A crackle glaze is a crazed glaze used on purpose. The same mismatch that is a fault on a mug is the whole point on a decorative vessel, and the network of fine lines can be stained afterwards so it reads as a deliberate pattern rather than an accident.
How the crackle is made
By choosing a glaze whose thermal expansion is higher than the body's, so it ends up in tension as the piece cools and cracks into a network. Commercial crackle glazes are formulated for it; the same effect appears accidentally when any glaze is mismatched. Cooling speed and glaze thickness both change the scale of the pattern, so the same glaze can give fine or coarse crackle.
Staining the lines
Fresh from the kiln the lines are almost invisible. Rubbing a stain into the surface while the piece is still warm, traditionally with ink, tea or a dilute oxide, fills the cracks and makes the pattern visible. The staining is done once and repeated over years by whatever the piece meets, which is why old crackle ware darkens gradually.
Why it is decorative, not functional
The cracks go through the glaze to the body. On a porous earthenware they let liquid in, and they hold whatever they let in, which means water, staining and anything else that gets there. That is a reason to keep crackle glazes for vases, panels and display pieces rather than for anything used with food or washed regularly.
Getting a pattern you want
Thicker glaze gives larger, bolder crackle; thinner gives a finer net. Cooling the kiln more quickly through the critical range exaggerates the effect and cooling slowly reduces it. Since both variables are easy to change and hard to predict, this is a glaze that repays a row of test tiles at different thicknesses more than most.
Questions people ask about crackle glaze
How is crackle glaze made?
By using a glaze that shrinks more than the body as it cools, so it ends in tension and crazes. Commercial crackle glazes are formulated to do it reliably.
How do I make the cracks show?
Rub a stain such as ink or dilute oxide into the surface while the piece is still warm from the kiln, then wipe the excess away.
Can I use crackle glaze on mugs?
It is not suitable for functional ware. The cracks reach the body and hold liquid and staining.