Penric observed the process as best he could with Des's Sight coming in and out. Starting fires is something he can do as well, so he is trying to compare and contrast what he sees with what Caleb is doing. Which doesn't work so well when he feels like one sense barely works.

"Just as long as you don't burn your own hand practicing. We can get some candles. But... magic with similar effects, like starting fire might be a useful test. Especially as the process is difficult."

Pen didn't have to speak or gesture to do so, though he did as a habit.