Potters & Sculptors - Making Rock from Mud
I have been testing porcelain bodies for nearly two years. Recently I purchased a 1600X lab scope to help me see what I had long suspected.
In going back over my flux tests in clay bodies, I began to notice a common COE pattern in pieces that were under-fluxed, or under fired.
The above picture is from the unglazed section of that bar, and the picture below is from the glazed area of the same bar. I used a monochrome filter to highlight the stress crack: hard to see in picture without it.
I have since learned from extensive testing that the flux in the clay bodies moved from the bottom (shelf contact) to the top of the bars. I made a series of test cylinders in various thicknesses: 3/8 to 1" to follow that flux flow through a body. Then a series of hollowed out cylinders to replicate an ovoid piece cavity walls. In every test: flux moved from the bottom to the top, or from the center to the outside of ovoid forms. As this example shows:
Sodium and potassium are in a gaseous state from cones 5-10; so simple physics explains the flux pushing upward or outward through the clay wall. You will notice the bottom of the sample shown is mature, while the top is not fully mature (some glassy matrix) but many voids. However, when you look through that same body on a cross section, the voids become more evident.