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 alot 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.
Albany Slip DFAC dried disk
This shows the soluble salts in the material and the characteristic cracking pattern of a low plasticity clay. Notice the edges have peeled badly during cutting, this is characteristic of very low plasticity.
Two plasticizers, two results
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.
Turbo-charge plasticity using bentonite, hectorite, smectite.
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).
Can you actually throw a Gerstley Borate glaze? Yes!
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.
Can you throw zircopax on the potters wheel? Yes!
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), a have amazing resistance to thermal shock. I could pour molten metal into them and they will not crack. I can heat one area red hot and it will not crack. I can throw the red hot piece into water and it will not crack!
The sun. Brought to you by Plainsman Polar Ice!
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.
A low fire talc body lacks plasticity when slip-mixed, but not when pugged
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.
Ridiculously plastic! How?
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.
With porcelains, poor plasticity gets worse at the leather hard stage
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).
When clay bodies are too sticky and plastic they do not release from bats
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.
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