Pottery clays can split at the plastic stage, sometimes ware can fall down right in front of your eyes. Why?
If clay bodies are made from too high a percentage of plastic materials they shrink too much during drying and crack. Thus it is common to incorporate aggregate materials, like silica sand, fine group, kaolinized sands or silts, the reduce the plasticity and cut the drying shrinkage. However, each grain of sand exposed at the surface during throwing is an opportunity for water to enter and begin a tiny split (that can grow to the point of failure if the clay is still soft enough). This problem is exacerbated when the plastic clays being employed are not fine ground and thus themselves contain a range of particle sizes above 200 mesh. The principle preventive action is to avoid situations where water or wet slip are left sitting on any surface that has been stretched or is under tensile stress (like the outer belly of a vase or where alterations are done). It is also helpful to use slip rather than water in forming processes.
Bodies having this issue can be improved by deairing during pugging. Or bentonite can be added to the recipe, it blocks the penetration of water and strengthens the plastic phase. Increasing the percentage of plastics at the cost of particulates in the body recipe will also help.
An example of splitting, where a thrown clay develops a split minutes after throwing. This often happens at stretch-points with sandy or groggy clay bodies or those that have a wide range of particle sizes (e.g. native clays not ground to 200 mesh). The larger particles create networks into which water can penetrate and begin and propagate a split. It is thus wise not to leave water or high-water-content slip on any surfaces experiencing tensile stress during forming.
|By Tony Hansen
Follow me on