Mugwort, Artemisia argyi, is described as bitter, acrid and warm, becoming hot when burned. Its virtue in moxibustion is not simply heat but the combination of heat with the aromatic, penetrating quality of the herb.
Older moxa is preferred because the volatile oils that make fresh moxa acrid and irritating diminish with storage, leaving a gentler and more even burn. The wool is refined by repeated sieving to remove stems and coarse fibre; the ratio of raw leaf to finished wool, expressed as 10:1 or 30:1, is a rough index of quality. Poorly refined moxa burns unevenly and produces more smoke and more discomfort.