How a clay is charged affects its plasticity: Kaolin has a neutral charge, which is why porcelain is prone to memory. Fireclay and ball clay have negative charges, which is why stoneware is always more plastic. Learn how to adjust your materials to make a clay body.
Defining the Terms
Anion: A negatively charged ion.
Cation: A positively charged ion.
Cation Exchange Capacity (CEC): The total capacity of a clay or other material to hold exchangeable charged particles.
Clay Memory: Tendency of clay particles, during drying and firing shrinkage, to either retain their shape or return to a previous forming shape. Can cause warping and cracking.
Isomorphic Substitution: The replacement of one atom by another of similar size in a crystal lattice without disrupting or changing the crystal structure of the mineral.
Rate Law: The amount of time, or rate at which cation exchange takes place. The pH of the clay body affects this rate.
SSA (Specific Surface Area): The total surface area per unit mass of a powder such as clay, typically given as square meters/gram.
WOPL (Water of Plasticity) Value: The amount of water 100 grams of dry clay will absorb to form a pliable ball. Higher values equal higher plasticity.
Positive and Negative Charges
Potters call it plasticity: but the chemistry behind it is known as cation exchange. Cation exchange is defined as the exchange of positive ions (cations) of different elements (but with the same net charge) between an insoluble solid and a solution in contact with it. In application to clay plasticity it refers to cations in a clay being replaced by cations from the water of plasticity.
Plasticity is created by a negative electrostatic charge, and clay memory is caused by a positive charge. Isomorphic substitution of elements within the lattice of individual clay crystals is the exact mechanism by which plasticity or memory is dictated. Kaolin has little such substitution, therefore a nearly neutral charge. This works against cation exchange and is why porcelain can be prone to memory. Fireclay and ball clay have negative charges of various values, which is why stoneware is always more plastic.
In porcelain, built around kaolin, which has a nearly neutral molecular charge, particles with negatively charged surfaces have to be added to the body in order to achieve plasticity. Negatively charged ball clays are substituted for some of the neutrally charged kaolin or plasticizers are introduced to obtain the negative charge. Clays are sheet silicates and hold their electrostatic charge on their outer faces. Plasticizers like bentonite or substitutes like ball clay have a net negative charge on their surfaces and facilitate cation exchange. In application to pottery, it is the net negative charge of the inner layers of bentonite due to isomorphic substitution of other metals for aluminum that attracts cations and permits cation exchange. So plasticity is improved by cation exchange.
To apply cation exchange to clay formulation you have to establish the cation exchange values given for each type of ball clay. Some mines supply this data; others do not. In lieu of cation exchange data, the water of plasticity (WOPL) value can be used to give some indication of cation exchange. Typically, the higher the WOPL value is; then the cation exchange will likewise be higher. If WOPL values are not available; then usually the smaller the particle is, the higher the cation exchange will be. Lastly, if none of these values are given: the lower the alumina content is, the higher the cation exchange will be.
Kaolin has a nearly neutral charge. Most kaolins average 37% alumina by weight. Ball clays have a range of 20–29% alumina. As the amount of alumina decreases, the negative charge increases.
The historic formulation theory is that: the smaller the particle, the more plastic it is. There is a certain amount of truth to that: because as the particle size decreases, so does the alumina level. So when there is no defined cation exchange value, or no WOPL value given: ball clays with the lowest amount of alumina will be the most plastic because they have higher cation exchange capabilities.
Like ball clay, the plasticity of bentonites is related to particle size. Bentonite can range in size from as large as 0.40 microns down to 0.25 microns. Although that may not seem like a lot of difference, 0.40-micron bentonite can have as much as 20% alumina, while 0.25-micron bentonite can have only 5%. The same principle applies to bentonites as does ball clay: the lower the alumina level is, the more plastic it will be. The major difference between the effectiveness of bentonites is whether they are sodium, calcium, or magnesium based. Most bentonite has a fairly high percentage of magnesium due to the way it was naturally formed. However, the alumina level predicts its ability to exchange cations, not the magnesium content. Sodium or calcium dictate how fast that cation exchange takes place, they create a rate law (differential exchange time) in application.
The table (2) below helps when evaluating the suitability of a clay for consideration in a recipe. The contaminants (iron, magnesium, and titanium) are also included in that consideration, but excluded from this table to focus on the relationship between alumina and Cation Exchange Capacity (CEC). As alumina levels decline, the CEC increases. It is not particle size that dictates overall plasticity; it is the CEC. If CEC values are not given, then look at the WOPL indices. Higher WOPL values follow higher CEC.
So in formulating a clay body, the first premise is to understand that porcelain bodies require much more negatively-charged ball clay or plasticizers than stoneware. Stoneware bodies consist primarily of clays with higher CEC, so the deficiencies found in kaolin are not present. EPK Kaolin actually has the highest CEC of all kaolin, as well as the highest WOPL value: which is why it is considered to be a plastic kaolin. The lower overall CEC in kaolin causes porcelain bodies to have memory. Excess memory is the primary indicator that an insufficient amount of negatively charged ball clay or plasticizers have been added to the recipe.
Judging Excess Plasticity
Potters generally define plasticity as a clay being along a continuum between either short or fat. Short clay, having a dry texture, tends to snap when formed tightly, and requires more mechanical pressure when throwing. Fat clay has a rubbery trait when handled, moves easily when force is applied, and creates more slip when throwing. In formulation, a short clay indicates a lack of adequate negative exchange, and a fat clay indicates an oversaturation of ball clay and/or plasticizers. There is a long held historic belief that plasticity can be achieved by simply making larger additions of ball clay. While there is some basis for that belief, there are formula limits to those additions. Excess ball clay can actually weaken the body due to higher absorption rates of water: remembering that all ball clays because of their higher CEC, absorb more water.
There is a fine line in formulation when it comes to additions of ball clay or plasticizers to achieve the desired level of plasticity. Obviously porcelain requires either higher amounts of ball clay, bentonites, and plasticizers, while stoneware typically only requires ball clay additions. Potters don’t usually have access to testing labs, so other methods of testing must be devised. A simple test of judging excess additions of plasticizers is to observe the amount of creamy slip that is generated when you throw. A clay body that forms excessive amounts of creamy slip, requiring constant removal during throwing is way past the amount of plasticizers like bentonite required to achieve plasticity.
Clay bodies that form heavy slip when throwing likewise are absorbing larger amounts of water in that process. As that happens, the body continues to decline in mechanical strength and slumping, or folding of the material occurs. This is especially detrimental when throwing wide-rimmed bowls or platters, because the water has weakened the clay’s ability to hold its own weight. So using the definitions of a short and fat clay can help to guide you in formulation. A short clay needs more negatively charged ball clay or plasticizers and a fat clay needs less. If your current ball clay does not provide the plasticity you desire; instead of adding more of it until it weakens the body, perhaps look at a different ball clay that has higher CEC.
the author Tom Anderson is a ceramic artist who has devoted the last ten years to crystalline glazes and clay chemistry research, including dedicating an extensive amount of research time to reading abstracts from various universities on the subject of clay chemistry and soil science. He kindly thanks Ron Roy for mentorship.