Grannus was especially revered at healing sanctuaries centered on thermal springs, which ancient peoples understood as manifestations of fire beneath the earth. Hot springs were seen as places where subterranean fire met sacred water, creating a powerful healing force.  Major cult sites include: Aquae Granni (modern Aachen, Germany), Grand (Andesina) in Gaul, and sites in Noricum and Raetia.  In these locations, the heat of the water—not flame—was the primary expression of sacred fire. Bathing in these waters was both a medical and religious act.  Under Roman rule, Grannus was frequently identified with Apollo through interpretatio romana. In inscriptions, he appears as Apollo Grannus, merging Celtic and Roman concepts of healing, light, and purifying heat.

     Apollo’s solar and purifying fire complemented Grannus’s role as a healer.  Apollo contributed  with a radiant celestial fire

as Grannus with a earth-born, therapeutic heat.  This fusion reinforced the idea that fire heals as well as burns, depending on its form and control.  While Grannus is not definitively a solar god, many scholars suggest a solar dimension to his cult with his association with Apollo. His name’s implication of radiance and his connection to warmth and vitality.  In this sense, Grannus represents gentle solar fire—the warmth that sustains life, promotes growth, and restores health, rather than the harsh or destructive sun.

    Rituals associated with Grannus likely included bathing in heated waters, votive offerings left near springs, and prayers for healing and renewal.  Inscriptions suggest that devotees credited Grannus with miraculous cures, viewing the healing heat as a divine presence working through fire-warmed water.  Fire here is mediated and softened, transformed into a force compatible with human life.  In Celtic religion, fire often symbolizes purification, vitality, transition, and protection.  Grannus fits within this framework as a god of restorative fire, paralleling other Celtic figures such as Belenus, with whom he is sometimes compared. However, Grannus is more specifically focused on healing heat rather than seasonal or ritual flame. 

    Grannus stands out for embodying fire as medicine.  Grannus represents a distinctive and compassionate expression of sacred fire in ancient religion. His fire heals rather than destroys and warms rather than consumes. Grannus restores balance rather than punishes.  Through Grannus, ancient Celtic and Gallo-Roman worshippers understood fire not only as a force of danger or transformation, but as a source of health, vitality, and renewal flowing quietly from the earth itself.

References:

• Anne Ross, Pagan Celtic Britain
• Archaeological remains from Aquae Granni (Aachen)
• Barry Cunliffe, The Ancient Celts
• Encyclopedia Britannica, “Grannus”

• Latin votive inscriptions to Apollo Grannus (CIL)

• Miranda Green, The Gods of the Celts

• Pierre Lambrechts, Les divinités celtiques