. But in clay bodies, the physical and fired properties are much more easily related to the physical properties of the materials in their makeup.

The ceramic world functions on "recipes" and often much less effort is put into understanding them that should be. An example of this is the pursuit of substitutes for materials in recipes. These substitutes can be straight -forward (e.g. switching one source of silica to another) but they often come with a complicated list of trade-offs. For example, switching from a English kaolin to an American one in a porcelain recipe might seem simple but it is not, these are quite different materials. One needs to consider impacts on body plasticity, degree of maturity (with associated fired hardness and durability, stickiness, fired color, translucency, effect on thermal expansion and drying performance).

Substituting materials becomes more complicated for secondary clays. These have chemistries, but often the chemical makeup is more difficult to connect with the physical and firing behaviour. This is often because the materials are not finely ground and their powders have populations of a variety of difficult mineral particles (which interact in complex ways). It is common for people to substitute materials in recipes simply because they have similar-sounding names! Red-burning clays can be particularly problematic. At high temperatures red-burning stonewares depend on a recipe that contains mostly refractory materials and a controlled amount of flux (e.g. a feldspar or high-feldspar clay). The color is achieved by finding a balance between an adequate degree of vitrification (for fired density and strength) but not too much (or the clay turns brown). If a refractory red burning clay (in the recipe) is switched from low fire red (e.g. a terra cotta material like RedArt) then the red color will be lost (the body will fire brown).

The most practical way to characterize clay materials is by:
-Firing test bars at various temperatures to profile the color, fired shrinkage and porosity (e.g. the SHAB test).
-Measuring the dry strength, dry shrinkage and drying performance (e.g. the DFAC test).
-Measured the particle size distribution (e.g. the SIEV test).
-Making ware using the material pure.

While the chemistry of glaze materials is their most important characteristic, it is also important to consider their other properties (this can be a determining factor in a choice). For example, feldspar and kaolin source Al2O3, but the kaolin suspends the slurry and hardens the glaze so at least 15% of it is needed. Calcined alumina also sources Al2O3 but its physical form is highly refractory and it does not dissolve into the melt readily. Talc and dolomite both source MgO, wollastonite and calcium carbonate both source CaO, but the first two have a lower LOIs.

Out Bound Links

  • (Articles) Low Budget Testing of the Raw and Fired Properties of a Glaze

    There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.

  • (Glossary) Physical Testing

    Ceramic engineers and technicians must take control of their production processes to trouble-shoot and have fewer problems, be able to adjust or improve bodies and glazes and formulate new ones. A long term program of quality control measurements is an essential context in which to deal with anomali...

  • (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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