Illegal research
It was easy for Footer to dismiss the purification as simply annoying, but as the one actually experiencing it, Nero was not having such an easy time. Although the pain was only skin deep, that did not mean it was light.
The itching got so bad he felt the urge to peel his skin, and the burning was so intense he wondered if he was actually being cooked alive. It had been a while since he had felt the effects of temperature.
But while the agonising sensation in his body demanded his attention, Nero kept his eyes on the man casting the spell. His expression never changed, even as Nero's body began to tremble. He maintained the same sense of focus he had when he was casting the spell on Footer.
So either Footer also went through the same thing, but did not let it show at all, or this was standard procedure.