Starting in the late Middle Ages rumors spread throughout Europe about blood miracles which concerned first the alleged relics of Christ, then those of the martyrs, and, finally, those of many other saints. Black blood, usually pulverized or solidified over the centuries, was said to return to its original bright red color, or it liquefied or bubbled in certain circumstances or on certain dates on the liturgical calendar. With the Reformation, in Protestant countries most of those relics were destroyed or simply forgotten, although for a few centuries there were cases of unspecified blood wonders or prodigies which would be much discussed in an attempt to interpret them in natural terms. In Catholic countries, on the contrary, the blood miracles multiplied, reaching a peak between the seventeenth and eighteenth centuries, when, even more than before (a fact which has never been properly emphasized by historians), they were used in the apologetic literature.
This paper aims to reconstruct the procedures with which, in the most heated moment of the debate, the properties of the ancient blood were tested and the results were made public, often not in publications, but through staging highly theatrical demonstrations. In the beginning, these were semi-clandestine performances which then, in what was the century of the birth of modern conjuring, were transformed into actual touring shows increasingly more popular and illusionistic. The time period that will be considered extends from the last two decades of the seventeenth century, when problems arose regarding the identification of reddish substances found in newly discovered Christian catacombs, to the 1790s, when the first round of testing the behavior of the blood of Saint Januarius of Naples was completed.
Particular attention will be paid to the miraculous Neapolitan bloods – St. Januarius, St. John, St. Stephen, St. Patricia, etc. In fact, in the eighteenth century they were studied by many scientists, who, throughout Europe (even in very Catholic Naples), developed recipes and equipment that replicated their "intelligent" behavior. In a period in which the canonization processes were undergoing a rapid evolution which was leading to medicalization, many asked themselves a question that was very tricky from both the theological and the scientific points of view: does demonstrating that a behavior similar to the miraculous can be produced naturally or artificially imply its non-supernatural nature? In other words, what role does analogy play in testing phenomena that are in some way unique and unrepeatable?