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The melts being compared here are our code number 6998, 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.
On the left is Plainsman M332, a sandy and coarse body dry ground at 42 mesh. On the right on a wet-processed body, sieved at 80 mesh and then filter pressed - it is porcelain smooth. Yet that glaze, GA6-C, on the smooth body is covered with blister remnants while the same glaze on the coarse body is glassy smooth (they were in the same firing). That smooth glaze is courtesy of the C6DHSC slow cool firing schedule. But why does it perform so poorly on the finer body? That body is being overfired. Pieces are warping. Although not bloating, it is beginning to decompose and generate gases, they are producing the blisters.
The glaze is G3948A iron red fired at cone 6 using the C6DHSC schedule. The bodies are Plainsman Coffee Clay and Polar Ice (the insides are different glazes). They were in the same kiln. These mugs demonstrate how much reactive glazes can interact with the body beneath and how much that affects their fired properties, especially when they have high melt fluidity like this one. On the left the glaze is drawing color out of the body. The porcelain on the right has no color to give but it does have sodium - and it is supplying enough to act as a catalyst to the creation of the iron crystals.
Glossary |
Body glaze Interface
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. |
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Glossary |
Melt Fluidity
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. |
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