What is an altar but, quite literally, a glorified stone? And which altar is more glorified, really, than that of St. Peter's Basilica? Saint Peter, the first apostle, the first link in the papal chain of the Catholic Church, ostensibly martyred by the emperor Nero, the man who bound heaven to earth through an altar in Rome. Never mind that the chair behind it, the venerated Cathedra Petri, was actually the seat of Charles the Bald. The illusions woven over these architectural elements by the great baroque artist Gian Lorenzo Bernini disguise in light and energy the small ironies of religious myth.
Commissioned by Pope Urban VIII in 1624, the turmoil of the Thirty Years' War carried the Baldacchino through to completion in 1633. The troubles brought by the Habsburg's and the Protestant faction seriously distressed the coffers, but Urban seemed to feel that the expense of spectacularizing the Catholic faith was a necessity in the face of these threats. So in a time when religious commissions were nearly the entirety of an artists work, Bernini was living very well, and there was little so spectacular as the works he was producing. Much of his effort went into the completion and detailing of St. Peter's Basilica, already a lavish project, and the Baldacchino was its centerpiece.
While the apse, the site of the purported Chair of Peter, was filled with angelic figures, illuminated through an ornate glass window whose warm rays coruscate off their shimmering bronze, the altar itself was decorated by an enormous canopy: a structural cover like a gaudy four-post pergola called a baldachin. Typical baldachin are cloth-covered and for use in outdoor processionals, but the Baldacchino of Bernini is an extravagant bronze affair, seamlessly blending sculptural and architectural techniques, and rising almost nine stories in height.
Four twisted bronze columns corkscrew upward, still coming far short of the basilica's dome, where they terminate in angels. These posts reference the tomb of St. Peter as constructed by Constantine, which used similar twisted columns called solomonicas after their supposed origin in Solomon's Temple. The intricate vine ornamentation on these columns is filled with the emblems of Urban VIII's lineage: the bees of the Barberini family. These and other minute details such as the scorpion-eating lizard are sometimes credited to the assistance of Francesco Borromini. To erect these enormous columns, Bernini was forced to excavate part of the basilica's floor to create foundation posts. Quite a delicate operation when you consider that Peter was under there somewhere, but since he would have been directly under the altar, and the columns were spaced at the points of a large surrounding rectangle, the work was allowed to proceed. The belief was substantiated, however, when the construction unearthed evidence of a ring of tombs around the altar space that could have been religious followers (or just other burials in an old pagan graveyard).
We shouldn't forget that the Baldacchino is topped off by a roof-like structure, complete with slightly disturbed imitation ruffles, that pitches to an apex capped in a bronze orb-and-cross detail making it, all in all, a very imposing presence. This piece, however much it is considered the paragon of baroque aesthetics, was not as appreciated as it could have been. Much of the bronze required for this enormous endeavor was stripped from the Pantheon and although these pilfered girders would have been, for the most part, invisible to the public eye, Romans were none too pleased with the looting. Urban VIII, spendthrift that he was, had meanwhile substantially depleted the church funds during his reign, and the Baldacchino was not exactly a sale item on that bill. But whatever financial troubles they had, you can turn on the church channel even today and, if the Pope is in Rome, Catholic services are still given in the basilica, on the altar over the tomb of St. Peter, under the enormous visual weight of the bronze baldachin.
Wallace, Robert. The World of Bernini: 1598-1680. Time-Life Books (NY: 1970)
Duffy, Eamon. Saints and Sinners: A History of the Popes. Yale Press (S4C: 1997)