Abstract: This invention relates to a method of manufacturing heat cells which are based on a specific iron oxidation chemistry and having specific physical dimensions and fill characteristics. This method uses direct compaction of powdered ingredients into granules, pellets, tablets, slugs, and/or the like. These heat cells, which can be incorporated into disposable body wraps, provide a controlled and sustained temperature for consistent, convenient, and comfortable heat application for treating temporary or chronic pain. This invention also relates to said exothemic compositions incorporated into said heat cells.