|Monthly Tech-Tip |
The melts being compared here are our code number 6880, a production run of Alberta Slip. The same sample batch and ball weight is being compared in these two flow testers fired side by side in a cone 10R kiln. Why are the flows behaving so differently? It is the clay from which the flow testers were cast. The one on the left is made from L4404A, a highly refractory casting slip. The one on the right is M370, a medium temperature porcelain (it survives pretty well to cone 10 but is obviously very vitreous). The difference in the flows (the width and length) is a product of the interaction with the material being tested and the tester itself. On the M370 tester the flow is adhering to the clay surface so well that it has spread and thinned enough so that few bubble-breaks are visible. This interaction has even slowed the flow. But the L4404A flow tester is clearly better, minimizing interaction and better revealing the fluidity of the melt.
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.