The internet will tell you to bake it in the oven to dry it out. Don’t do this. It’s a fire hazard, and it won’t bring back the stored energy. Once the volatile compounds have evaporated or absorbed too much water, they’re gone forever. Like your ex’s trust.
Your best bet is to mix old charcoal with fresh stuff. Use a 50/50 ratio. The fresh charcoal will get the fire going, and the old stuff will contribute some heat—just not the star player. Kind of like a mediocre backup dancer.
