Trial by ordeal is a judicial practice by which the guilt or innocence of the accused is determined by subjecting them to a painful task. The determination of innocence coming from either completing the task uninjured or by the quick healing of the wounds taken. Like trial by combat, it was a judicium Dei: God would help the innocent.
Common throughout the world in the past, the process survived until The Age of Enlightenment in western Europe and longer elsewhere.
"The Ordeal by Fire" by Dierec Bouts the Older
The ordeal was, in Europe, often by fire or water -- red-hot metal in the first instance and boiling water in the second. The exact use of the ordeal varied considerably, a Scandinavian and Anglo-Saxon practice was for the accused to walk nine paces with a red-hot iron bar held in both hands. Depending on the custom of the time innocence would be shown by a complete lack of injury from the ordeal or the wounds would be bound and regularly examined for healing or festering. An English version had nine red-hot ploughshares placed on the floor, the accused was blindfolded and if they successfully crossed the floor without injury they were judged innocent.
Ordeal by water varied. Gregory of Tours (died 695) recorded the common expectation that with a millstone round their neck, the guilty would sink: "The cruel pagans cast him [Quirinus, bishop of the church of Sissek] into a river with a millstone tied to his neck, and when he had fallen into the waters he was long supported on the surface by a divine miracle, and the waters did not suck him down since the weight of crime did not press upon him." (Historia Francorum i.35)
A variant on the ordeal by water was the requirement to remove a stone from a pot of boiling water, the injury sustained indicating guilt as in the trial by fire, sometimes the liquid medium used could be oil or molten lead. Trial by water could also be the consumption of 'bitter water' without harm. Or, it could be a variant on the dunking of witches, the accused would be bound and thrown into water - an innocent person would sink and a guilty person would float. The innocent person would then be rescued, not left to drown - not always successfully. Witches were imagined to supernaturally float above water because they had renounced baptism when entering the Devil's service. Some researchers theorise that specific diet was used to cause witches to float by increasing the amounts of gas within their intestines. By other theories of the time, innocent person would float with God's aid while the guilty one would sink. Either way the accused had little chances to survive this ordeal.
The elements in the trial by fire and the trial by water were usually under the control and supervision of the local clergy. Judicial records of these proceedings indicate that a fair number of those accused were in fact acquitted by the ordeals. Since the priests of the era knew their parishoners, knew of their reputations, and heard their confessions, it seems probable that the ordeals were rigged in some manner to yield a verdict that the priests thought were just.
The clergy were unwilling to submit to the hazards of the ordeals by fire and water themselves, so their ordeal was literally a piece of cake. Called the corsned ("cursed morsel") a piece of cake, bread, or cheese was placed on the altar of a church. The accused was taken to the altar and there made to recite a prayer to the effect that God would send the archangel Gabriel to stop his throat and make him choke if he was guilty. Few were convicted at these trials. Although it is plausible that the selection of the size of the morsel was up to the inquisitor, who likewise would have secret knowledge of the behavior of the accused from the confessional.
Some ordeals were less painful, both coscinomancy and axinomancy were considered ordeals but without harm being caused during the process of determining the guilty. Trial by bier, where it was believed that the wounds on the body of a murdered corpse would reopen and bleed in the presence of the murderer, was also an ordeal, last used in England in 1628. The practice of dunking for witchcraft was practiced well into the seventeenth century. Pricking an accused witch to find the Devil's mark can also be considered a type of ordeal. Matthew Hopkins, the notorious "Witch-Finder Generall," made his victims submit to these ordeals. These latter ordeals were more in the manner of divination used as a tool for investigating crimes rather than actual trials.
In the common law of England, ordeals began to fall into disuse beginning with the Assize of Clarendon established by Henry II. It continued to be used, either judicially or extrajudicially, in cases where no other proof was thought possible, as in the case of secret and unwitnessed murders or supernaturally concealed crimes such as witchcraft. Another blow was struck against trial by ordeal in 1215, when the Fourth Lateran Council forbade the Roman Catholic clergy from administering the ordeals. Since circumstances suggest that the intervention of the clergy was pivotal in the outcome of the ordeals, this prohibition severely hampered the practice. In obedience to this principle, in 1220, during the reign of Henry III, trial by ordeal was abolished in England, and jury trials were substituted in every case that was formerly tried by ordeal. When the witch persecutions finally died out at the end of the seventeenth century, the last vestiges of ordeal ended with them.
The trial by ordeal was something different from the peine forte et dure, which involved pressing an accused person who refused to enter a plea of guilty or not guilty with heavy weights until he relented or died.