London seems to have an obsession with clay here. Or, more accurately, an obsession with souls who get molded like clay. Just check out these characters, along with London's descriptions:
Yeah, there's a whole lot of molding going on. (Or moulding, as it were.)
So what's all that molding about? Well, for one thing, it emphasizes the novel's belief that we're a product of our environment. The folks who get molded often seem to be bad sorts who do awful things. London's clay metaphor suggests that it's not all their fault, that bad circumstances forced them into a life of evil and depravity, rather than just being bad to the bone.
This plays out most strongly in White Fang, who settles into to his circumstances and becomes evil or good depending on how he's treated. (Never under estimate the power of a good belly rub.) The fact that the metaphor extends to humans like Beauty suggests that maybe even these horrible monsters aren't entirely to blame for their horrible monsterness. Rather, their general awfulness comes about because of the generally awful way life has treated him.
We're malleable like clay; we can be shaped and molded into baddies by horrible circumstances, or heroes by good ones. We really don't have a say in it, at least as far as London is concerned. We are who the world makes us. And free will? That just doesn't seem enter into London's equation. It's kind of scary, thinking that we don't have a say in who we are, but this novel suggests that that's just the way the world works. Natch.