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The term 'ball' traces to historic mining in England where large chunks of the clay were cut from the bank in ball shapes for transport to processing.
There are hundreds of different ball clays available and they vary widely in plasticity, particle size, raw color, and drying properties. A typical ball clay powder is light grey (from lignite) or cream color and fires to a buff or cream white color with some soluble salt deposits on the fired surface. While potters only buy ball clay as a powder, industrial users have access to the material in many forms (shredded, noodled and partially deflocculated, chopped filter press cake, vacuum pugged extrusion, crumble with about 10% moisture). These forms enable a more consistent supply and alot of flexibility, especially when making casting slips (since ball clays are the most difficult to disperse materials in the bodies).
Ball clays are very plastic and much finer grained than kaolins. They are easily slaked in water when dry. Few people fully appreciate how 'sticky' and plastic these materials are until they mix some with water and work with it pure. The fine particle size also makes them impermeable to the passage of water (a small test bar can take a very long time to dry).
They are typically unvitrified at cone 10. There are a wide range of ball clays used in traditional ceramic manufacture in North American and they have surprisingly similar firing characteristics (maturity and color). In one test of 6 different ball typical clays from a range of suppliers (blended with 50% feldspar and fired from cone 6 to 10), all had the same porosity and fired appearance. Drying shrinkage and water of plasticity also was quite similar. Ball clays intended for casting have a lower drying shrinkage but also fire very similar.
Ball clays are used in ceramic bodies (porcelains, stonewares and earthenwares, casting slips, pressing bodies) because of their plastic nature combined with high firing temperature. Ball clays have very high dry shrinkage combined with high green strength and slow drying. Were it not for their iron and coal impurities, ball clays would be ideal ceramic materials. However, in practical terms, they are employed to achieve desired plasticity, but are minimized to reduce the detrimental effect on fired whiteness and drying properties.
A common starting recipe for a high temperature general purpose porcelain (as is used in electrical porcelain or extruded pottery porcelain is 25% each of ball clay, kaolin, feldspar and silica). The ball clay:kaolin mix can be altered to change body plasticity without significantly affecting the maturing temperature.
In North America, most commercial ball clays are mined in the southeastern US. Ball clay deposits are common and were laid by the action of slow moving water with an acidity that tended to flocculate and settle the clay. It is common to find lignite associated with ball clay, and this accounts for the almost black appearance of many varieties (when wet).
Ball clays tend to be quite refractory (PCE 28-34) and some dirtier deposits are sold as fireclays. Ball clay is not a clay mineral in itself, but contains other minerals, primarily kaolinite (but also montmorillonite, halloysite, and illite). Mica and quartz are also normally present in substantial amounts (e.g. 10-20% for Tennessee ball clays).
Ball clays vary widely in their plasticity, and it is difficult to compare them by quantitative tests because pure samples are difficult to mix and form and crack badly during drying. Thus, it is common to mix ball clay and silica 50:50 and prepare dry shrinkage, dry strength and fired strength bars (from this mix) for comparative testing. Another technique to produce a workable material is to calcine half of a sample to destroy its plasticity, then mix virgin:calcine 50:50. However most technicians find that silica dilution is advantageous for comparing color and solubles contents.
In general it may be said that English ball clays tend to have a higher dry strength (and thus drying shrinkage) than American ones, Kentucky ball clays have the lowest carbonaceous matter, English ones vitrify lower, Tennessee ones fire whitest.
Although some ball clays resist deflocculation because of hostile soluble impurities, most deflocculate very well with sodium silicate and other equivalent dispersants. A wide range of ball clay slurries and slips are used at all temperature ranges in casting processes. One common recipe uses a simple 50:50 ball clay:talc mix. This recipe and close derivatives are used in large quantities in the hobby casting market. The same mix is also dry pressed in the tile industry, and extruded for jiggering and wet processing in artware.
The refractories industry is a large user of ball clay. Common refractory materials lack plasticity and ball clay is used to help in forming and shape retention and to impart dry strength. The abrasives industry likewise uses it to bond aggregates until firing fuses the mass.
