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.
This term is used in reference to clays (or more often bodies which 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). 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). It's nature is also a product of how the surface chemistry and the electrolytic charge of the particles express themselves, 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 anywhere near 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 big 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.
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. If a material is plastic that means it has a fine particle size and is active electrolytically, because of this it will suspend particles in a slurry and it will contribute to dry hardness.
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).
These crucibles are thrown from a mixture of 97% Zircopax (zirconium silicate) and 3% Veegum T. The consistency of the material is good for rolling and making tiles but is not quite plastic enough to throw very thin (so I would try 4% Veegum next time). It takes alot of time to dewater on a plaster bat. But, these are like nothing I could make from any other material. They are incredibly refractory (fired to cone 10 they look like bisqued porcelain). However I have had mixed results for thermal shock resistance.
The walls are very thin, yet no trimming was done to make them thin. Why? It is super plastic. Others claim to be plastic, but they use the word in a relative sense. They mean a little less flabby than that other really flabby porcelain! Polar ice, when it has the right water content (dewater it on a bat if needed), is tough enough to throw as large as even the most plastic stonewares. It might seem impossible that a body this translucent can be as plastic as it is, read its data sheet to find out how they did it.
Wow, just threw this mug from a porcelain having 10% Veegum plasticizer (of course no one could afford that, it is $15 a pound). But anyway, I was testing the extreme. These mugs did not twist during throwing, I could have pulled the wall thinner at the middle and top. The wall thickness at the bottom is 2.3mm (less than 3/32")! This mug is 15cm (6 in) tall. One problem: It takes forever to dry.
This clay was slurried in a mixer and then poured onto a plaster table for dewatering. During throwing it is splitting when stretched and peeling when cutting the base. Yet when this same clay is water-mixed and pugged in a vacuum de-airing pugmill it performs well. One might think that the slurry mixer would wet all the particle surfaces better than a pugmill, but it appears the energy that the latter is putting into the mix is needed to develop the plasticity when there is a high talc percentage in the recipe.
This porcelain becomes quite brittle as it gets stiffer making it difficult to make these cuts in the foot ring. This creates extra sponging work when it is dry. It also means that dry strength will be low. Porcelains do not need to be this way, plenty of white burning bentonites are available (although they increase cost).
This thrown vessel has sat on this plaster bat for almost 24 hours and yet still has not released. The bat was dry. It had to be slowly pried off with a flat scraper (which deformed it somewhat). When clay bodies are high in ball clay and bentonite they dry slower. If this is taken to an extreme, it can slow down production.
Boxes are 20kg (22.68 lbs). Plus there are enough trimmings to make about two more. That is about 500g of pugged clay per mug. These have been trimmed and engobed (using our standard cone 6 engobe) and are drying. Notice I have waxed the outers of some of the handles to slow their drying down (to keep it in sync with the mug itself). M390 is likely the most plastic native Plainsman body. Although it was not overly soft I stiffened up the clay for ten minutes on a plaster bat to make it my ideal throwing stiffness.
This is from half a box! 21 mugs from 10 kg ( (all scrap was reclaimed). Polar Ice porcelain double the price of others. Why use it? Because it is so plastic that you can make more pieces, many more, enough to more than pay the extra clay cost. And you can charge more for each piece. These have a weight-to-capacity ratio of 1.09. That means the mug itself is lighter than the weight of water it can hold (each 1 gram of fired porcelain can contain 1.09 grams of water). This is much better than most other clay bodies.
This item was made in India. Their pottery tradition expects that ware can be sun-dried immediately after it is made. To make that possible their clay bodies have low plasticity and lots of large particle sizes. They have learned to work with these and regard them as normal. However potters in the west would find bodies like this unusable (they are accustomed to taking lots of care in drying in exchange for having much more plasticity). However, if Indian potters let the fine plastic clay portion of the body recipe decline too much then this type of surface-cracking issue can occur.
Worthington Clear is a popular low fire transparent glaze recipe. It has 55% Gerstley Borate plus 30% kaolin (Gerstley Borate melts at a very low temperature because it sources lots of boron). GB is also very plastic, like a clay. I have thrown a pot from this recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay the shirinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries. Countless potters struggle with Gerstley Borate recipes. How could we fix this one? First, substitute all or part of the raw kaolin for calcined kaolin (using 10% less because it has zero LOI). Second: It is possible to calculate a recipe having the same chemistry but sourcing the magic melting oxide, boron, from a frit instead.
These are porosity and fired shrinlage test bars, code numbered to have their data recorded in our group account at Insight-live.com. Plainsman P580 (top) has 35% ball clay and 17% American kaolin. H570 (below it) has 10% ball clay and 45% kaolin, so it burns whiter (but has a higher fired shrinkage). P700 (third down) has 50% Grolleg kaolin and no ball clay, it is the whitest and has even more fired shrinkage. Crysanthos porcelain (bottom, from China) also only employs kaolin, but at a much lower percentage, thus is has almost no plasticity (suitable for machine forming only). Do H570 and P700 sacrifice plasticity to be whiter? No, with added bentonite they have better plasticity than P580. Could that bottom one be super-charged? Yes, 3-4% VeeGum or Bentone (smectite, hectorite) would make it the most plastic of all of these (at a high cost of course).
A comparison of the plasticity of Volclay 325 Bentonite:Silica 25:75 (top) and Hectalite 200:Silica 50:50. Both are mixed with silica powder. The latter (a highly refined bentonite) is much less plastic even though it is double the percentage in the recipe.
This shows the soluble salts in the material and the characteristic cracking pattern of a DFAC test disk when made from a low plasticity clay. Notice the edges have peeled badly during cutting, this is characteristic of very low plasticity clays.
The 20cm vase on the left is thrown from what I thought was a very plastic body, I achieved close to the same thickness top-to-bottom (5mm). The one on the right was the same original height, 20cm. But it has dried down to only 18cm high, it shrinks 14% (vs. 6% for the other). The thinnest part of the wall is near the bottom, only 2mm thick! How is it possible to throw that thin? The body is 50% ball clay and 50% bentonite. Bentonite, by itself, cannot be mixed with water, but dry-blended with fine-particled ball clay it can. It took about 4 days to dewater the slurry on my plaster table. This is the poorest drying body one could possibly use. But there is the lesson here: Even this can be dried crack-free. How? One month under cloth and plastic to assure water content throughout! This means that pretty well any other body can be dried without cracks if done sufficiently evenly.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
Clays used in ceramics shrink when they dry because of particle packing that occurs as inter-particle water evaporates. Excessive or uneven shrinkage causes cracks.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.
|Glossary||Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.
Methods of Measuring the Plasticity of Clays
Atterberg Plastic and Liquid Limit Tests
|Articles||How to Find and Test Your Own Native Clays
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.