1.4 Tests for Clay Bodies

Test tiles

Plasticity – is necessary for throwing and hand building. Simple way is to roll out a pen-size coil, and then wrap around one’s finger. Coil with excessive cracks means lack of plasticity. Adding plastic components such as ball clay will help improve the plastic. It is also important to wedge properly before throwing for even moisture.

Porosity – is related to the hardness and the verification of the clay. Simple way is to weight a unglazed fired clay simple, then soak in water. After overnight, the sample is wiped off excess water then weight again. The increase after soaking will be the percentage of porosity of the clay body. In general, the porosity of fired clay bodies directly connected to their mature temperature (earthenware in 4-10%, stoneware in 1-6%, porcelain in 0-3%).

Shrinkage – is associated to the plasticity of clay. The more plastic will often shrink the most. First is to cut a piece of rectangle slab and draw a line of 10cm in the center, then measure it before and after firing. The decrease after firing will be the percentage of shrinkage of the clay body. For large sculptural or thicker form, it is beneficial to include 20-30 percent of fired grog for less shrinkage, it will avoid crack during drying or firing. In general, the shrinkage rate of earthenware, stoneware, procelain are in 7-10%, 10-15% and15-20%.