$8.26
DESCRIPTION-> Click "Learn more about this item" for article & instructions!
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Ball clay is widely known among ceramists, and available in many variations. They are very plastic clays having very fine particle sizes. Digital Fire has a list of 72 (if I counted right) different ball clays at the end of their article (and says there are hundreds of them). The one in this listing ("Old Mine #4) is a very popular one and is found at 5 ceramics suppliers I looked at. One says: "A fine-grained ball clay with excellent plasticity and strength. Old Mine #4 is an 'industry standard' based on its popularity in both casting and plastic formed bodies. It is also widely used as a suspension aid in glazes."
This time Wikipedia says it best and most concisely- { https://en.wikipedia.org/wiki/Ball_clay } :
"Ball clays are kaolinitic sedimentary clays that commonly consist of 20–80% kaolinite, 10–25% mica, 6–65% quartz. Localized seams in the same deposit have variations in composition, including the quantity of the major minerals, accessory minerals and carbonaceous materials such as lignite. They are fine-grained and plastic in nature, and, unlike most earthenware clays, produce a fine quality white-coloured pottery body when fired, which is the key to their popularity with potters."
The CAMEO art materials database even mentions ball clay- { http://cameo.mfa.org/wiki/Ball_clay }
"A fine, soft, very plastic clay. Ball clay is a relatively pure, secondary clay composed of kaolin (20-90%), mica (5-45%), and quartz (1-70%). The high quality clay is found in England (Wareham, Dorset, Devon) and the U.S. (Kentucky, Tennessee). Originally, ball clay was dug up and made into 30-35 pound balls for transportation by horses. The fine-grain clay contains small amounts of carbon and other organic materials that give it an initial dark color. These materials are combusted during firing leaving a white vessel. Ball clay is very plastic and has a high shrinkage rate of up to 20%. It is often mixed with other clays to increase their plasticity. Ball clay is used in the manufacture of ceramics, whiteware, porcelain, glass, and tiles.
The Digital Fire resource has a long article on ball clay since it is so well known and used-
{ https://digitalfire.com/4sight/material/ball_clay_80.html } :
"The term 'ball' traces to historic mining in England where large chunks of the clay were cut from the bank in ball shapes for transport to processing.
There are hundreds of different ball clays available and they vary widely in plasticity, particle size, raw color, and drying properties. A typical ball clay powder is light grey (from lignite) or cream color and fires to a buff or cream white color with some soluble salt deposits on the fired surface. While potters only buy ball clay as a powder, industrial users have access to the material in many forms (shredded, noodled and partially deflocculated, chopped filter press cake, vacuum pugged extrusion, crumble with about 10% moisture). These forms enable a more consistent supply and alot of flexibility, especially when making casting slips (since ball clays are the most difficult to disperse materials in the bodies).
Ball clays are very plastic and much finer grained than kaolins. They are easily slaked in water when dry. Few people fully appreciate how 'sticky' and plastic these materials are until they mix some with water and work with it pure. The fine particle size also makes them impermeable to the passage of water (a small test bar can take a very long time to dry)...
Ball clays intended for casting have a lower drying shrinkage but also fire very similar.
Ball clays are used in ceramic bodies (porcelains, stonewares and earthenwares, casting slips, pressing bodies) because of their plastic nature combined with high firing temperature. Ball clays have very high dry shrinkage combined with high green strength and slow drying. Were it not for their iron and coal impurities, ball clays would be ideal ceramic materials. However, in practical terms, they are employed to achieve desired plasticity, but are minimized to reduce the detrimental effect on fired whiteness and drying properties.
A common starting recipe for a high temperature general purpose porcelain (as is used in electrical porcelain or extruded pottery porcelain is 25% each of ball clay, kaolin, feldspar and silica). The ball clay:kaolin mix can be altered to change body plasticity without significantly affecting the maturing temperature....
Ball clays tend to be quite refractory...Ball clay is not a clay mineral in itself, but contains other minerals, primarily kaolinite (but also montmorillonite, halloysite, and illite). Mica and quartz are also normally present in substantial amounts (e.g. 10-20% for Tennessee ball clays).
Ball clays vary widely in their plasticity, and it is difficult to compare them by quantitative tests because pure samples are difficult to mix and form and crack badly during drying. Thus, it is common to mix ball clay and silica 50:50 and prepare dry shrinkage, dry strength and fired strength bars (from this mix) for comparative testing...
Although some ball clays resist deflocculation because of hostile soluble impurities, most deflocculate very well with sodium silicate and other equivalent dispersants. A wide range of ball clay slurries and slips are used at all temperature ranges in casting processes. One common recipe uses a simple 50:50 ball clay:talc mix. This recipe and close derivatives are used in large quantities in the hobby casting market. The same mix is also dry pressed in the tile industry, and extruded for jiggering and wet processing in artware.
The refractories industry is a large user of ball clay. Common refractory materials lack plasticity and ball clay is used to help in forming and shape retention and to impart dry strength. The abrasives industry likewise uses it to bond aggregates until firing fuses the mass.
Engobes in the tile and brick industries are suspended, hardened, and adjusted to match body shrinkage by the addition of ball clay. Many pottery glazes contain ball clay to help suspend and harden them and control their shrinkage during drying...
If the iron or lignite content of ball clay is a problem, it is common to employ bentonite to reduce the ball clay requirements (5% bentonite can provide as much improvement in plasticity and dry strength as 25% ball clay). However, care is recommended to make sure a fine grade of bentonite is used to avoid fired specks (bentonite also burns darker).
If you use ball clay in your production there is good reason to be doing routine quality control to make sure it is remaining consistent. Ball clays are likely the most variable material you will have to deal with. They can sometimes have particulate impurities (especially lignite) and exhibit differences in soluble salts content, drying shrinkage, drying performance, fired maturity, fired color and behavior in slurries...
Adequate quartz content is an important factor in porcelain and whiteware bodies (it is an important structural element in the fired matrix and it is needed to prevent glaze crazing). Since ball clays contain quartz, it is possible to use less raw silica (quartz) powder in the recipe. Of course, the quartz grains in the ball clay are finer, so they will dissolve into the feldspar glass more readily." Funny, I was just reading up on magnesium sulfate (epsom salt), new to me as a clay additive; it's similar in that it does several of the things cited above that ball clay does.-
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Sep 27, 2022
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Jan 4, 2020
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