|Monthly Tech-Tip |
The green strength of clay bodies is an important property, it makes them resistant to breakage or damage during handling in production.
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 clay on the left shrinks 7.5% on drying, the one on the right only 6%. Yet it consistently cracks less! Not the slightest hairline crack, not even at the handle joins. Why? Green or dry strength. If the dry clay matrix has the strength it can resist cracking even if there are stresses from uneven drying. The clay on the right is made using Kentucky ball clay, which has good plasticity but fairly low drying strength. The clay on the left is a native terra cotta, very plastic and very strong in the green state (likely double or triple the white clay). To demonstrate further: If paper fiber were added to the white clay, it would not crack. Why? Not because it would shrink less with the added fiber, no, the shrinkage would stay the same. Increased strength imparted by the fiber would give it the power to resist cracking.
This measuring cup contains 30 squares of toilet paper or 11 grams (which has disintegrated quickly and has been propeller-mixed). I am about to dump the paper fiber and 1000 grams of plastic porcelain powder into the water and then mix that up and pour the slurry onto a plaster bat. Although the fiber is only 1% by weight of the dry mix, this completely changes the working properties of the clay. It is still plastic, but much more difficult to cut with a knife or wire. It rolls out nicely into very thin slabs and they are very tough and easy to manipulate and build with. As it hardens it is still pretty plastic.When forced to bend it slowly breaks as the fibers release across the boundaries. Two dry pieces of this clay can be joined using only water and they stick together! Of course this paper needs to burn out during firing, so you need good ventilation on your kiln. You might think that this paper clay shrinks much less than the non-paper version. Actually, it shrinks more (likely because of the increased percentage of water needed). The paper is imparting strength, that strength is enough to resist cracking on drying.
Binders are glues that harden ceramic powders as they dry. They enable improved surface adherence. And slower drying.