The linked video is not a very good or informative short piece on the castrati.
1) It was acknowledged in Alessandro Moreschis lifetime that his voice was not very good, even when he was younger. The recording that is played during the segment was made on a wire recorder (poor fidelity) when he was quite old, and what little power he had was long gone.
2) His voice is compared to that of a mediocre mezzo-soprano, rather than to one of the very good sopranos whose voice might better represent the possibilities of a true castrato voice.
3) While it is usually claimed that Moreschi was the LAST of the castrati, many scholars believe that Domenico Mancini was an unacknowledged castrato in the Vatican as late as 1959. Officially he was a falsettist, not a castrato, though the director of the Vatican Choir spoke of him as a castrato.
4) Even though the castrati existed until fairly recently in European history, I have found no scholar who has given a comprehensive view of them. Each seems to work on a small piece of the puzzle. There is little about their early lives or, except for a very few individuals, about the reasons for the decision to castrate them. Below are a few small pieces of the puzzle. Full citations for the books and articles mentioned can be found in the Bibliography (
http://www.eunuch.org/vbulletin/showthread.php?t=17583) on the Non-Fiction Articles board.
Mary Frandsen (2005) does not begin her history of castrati with the castrated praise-singers to the gods of ancient Sumer, or even with the castrati of Byzantium, but rather begins her discussion of the line of descent that led to the Italian church castrati and the castrati of the opera with the Spanish castrati who were the earliest to be recruited to the Vatican choir. She attributes them, and the early French castrati (who seldom appear in the historical record, though there were apparently quite a number of early ones) to the use of eunuchs in Moslem Spain. Many of the Spanish eunuchs were Slavic slaves, captured in Eastern Europe and castrated in Verdun (France), before being sold into the Moslem world. There were plenty of eunuchs still around in Spain after the Reconquista in 1492 and frequent castration apparently continued locally well into the 16th century, though she gives no reasons for the practice. The 1587 papal brief of Pope Sixtus V, Cum frequenter, which forbid the marriage of castrati was written to end the common practice of married eunuchs in Spain. The brief was apparently controversial at the time and was sometimes ignored by local bishops.
Valeria Finucci (2003) focuses on Southern Italy as the source for most of the early Italian castrati. She discusses two intersecting causal threads. The earlier Byzantine, followed by Moslem, rulers of Sicily and the southern part of the peninsula used many eunuchs, including singers in the Byzantine era. Hence castration and castrati were familiar in the region. During the early period of Italian church castrati, Southern Italy was sunk in poverty and disease. Finucci claims, based on church parish records, that orphans were often disposed of by relatives such that they could not return to claim their parents lands. Girls were sent to convents and young boys were castrated by their extended families and sent to the church as potential clerics or singers (musical talent not required).
John Rosselli (1988) points to the early 17th century as a period of deindustrialization and of war and plague throughout Italy. Families became concerned about the division of wealth among multiple sons. Entry into the celibacy of the church or castration became a way to reduce the number of claimants to the family fortune. This, together with the early modern theory of sex as a hierarchy from female at the bottom to male at the top, left prepubescent boys and eunuchs in the middle ground, with castrati frozen on the journey to manhood. As much as 10% of the male population in some parts of Northern Italy were entered into the clergy. Some families placed their young sons with music teachers who, as part of their contractual obligations, were to pay for the castration of the boy when he reached the appropriate age. Rosselli quotes from several such contracts, including one contract relating to an 8 year-old boy, which explained that the boys father had resolved, for the greater benefit of [himself], of his family, and of [his son] to have him perfectly taught the profession or art of canto figurato, and in due time to have him castrated, but that castration was to be paid for by the singing teacher. Other contracts required the family to pay for the surgery when the time came. Some local rulers, such as the dukes of Mantua and Modena paid for the castration of promising young singers when their families were unable to afford the surgery.
Roger Freitas (2003; 2009) builds on Rosselli to emphasize the early modern ideal of Christian chastity and abstinence from sexual activity. Even intact males, if not the family heir, were expected to remain celibate and the sexual activity to produce heirs was considered a burden. Freitas biography of Atto Melani notes that he was one of seven boys in his family at least four of whom were castrated in childhood to become church castrati. One more never married and took an occupation usually reserved for eunuchs at the time, though there is nothing in the historical record to prove that he, too, had been castrated. One more son, who was definitely not castrated, never produced children, so that only one of seven produced heirs to the family line.
Another author (though I cant remember who, nor find his article in a quick scan of the 12 feet of catalogued books and articles on my shelves) noted that castration was a common treatment for many ailments in the 16th and 17th centuries. It was considered minor surgery with a very short recovery time and folk practitioners often performed it. He quotes the memoirs of one folk surgeon who wrote that he castrated about 200 males per year during his career. Most of them would, of course, have been adults, but many would have been children. Eunuchs and castrati were not all that rare in society.
All of this would indicate that 4,000 boys castrated per year in the early years of the castrato boom seems perfectly reasonable. By the late 18th and 19th centuries, however, there seems to have been a change in attitude and the numbers began dropping.
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