|Monthly Tech-Tip |
This is a slip-cast vase. The body is a typical 50:50 talc:ball clay blend. Only the outside of the vessel is glazed. The talc increases the thermal expansion so that during cooling in the kiln the body contracts more than the glaze, putting it under compression (and thereby preventing crazing). But, these bodies have no flux, they typically have 10%+ porosity and often are not strong enough to resist for long the tensive forces the glaze can put them under. This is especially so when walls are thin and only the inside or outside is glazed (or when a glaze is under compression on one side of the wall and tension on the other). This issue is more important at low fire but people do something very unfortunate: they use glazes from one manufacturer and trust they fit bodies made by another without doing any fit-testing.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.