Engobes in the tile and brick industries are suspended, hardened, and adjusted to match body shrinkage by the addition of ball clay. Many pottery glazes contain ball clay to help suspend and harden them and control their shrinkage during drying (although some technicians prefer cleaner kaolins for this, claiming they gel the slurry better and prevent drip-drip during drying). It makes a big difference what ball clays are used, some produce a syrupy mess that settles while others produce a beautifully suspended creamy slurry. In North America, Old Hickory #5 and Old Hickory No. 1 Glaze ball clays work well in glazes. 10-15% ball clay should be enough to suspend a glaze, if there is any less add 1-2% bentonite. Recipes with 20% or more ball clay risk shrinking and cracking during drying.
If the iron or lignite content of ball clay is a problem, it is common to employ bentonite to reduce the ball clay requirements (5% bentonite can provide as much improvement in plasticity and dry strength as 25% ball clay). However, care is recommended to make sure a fine grade of bentonite is used to avoid fired specks (bentonite also burns darker).
Unlike a kaolin, it is difficult to establish a generic or theoretical analysis, we have provided one for a typical Kentucky ball clay.
If you use ball clay in your production there is good reason to be doing routine quality control to make sure it is remaining consistent. Ball clays are likely the most variable material you will have to deal with. They can sometimes have particulate impurities (especially lignite) and exhibit differences in soluble salts content, drying shrinkage, drying performance, fired maturity, fired color and behavior in slurries. Consider the SHAB test (see link below).
Adequate quartz content is an important factor in porcelain and whiteware bodies (it is an important structural element in the fired matrix and it is needed to prevent glaze crazing). Since ball clays contain quartz, it is possible to use less raw silica (quartz) powder in the recipe. Of course, the quartz grains in the ball clay are finer, so they will dissolve into the feldspar glass more readily.
Example of four different north American ball clays fired to cone 10R, cone 11 and cone 10.
Ball clays are normally refractory, none of these are vitrified to any extent. The cone 10R bar is yellow because it is stained by the soluble salts present in the material.
Ball clay and kaolin test bars side-by-side fired from cone 9-11 oxidation and 10 reduction.
This is a ball clay. They are known to produce this type of soluble salts when fired at high temperature reduction (the inner salt-free section is such because that part of the tile was covered during drying, so the soluble salts from there had to migrate to the outer exposed edge). If soluble salts fire to a glassy surface they can affect the overlying glaze. But in this case they are not and have a minimal effect.
A broken test bar of ball clay fired to cone 10 reduction. Notice the black carbon core. Ball clays commonly contain carbon, many have a noticeable grey color in the raw state because of this. Notice it has not burned out despite the fact that the clay itself is still fairly porous, the firing was slow and the temperature reached was high. Ball clay typically does not comprise more than 30% of a body recipe so its opportunity to burn away is sufficient. However some specialized bodies have a much higher percentage.
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with that same material ball milled for an hour (right). The 10 mesh designation is a little misleading, those are agglomerates. When it is put into water many of those particles break down releasing the ultimates and it does suspend fairly well. But after 24 hours, not only has it settled completely from the upper section but there is a heavy sediment on the bottom. But with the milled material it has only settled slightly and there is no sediment on the bottom. Clearly, using an industrial attrition ball mill this material could be made completely colloidal.
This test shows the incredible dry shrinkage that a ball clay can have. Obviously if too much of this is employed in a body recipe one can expect it to put stress on the body during drying. Nevertheless, the dry strength of this material far exceeds that of a kaolin and when used judiciously it can really improve the working properties of a body giving the added benefit of extra dry strength.
Large particle kaolin (left) and small-particle ball clay (right) DFAC drying disks demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.
I poured 4 teaspoons of two glazes onto a board and let them sit for a minute, then inclined the board. The one with Gleason Ball clay (right, much higher in coal and finer particle size) has settled and the water on the top of running off. The one with Old Hickory #5 ball clay has not settled at all and the whole thing is running downward. Below I have begun to sponge them off. Old Hickory No. 1 Glaze Clay is even better than #5 for suspension. The most amazing thing about this: There is only 7% ball clay in the recipe.
