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Plasticity

Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.

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Details

This term is used in reference to clays (or more often bodies that are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). That being said the term is also used here in indirect ways - a clay might be said to be highly plastic as a way to imply other properties it would thus have to have: e.g. high dry strength and shrinkage, good slurry suspension and fine particle size.

With plastic clay you can do amazing things!

Simplistically, we can say that plasticity is a product of the electrolytic character of flat clay particles (they have opposite charges on the faces and edges), it gives them an affinity for water, water becomes both the glue holding particles together and the lubricant that imparts the plasticity. There are many finer points to understanding the dynamics of plasticity and it is difficult to measure using test equipment. It is only by a lot of hard work testing many material combinations that one can start to get an understanding of the complex factors that interplay to create the different kinds of plasticity we can detect and how these relate to the other properties of the body or material (e.g. dry strength, drying shrinkage, hardness, LOI, etc.). When one understands his/her materials well (especially the ball clays, kaolins and bentonites available to him), bodies of more plasticity that have less drying shrinkage and better drying performance can be created.

In industry plasticity is often just gauged by the way a clay behaves in forming machines, how it dries and by its stickiness. Technicians look at particle size, shape, surface area information on data sheets to extrapolate plasticity. However potters find that simply throwing two samples of clay on the potters wheel a more practical way to compare plasticities. Lab workers at manufacturing companies see plasticity as numbers on paper, they often make the assumption that plasticity is only a function of particle size (not particle identity) and are thus often not as fully aware of it as potters. Plastic clays are responsive, large thin pieces can be made (and made faster), wet pieces can be moved without excessive deformation and plastic clays center more easily during throwing. Non-plastic clays tend to split at edges during wedging and rolling, they generate a lot of slip, they are more difficult to center during throwing, they are more flabby and unresponsive and require more finicky refining work in the latter stages of the process.

Again, plasticity is mainly, but not only, a function of particle size (normally clays of finer ultimate particle sizes are more plastic). Its nature is also a product of how the surface chemistry, the particle shape, the mineralogy of the particles and the presence of impurity non-plastic particles in the matrix. The fact that particle identity (rather than particle size) is the key factor in plasticity can be demonstrated. Ball milling a non-clay material to sub-micron particle sizes does not make it plastic. Ball milling a kaolin to bentonite particle sizes will not give it the plasticity of a bentonite. For another example that demonstrates that the nature of particles is as, or more important as their size: You can plasticize zirconium silicate or calcined alumina with only 3-4% Veegum! Clearly, the nature of the particles of the host material mix to which the clay is added make a difference in how effective the plasticizing action is.

Clay particles of different sizes require differing amounts of water to plasticize them. But particles of differing nature, that is, whose electrolytic surfaces react differently with water, also require differing amounts of water. This is another area where a potter sees plasticity much differently than a technician in a tile company lab. The former looks for WOPL (water of plasticity) values on data sheets (an indicator of the amount of water needed to make a raw material plastic, e.g. 26 grams per 100 grams of clay). However a potter sees water content as simply an indicator of how stiff a clay body is, he does not really care if a body has 21% or 23% water, as long as it is the stiffness he needs. Looking at WOPL numbers for various clays suggests that porcelains require much more water than stonewares and earthenwares. But this is not the case, this author has noted that almost all de-aired clay bodies, from coarse stonewares to fine porcelains need about 20-22% water for throwing consistency. Also that WOPL numbers are not a great indicator of plasticity. This can be verified by simply throwing on the potters wheel with a range of ball clay, kaolin, stoneware and earthenware materials. The degree of observed plasticities will not relate well to the WOPL numbers on the data sheets (e.g. drastic differences in plasticity will only be small differences in WOPL).

Typically 50% or more clay is needed to create a body or porcelain for plastic forming (by "plastic forming" we mean a body that is plastic enough that a potter could pull it up into a tall thin cylinder with little trouble). Augmenting with bentonites can reduce the clay requirement to 40% of the total.

A few drops of water on top of a tiny pile of bentonite powder. The water just sits there in a little lake, it doesn't soak in because an impermeable layer forms.

Bentonites are the most plastic common clay. Kaolins the least plastic. Clays of different plasticities exhibit vastly different properties. For example, ball clays are very plastic but they shrink so as a pure material they often so much on drying that cracks cannot be prevented. Bentonites have such a high affinity for water that it can take a week to dry a specimen and it can shrink to half the size. Kaolins can dry in a short time and have little shrinkage, but they can have much less dry strength (some plastic kaolins are available but their plasticity is usually because they contain bentonite or have a mineralogy that is bordering on ball clay). Thus a typical white-burning clay body might employ as much kaolin as possible for whiteness, enough ball clay to achieve the needed plasticity, and possibly a small addition of bentonite to provide fine control or minimize the amount of ball clay needed. A white stoneware pottery clay might have as much ball clay as possible to achieve lots of plasticity but some kaolin will be present to reduce firing shrinkage and get better water permeability and drying properties. White casting porcelains are normally made using only kaolin (since they do not require alot of plasticity).

Two clay bodies can have equal plasticities, but require different treatment during forming. For example, plasticity x (in a porcelain body) could be achieved using all clay and a little bentonite (in the recipe to make up the clay portion of the mix) while plasticity y could be a product on only kaolin with a high percentage of bentonite. While it might be possible to pull up a piece of similar height and wall thickness from both, there are important aspects of the second that need consideration. This type of mix cannot be moved as quickly when centering and pulling (during throwing), especially when stiffer. Handles must be pulled with more care to avoid ripping. While not always the case, this type of body will often generate very little slip during throwing.

In general, plastic clays are very sticky. The term 'gumbo' refers to clays that are very sticky (and therefore build up on the tires of cars). However it is also true that certain non-plastic clays can also be very sticky. New Zealand kaolin is an example of this.

Potters often speak about aging to improve the plasticity of clay bodies. But the author has observed that the only time aging is important is if a porcelain, for example, is so non-plastic that one is willing to do just about anything to improve it a bit. But, as noted, we have fine bentonites now so any body can be as plastic as needed. Right out of the pugmill.

The plasticity of a material is sometimes mentioned in the context of its use in glaze recipes or ceramic slurries. This is because plastic materials have very tiny particles that are active electrolytically (like tiny magnets). That property enables them to interact in a slurry, forming a network that suspends other particles. And the clay particles harden the slurry during drying. Interestingly, the drying and hardening properties of plastic clays are reversible. A raw glaze can be scraped off, rewetted and applied again, it will work just as well the second time. And third time. Clay bodies can likewise be reused. This is in contrast to ceramic powders that are hardened using organic binders, they can only be formed or applied once.

Atterberg plastic and liquid limit tests are performed on soils but are normally not applicable to most ceramic clays (since they are so much more plastic).