Incorruptibility is the failure of some human bodies to decompose after death. The term comes from the Catholic Church, which considered the phenomenon a sign of sainthood in the Middle Ages. The Catholic Church no longer uses this criteria for canonization, however, though the bodies of some incorruptible saints are still on display and visited by the faithful. In other cultures, incorruptibility may be a sign of evil rather than holiness, suggesting that the deceased is a vampire. Though incorruptibility has never been sufficiently explained, some people believe it to be an imperfectly understood scientific phenomenon.
The great majority of incorruptibility cases are associated with Catholic, Anglican, and Orthodox Christianity, but this is possibly because people of these faiths regularly exhumed the bodies of alleged saints in order to check for incorruptibility, while people of other cultures rarely unearthed their dead. Incorruptible bodies were considered very powerful relics in the medieval era and often placed on display in large glass reliquaries in churches, where many remain to this day. An incorruptible body does not exactly resemble a living body, as the skin is usually discolored and desiccated to some degree, so some incorruptible saints are enhanced with wax masks.
It is important to note that incorruptibility is distinct from natural mummification, which may happen to a corpse in a bog setting, for example. Also, a corpse that has undergone any type of embalming may not be designated as incorruptible. Incorruptibility does not seem to be caused by unusual soil types, temperatures, or other conditions of the burial. Some incorruptible bodies were buried next to corpses that decayed normally, and others had clothes that decayed while the body remained intact.
In religious lore, incorruptibility is often said to be accompanied by other supernatural phenomena, including a sweet smell known as the odour of sanctity, a lack of rigor mortis, stigmata or martyrdom wounds that continue to bleed, physical warmth long after death, and even movement. However, such cases are much less well documented than incorruptibility itself. Some incorruptible saints exuded a sweet-smelling oil known as "Oil of Saints" that is believed to have miraculous healing powers.
The scientific phenomenon behind incorruptibility is sometimes said to be saponification, in which the body's fats are converted to adipocere, a soap-like substance. Saponification is more likely to occur in corpses with large amounts of fat and in alkaline soils, and many bodies experience it to some degree, but not to the extent of preservation seen in incorruptibles. The incorruptibles known to the Catholic Church, however, are not particularly fat, and as mentioned above, the soil does not seem to be a significant factor.