Stella had noticed Vale's actions from the very beginning.
She saw how he calmly prepared his Ritual Prayer for the first ten minutes. During that time, she saw him revising his prayer multiple times before he felt satisfied.
Then, from the five bottles of bone powder she had prepared, only Vale took one.
In her class in Elementary Spirit Law, she taught them that using bone powder easily attracts evil spirits, especially if it's human bone powder.
If it's an animal's bone powder, the chances of attracting evil spirits might be lower, but there's a higher chance that you'll summon a spirit with low intelligence… It means that assessing it, whether it's friendly or not, will be tougher.