These are the oversize particles (from the 70, 100, 140 and 200 mesh sieves) from 100 grams of a commercial ball clay. They have been fired to cone 10 reduction. As you can see, this material is a potential cause of specking, especially in porcelain bodies. It is not only wise to check for oversize particles in clays, but firing these particles will tell you if they contain iron. A 200 mesh screen would be a good start for this test, it would catch all of these.
These cone 6 clear-glazed porcelains demonstrate just how white you can make a porcelain if you use white burning kaolins and bentonites instead of ball clays. Both contain about 40% clay. The one on the left employs New Zealand kaolin and Veegum plasticizer, the one on the right Kentucky ball clays (among the whitest of ball clays in North America) and standard bentonite. Both are zero porosity. The glaze surface is a little more flawless on the right one (possibly because ball clays have a lower LOI than kaolins).
Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in production either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers.
These particles contaminating particles are exposed on the rim of a bisque fired mug. The liqnite ones have burned away but the iron particle is still there (and will produce a speck in the glaze). Remnants of the lignite remain inside the matrix and can pinhole glazes. Since ball clays are air floated (a stream of air takes away the lighter particles and the heavier ones recycle for regrinding) it seems that contamination like this would be impossible. But the equipment requires vigilance for correct operation, especially when there is pressure to maximize production. Ores in Tennessee are higher in coal than those in Kentucky. North American clay body manufacturers who confront ball clay suppliers with this contamination find that ceramic applications have become a very small part of the total ball clay market, complaints are not taken with the same seriousness as in the past.
Typical porcelains are made using clay (for workability), feldspar (for fired maturity) and silica (for structural integrity and glaze fit). These cone 6 test bars demonstrate the fired color difference between using kaolin (top) and ball clay (bottom). The top one employs #6 Tile super plastic kaolin, but even with this it still needs a 3% bentonite addition for plasticity. The bottom one uses Old Hickory #5 and M23, these are very clean ball clays but still nowhere near the whiteness of kaolins. Plus, 1% bentonite was still needed to get adequate plasticity for throwing. Which is better? For workability and drying, the bottom one is much better. For fired appearance, the top one.
Left: A porcelain that is plasticized using only ball clays (Spinx Gleason and Old Hickory #5). Right: Only kaolin (in this case Grolleg). Kaolins are much less plastic so bentonite (e.g. 2-5%) is typically needed to get good plasticity. The color can be alot whiter using a clean kaolin, but there are down sides. Kaolins have double the LOI of ball clays, so there are more gasses that potentially need to bubble up through the glaze (ball clay porcelains can produce brilliantly glassy and clean results in transparent glazes even at fast fire, while pure kaolins can produce tiny dimples in the glaze surface if firings are not soaked long enough). Kaolins plasticized by bentonite often do not dry as well as ball clays even though the drying shrinkage is usually less. Strangely, even though ball clays are so much harder and stronger in the dry state, a porcelain made using only ball clays often still needs some bentonite. If you do not need the very whitest result, it seems that a hibrid using both is still the best general purpose, low cost answer.
You may not fully appreciate what your clay body manufacturer has to go through to make clean porcelain for you. Every load of material that they receive has to be checked. We now have to check every pallet. This is the third semi-trailer load of material we have had contaminated (ball clays and kaolins are most vulnerable). When we phoned another manufacturer they checked their supply and it was contaminated also! Materials can also be contaminated by larger clay particles that disrupt the fired glaze surface. These chunks of metal were pulled out by magnets in the production line, a thousand boxes of porcelain are now garbage. It is too expensive to return a load, so it just becomes a loss.
These test bars are fired at cone 10R. The top one is EP Kaolin, the bottom one is Old Hickory M23 Ball Clay (these materials are typical of their respective types). It is interesting that although the kaolin has a much larger ultimate particle size it is shrinking alot more (23% vs. 14%). This is counterintuitive to what should be happening. And another thing: the kaolin has a porosity of 0.5% and the ball clay is 1.5%. This is also counterintuitive. The kaolin should be more refractory since it contains less fluxing oxide impurities. In fact, both should theoretically be a lot more refractory than they actually are.
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