What happens to Helen’s apron when she tries to dry it? How does this happen?

Helen's apron caught of fire because she threw it over the hot ashes to make it dry faster.

One day I happened to spill water on my apron, and I spread it out to dry before the fire which was flickering on the sitting-room hearth. The apron did not dry quickly enough to suit me, so I drew nearer and threw it right over the hot ashes. The fire leaped into life; the flames encircled me so that in a moment my clothes were blazing. I made a terrified noise that brought Viny, my old nurse, to the rescue. Throwing a blanket over me, she almost suffocated me, but she put out the fire. Except for my hands and hair I was not badly burned.