The green strength of clay bodies is an important property, it makes them resistant to breakage or damage during handling in production.
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This refers to the strength of the clay body in the dried form (greenware). The strength is a product of the degree to which the mass has been compressed and the surface area of the particles (great surface area means more points-of-contact and thus more strength). Clays of high green strength handle well during production, thus lowering losses in handling. Porcelains often have very little green strength whereas stonewares and fireclays typically have alot. Ball clays and bentonites are often added to bodies to impart green strength. Kaolins, on the other hand, have very low green strength. Binders are also added to bodies to increase green strength. Green strength, when imparted by clays in the recipe, is normally associated with higher drying shrinkage. However the green strength can more then compensate to enable crack-free drying. Bodies of high green strength dry slower.
The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.
Two mugs have dried. The local terra cotta native clay on the left shrinks 7.5% on drying, the porcelain one on the right only 6% (it is made using Kentucky ball clay). Yet few pieces of the terra cotta are ever lost due to drying cracks, even if it is uneven! For example, in a batch of a dozen mugs none of these will be lost whereas one or two of the white ones will always crack. Why? Dry strength. The clay on the left is very strong in the dry state, likely double or triple the white clay (the strength is a by product of its high plasticity and particle size distribution profile). That strength is enough to more than counter the extra shrinkage.
Paper clay is easy to make. And really different!
Binders are glues that harden ceramic powders as they dry. They enable improved surface adherence. And slower drying.
|By Tony Hansen
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