We remodeled a few years ago and I tore down the old stone Fireplace & chimney. It Was about dead center in the place and the decision was made to open the floor plan. This fireplace was at least 100 yrs old and had a stone And masonry base. The attached photos show the sub-floor once the hearth was replaced. We never had on open fire in as there was an Encore to a Dura-liner flue. How many fireplace fires would have been necessary to reach the reduced kindling temp caused by repeated heating and cooling of the subfloor?