Fantasy and fact clash when it comes to the undead and authors of occult fiction would sometimes have us believe hair and fingernails and toenails continue to grow after death. Be ready then for a myth bust. nails and hair do not grow after death. In life, the toe nails grow at a regular rate of between one fifth to one third of a mm, per day; in death, the illusion is the result of the surrounding tissue desiccating (drying out and dehydrating). The shrinking of tissue away from the nail folds and hair shafts, gives the impression of growth.
From antiquity women prepared the deceased for burial which would include preserving locks of hair and pairing the nails, today professional undertakers will make up the corpse and use moisturisers to help reduce skin shrinkage. Occult practice predates Christian belief but gradually many of the pre-Christian rituals were absorbed into theological practice. The concept of a thaumaturge (wonder woker), such as a Saint or Magician harnessing sacred power through ritual would ensure a supernatural outcome is an accepted part of the belief set.
It was common to treasure relics of the Holy and use them as a focus of worship or practice magic. Relics were divided into primary (or part of the individual i.e. bones); or secondary relics such as clothing e.g. Saint Teresa of Avila’s sandal. Most relics were collected after death, both for practical reasons as well as for the purpose of authentication. Selling Holy relics was big business in the Middle Ages and a major source of revenue for the Church but not all relics were genuine.
Morbid hair and nails provided an interesting souvenir (or Mana), which was frequently counterfeited. There are accounts from 14th century Inquisitional records that refer to the clipping of nails and hair of the newly deceased and it was common practice for families to keep morbid hair and nails clippings as momento in the form of Mourning Jewelry. It is also well established hair and nails were frequent ingredients for magical spells.
Picasso for example, kept all his hair and nail clippings safe and would document these to prevent those who might use them in the practice black magic, against him. Many spells are purportedly enhanced by affixing the hair or nail clippings of the spell’s subject into the candle used in the rutual.
Similarly in the practice of Vodun , voodoo dolls may contain hair or nail clippings. Love spells intended to influence or control other are based on ages old practices and many involve locks of hair and nail clippings.
Rituals involving nails are not restricted to occidental society and in the Loi Kratong (Light Festival) of Thailand, people put money, strands of hair and nail clippings on a handmade raft then set it afloat in the river with thousands of others. The sky is filled with paper lanterns and fireworks and the hope is evil spirits within, will leave the body in this cleansing ritual.