|Monthly Tech-Tip |
The super-smooth clay on the left is "Industrial Crank" from Potclays in the UK, I have removed the grog (our test code number L3868). First I did our standard tests on that body, then I dried some, slaked it in water, screened the grog out and then dewatered the remaining clay to plastic form. The grog yielded weighed the same as the clay portion of the recipe yet this body is known for amazing workability and toughness! How is that possible with so much aggregate in the recipe? The base clay body is extremely smooth, sticky and highly plastic. Mixtures of just ball clay and feldspar (e.g. 75:25) will yield this type of material, some ball clays will produce as little as 6% drying shrinkage using this ratio. The grog they use is also special: The particles have angular shapes and a narrow range of sizes, from about 35 mesh to 70 mesh (most is of the coarsest size). This narrow range of sizes means dramatically less particle surface area. These factors produce much less disruption of the plastic properties of the base clay. How could you make a body like this? Slurry up about 75% ball clay and 25% feldspar and wedge the grog into that.
This is Plainsman M350. A freshly trimmed foot and the same piece after firing at cone 6. Although this amount of grog is enough to make this body impractical for throwing it is not enough to significantly reduce the drying shrinkage (it dropped by at 0.5%). However, it does speed up drying, providing more channels for water exit. This body has very high dry strength, that alone eliminates a lot of drying cracks that would otherwise occur.
In ceramics, clays with added aggregate are free formed or molding into shapes that are slowly dried and fired in a kiln. The process requires much skill.
Grog is a term used in ceramics to describe crushed brick (or other fired ceramic) aggregate that is added to sculpture and structural clays to improve drying properties.
Drying Ceramics Without Cracks
Anything ceramic ware can be dried if it is done slowly and evenly enough. To dry faster optimize the body recipe, ware cross section, drying process and develop a good test to rate drying performance.
Clay Cracking During Drying
The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.