From The Art and Popular Culture Encyclopedia
Apotropaic magic is a ritual observance that is intended to turn away evil. It can be as elaborate as the use of magical ceremonies or spells, or as simple as the vaguely superstitious carrying or wearing of a "good luck" token or "charm" (perhaps on a charm bracelet), crossing one's fingers or knocking on wood.
"Apotropaic" is an adjective that means "intended to ward off evil" or "averting or deflecting evil" and commonly refers to objects such as amulets or other symbols. The word is of Greek origin: apotrope literally means "turning away" or averting (as in "averting the evil eye"). The Greeks propitiated the chthonic "Gods of Aversion"—the apotropaioi.
Among the Ancient Greeks the most widely-used image intended to avert evil was that of the Gorgon, the head of which now may be called the Gorgoneion, which features wild eyes, fangs, and protruding tongue. The full figure of the Gorgon holds the apex of the oldest remaining Greek temple where she is flanked by two lionesses. The Gorgon head was mounted on the aegis and shield of Athena.
Curiously, eyes were often painted to ward off the "evil eye". An exaggerated apotropaic eye was painted on Greek drinking vessels in the 6th century BC to ward off evil spirits while drinking. Fishing boats in some parts of the Mediterranean region still have stylised eyes painted on the bows. A Turkish budget airline has adopted the symbol (known as Nazar boncuğu or Nazar bonjuk) as a motif for the tailfin of its aeroplanes.
The doorways and windows of buildings were felt to be particularly vulnerable to evil. On churches and castles, gargoyles or other grotesque faces and figures such as Sheela na Gigs and Hunky Punks would be carved to frighten away witches and other malign influences. Those other openings, fireplaces or chimneys, may also have been carved. Rather than figural carvings, these seem to have been simple geometric or letter carvings. Where a wooden post was used to support a chimney opening, this was often an easier subject for amateur carving. To further discourage withcraft, rowan wood may have been chosen for it.
Similarly the grotesque faces carved on Pumpkin lanterns (and their earlier counterparts, made from turnips, swedes or beets) at Halloween are meant to avert evil: this season was Samhain, the Celtic New Year and, as a "time between times", it was believed that souls of the dead and other dangerous spirits walked the earth at this time. (See also: Celtic calendar)
Mirrors and other shiny objects were believed to deflect the evil eye. Traditional English "Plough Jags" (performers of a regional variant of the mummers play) sometimes decorated their costumes (particularly their hats) with shiny items, even to the extent of borrowing silver plate for the purpose. "Witch balls" are shiny blown glass ornaments, like Christmas baubles, that were hung in windows.
In ancient Greece, phalli were believed to have apotropaic qualities. Often stone reliefs would be placed above doorways, but there were also many three-dimensional renditions erected across the Greek world. Most notable of these were the urban monuments found on the island of Delos. Grotesque, satyr-like beaded faces, sometimes with the pointed cap of the workman, appeared often over the doors of ovens and kilns, to protect the work from fire and mishap. A similar use of phallic representations to ward off the evil eye remains popular in modern Bhutan and is associated with the 500 year old Buddhist tradition of Drupka Kinley, and is paralleled by other south Asian uses of the lingam symbol.