The meaning of "glass behind driven flame"

Question

‘‘Thank you.’ Then with a sudden swoop into the humility of an abashed boy—‘’Sorry if I haven’t said the proper things. You see—there’s Harvey looking at us again. Oh, I want to say—if ever you want anything in the way of orchids or goldfish or—or anything else that would be useful to you, you’ve only to come to me for it. Under the will I’m perfectly independent, and we’re a long-lived family, worse luck!’ She looked at me, and her face worked like glass behind driven flame. ‘I may reasonably expect to live another fifty years,’ she said.

This is from "The Dog Hervey " by Rudyard Kipling.
https://www.kiplingsociety.co.uk/tale/the-dog-hervey.htm

I can't Understand the meaning of "her face worked like glass behind driven flame. "
I am glad if someone would kindly teach me.

Answer 1

The phrase "worked like glass" sounds to me like a simile comparing her facial movements to glassworking or glassblowing (with "behind driven flame" specifically implying hotter, faster, more-liquid-like movement).

So, I imagine this means the author is commenting that the lady's facial expressions are quite animated.