unburn

From un- +‎ burn.

unburn (third-person singular simple present unburns, present participle unburning, simple past and past participle unburned)

1. (intransitive) To undergo the process of burning in reverse.
   • 2002, Jim Al-Khalili, Black Holes, Wormholes and Time Machines, page 122:

     Similarly, you cannot 'unstir' the sugar from a cup of coffee once it has been dissolved, and you never see a pile of ash in the fireplace 'unburn' to become a log of wood again.

   • 2009, Bradley Harris Dowden, The Metaphysics of Time: A Dialogue, page 122:

     In your class last year we learned that increase in entropy is why matches burn but never unburn, why cans rust but never unrust, and why engines are never perfectly efficient.