"smoldering ash" is correct and usable in written English.
You can use it when referring to the ashes of a fire that are still giving off heat or smoke. For example, "The last of the fire had burned down to smoldering ash."
This playground and everything around it will deteriorate into a dystopian war zone, a lawless heap of smoldering ash.
The fire in Breezy Point, fueled by the storm's unrelenting winds, reduced more than 80 homes to smoldering ash.
THE Paris Commune of 1871 is one of those smoldering ash heaps in French history, like the 1968 riots, stirred habitually by left and right to inflame old class allegiances and fears.
— SHERI FINK AND ANEMONA HARTOCOLLIS Dozens of homes in the Breezy Point section of the Rockaways — many of them elaborate year-round homes, some of them seasonal bungalows — were turned into acres of smoldering ash by a wind-whipped fire that burned for perhaps nine hours throughout Monday night and early Tuesday morning.
Small piles of smoldering ash, each with the potential to flare.
Fire officials were able to board the flight and deal with whatever smoldering ash was left after the OJ bath and no injuries were reported.
Do not underestimate the regenerative power of smoldering ashes.
With no insurance and now just a pile of smoldering ashes for a home, the K.D.H.E.
Shea Stadium rose on the ashes of Ebbets Field and the Polo Grounds, and the smoldering ashes of the Corona Dumps, memorialized in "The Great Gatsby".
As the Afghan laborers tried to extinguish the flames with their water bottles, at least one laborer plunged into the smoldering ashes to retrieve the books, Mr. Zafar said.
Angela's ashes represent her crumbling hopes: her dreams of raising a healthy family with a supportive husband have withered and collapsed, leaving her with only cigarettes for comfort and the smoldering ashes of a fire for warmth.
