How did Pat Perrin shape and colour her corks

So It's always been a mystery to me how Patricia Perrin made her very distinctive corks. Did she ever give a class on it or talk about them. I've just read a story by Brickell that she used fire in some way. But apart from that not much else. I have a faint memory she stained them too. Does anyone recall anything?

Way back we used Condy's Crystals to colour corks and rope for macrame work.
Potassium permanganate.