An example of a highly fluid cone 6 glaze that has pooled in the bottom of a mug (and crystallized). Glazes normally need to be under some compression to avoid crazing (by having a lower-than-the-body thermal expansion), but if they are thick like this the body does not have the strength to resist the extra outward pressure the glaze can be exerting at the base from the inside. The result here is a separated base. Conversely, if the glaze is under tension (having too high an expansion), the cracks that develop within it to relieve the tension are deep and wider and thus more likely to propagate into the body. The ultimate result: Poor ware strength and durability. A better solution is to use a liner glaze.
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.
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
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.