Olivine Fosterite

Pyrite Fools Gold

Soapstone Steatite Carving

Beryl Chryso

Boracite Crystals

Beryl Feldspar Mica

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.

OM#4 ball clay test bars fired from cone 4-10 oxidation and cone 10 reduction. The yellow on bar 12 is iron stained soluble salts.

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.

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and diring shrinkages vary widely. Materials normally acting as fluxes (like dolomite, talc, calcium carbonate) are refractory here because they are fired in the absence of materials they react normally with.

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.

These are DFAC drying performance tests of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.

Stockpile of crude feldspar from MGK Minerals (India) deposit

Closeup of feldspar deposit in the MGK Minerals quarry in India.

Example of four different north American ball clays fired to cone 10R, cone 11 and cone 10.

This disk has dried under heat (with the center part protected) for many hours. During that process it curled upward badly (flattening back out later). It is very reluctant to give up its water in the central protected section. Obviously it shrinks alot during drying and forms a network of cracks. When there are this many cracks it is difficult to characterize it, so a picture is best.

Notice the water just sits there in a little lake. It does not soak in because the bentonite gels in contact with the water and that gel acts as a barrier. This water-barrier property of bentonite is a key to its use in many products but can be a problem in ceramics (because it slows down the drying speed of bodies and glazes that contain it).

These three materials also fire to a similar color. Grolleg is the most plastic, Dragonite the least.

Halloysite forms over long periods as kaolin sheets roll into tubes.

These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material, alumina has the lowest. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!

Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.

Bentone (AKA Macaloid) is a super plastic additive used to modify rheolgy in many consumer products. It is made by refining Hectorite. It is very difficult to mix pure Bentone with water, it is just so sticky and the water content is so high, it takes a week to dry a sample and it cracks into pieces during drying. I am studying five different grades for use as a plasticizer in premium porcelains. I am interested in how they stack up against the king: VeeGum T (both in price and performance). The first step was to fire square tiles of the powder on small porcelain tiles at cone 6 to compare the iron content. Three sintered into a solid mass, shrinking to about half the size. The CT grade is the natural, untreated Hectorite clay (accounting for its darker color), the processing to purify the material obviously increases its affinity for water, shrinkage and fired maturity.

The stoneware has a higher silica content and is not vitreous. This means there are more quartz particles to impose their high expansion because fewer are taken into solution by the feldspar.

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