Clark took the lock of hair and incinerated it. The lead box was something else that was in the vault.
You have to remember the timeline here. Later in the series, Clark might have been more careful, but as of this date, he has only seen Kryptonite once -- at his parents' farm. The piece that Irig sent to the lab in Wichita disappeared, but he has no reason to think that Luthor might have it.
Now I'm not saying that's what is in the box, but this is why Clark is less careful than perhaps he should be when opening a mysterious lead box.
Nan
