For me, good clay comes purely down to its affect on tea. For me, in my experience, with yixing wares ranging from LQER all the way up to modern, including f1 60s, 70s, 80s, and white label, excluding laser label. I have found that most prominent effect I notice is textural, with actually a modern zhuni being the most pronounced and then 60s and 70s hongni. Some zini clays I've found to be close to 60s, 70s, hongni, but the effect being more thickening than smoothing. Based on this, I look at the clays which make very nice tea and inspect their visual characteristics. I think it is general consensus (is that even possible in the yixing world?
) that the clay of the earlier f1 eras were more pure, and higher quality than the later eras. If we believe that, we can look at their texture and physical clay character to establish some sort of idea of what a good clay might look like.
Idk really how to explain it, other than it just takes a certain amount of experience and seeing, handling, and using pots, but you can get an eye for whether a pots clay is good or not. I guess a very easy example is look at F1 white label hongni and 60s hongni, they are visually quite different, and once you have one of both, and brew with them, brewing performance (textural effect by my definition), you can see that white label is clearly inferior.