Related: Coning up and pushing down properly: aka power wedging is the secret to never getting an s-crack.
Zamek did a nice paper on the subject. Even with the most finiky clay the power wedge prevents these pesky little devils: important to really engage the clay down at the bottom area of the cone:
I have seen really experienced throwers start to get s-cracks and think it is a clay problem, but always once they read this, they realize that with age and arthitis etc, they really are not engaging the clay in the coning process, especially down low, like they used to. I have never seen this not solve the problem.
Some clays are more prone to s-cracks, but this is a good thing to assimilate into one's practice even if s-cracks are not a problem (at the moment)
here's the paper: