In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.
Key phrases linking here: characterization, characterizing, characterize - Learn more
The first hundred pages of almost any textbook on ceramic manufacturing are on clay properties. Material knowledge is thus all important. When ceramic materials are "understood" it is possible to control the properties of the bodies, glazes and engobes used in traditional ceramics. Characterizing materials is about understanding them. It is about being able to describe what a material is in terms that enable the determination of its suitability (often compared to alternatives rather than in absolute terms). Product datasheets typically highlight material properties of interest to a specific market. But these are often focussed at purchasing agents who want absolute numbers to compare brand names. But to technicians, data sheets full of numbers and acronyms often seem inadequate. At times it even appears that companies do not really understand products they manufacture for ceramics!
The ceramic world functions on "recipes" and often much less effort is put into understanding them than should be. An example of this is the pursuit of substitutes for materials. These substitutes can be straightforward (e.g. switching one source of silica to another) but they often come with a complicated list of trade-offs. For example, switching from an English kaolin to an American one in a porcelain: These are quite different materials. One needs to consider impacts on body plasticity, stickiness, drying performance, degree of maturity (with associated fired hardness and durability, fired color, translucency, thermal expansion).
Substituting materials becomes more complicated for secondary clays. Although these have chemistries, the chemical makeup is more difficult to connect with the physical and firing behavior. This is often because the materials are not finely ground and their powders have populations of a variety of different mineral particles (which interact in complex ways). It is common for people to substitute materials in recipes simply because they have similar-sounding names! Consider red-burning stonewares: They depend on recipes that contain refractory red clays and a controlled amount of feldspar or high-feldspar clays. Using low fire red clays with less feldspar Fired color is achieved by finding a balance between vitrification (for fired density and strength) and refractoriness (for red rather than brown coloration).
Consider a good example of the value of characterization of clays: Suppose you need a plastic body that fires around 1200C (cone 6). You have a fine-grained silty clay that is somewhat plastic and fires more vitreous than required. You also have a second fine-grained clay is highly plastic and is too refractory for the required temperature. That makes it likely that a mix of these two materials will produce a usable body. Studying data sheets of these two materials would not make that evident, but knowing them, via your own characterization efforts, would make it so.
Textbooks treat the subject of characterization in an intimidating way: They list a range of lab equipment used. Terms like “photon scattering”, “RM diffraction”, “scanning microscopy” or “mass spectrometry” can seem pretty distant and expensive when all one needs is simple physical tests. Knowing about these things typically falls in the realm of people designing the process not using it. The test procedures I am about to recommend will not be listed or studied in any college course. That being said professors are pointing students here for practical testing methods.
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).
-Measuring the particle size distribution (e.g. the SIEV test).
-Making ware using the material pure.
In glazes, the focus is often on the chemistry of the materials. 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 what oxides they source. Feldspars are a similar story.
While the chemistry of glaze materials is their most important characteristic, it is also important to consider other properties. Consider some examples. Feldspar and kaolin source Al2O3, but the kaolin suspends the slurry and hardens the dried glaze (at least 15% of it is normally 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 (respectively) have lower LOIs.
These are raw clays behind the Plainsman Clays plant. The top one is a middle temperature stoneware. All it needs is a little bentonite (about 2-3%) to be a plastic, smooth, vitreous throwing body. If it was not mature at cone 6, that would be easy to fix by the addition of a little feldspar. Any fired-speck-producing impurities can be removed by using a propeller mixer to slurry it and then putting it through a screen (e.g. 60 mesh). After dewatering on a plaster table I am ready-to-go. And that bottom pile? That is the main ingredient in Ravenscrag Slip. All it needs is some feldspar and frit to be a base glaze at cone 6. It is non-plastic and easy to screen (although not really needed since it has few particulate impurities). Likely there are clays in your area you could use to make your own clay bodies and glazes also. The key is to characterize the material first so you know what type of body it would be best for and what to add to get it there.
Let's suppose you need strength and density for utilitarian ware. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.
If you are trying to use local clays for brick or tile or even pottery production, characterizing the available materials is the first step. But how? This is the kind of data a lab might submit and perhaps you wonder about its value? We feel traditional ceramics technology is fundamentally relative. A history of many reports like these, in context with other data, might be good for mining companies to determine if new stockpiles have any shifts in certain specific properties. But as a way to understand the utility of a clay for a specific purpose, this contextless report is of little use. It is also a tunnel vision view, looking at only one temperature. On the other hand, simple procedures like the SHAB test provide a hands-on way to understand what a clay actually is.
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Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
Formulating a body using clays native to your area
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In ceramics, glazes and bodies have a chemistry, a mineralogy and a physical presence. All of these need to be understood to adjust and fix issues.
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
A term used by potters and in the ceramic industry. It refers to the earthenware, stoneware or porcelain that forms the piece (as opposed to the engobe and covering glaze).
A clay that a potter finds, tests and learns to process and use himself. To reduce the costs of importing materials manufacturers, especially in Asia, often develop processes for clays mined in their locality.
|By Tony Hansen
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