Just chanced on this helpful site while asking Mr Google, What is ‘crank’ clay? –
www.tombutcherceramics.co.uk/how-its-made explains that crank is a hand building clay body with coarse grog.
Grog: The grog is pre-fired pottery that has been ground down to varying degrees with textures ranging from flour to granulated sugar. This is added to clay body to give it structure, speed up the drying process and reduce shrinkage during drying. The grog makes the pot more stable and therefore stronger.
So that may explain why some of my clay bodies for making low-bisque, smoke-fired planters are still cracking after firing, even though the clay body I use is a grogged body. It’s probably got fine too fine a grog instead of a good, thick, granulated-sugar, coarse type of grog. Of course!
Next step is to buy some coarse-grogged ‘crank’ clay…