The Baths of Diocletian, the largest in ancient Rome, are, paradoxically, both more and less altered than those of Caracalla. Much of the original vaulting still covers the central hall, resting upon eight ancient columns of red granite topped by rich Corinthian capitals. But this hall has been used as a church for about four hundred years, while other parts of the baths have been turned into cloisters, now included in the Terme Museum, and into a planetarium. One of four round buildings which occupied the corners of the outer enclosure was converted into the church of San Bernardo in 1594 and another has been used as a girls' school.
The baths were built between A.D. 298 and 306. Just when they ceased to function is not certain--probably not long after the aqueducts were cut during the Gothic wars of the sixth century. Like the Baths of Caracalla, which they resemble in plan, they were supplied with water by a branch of the Aqua Marcia. Like these baths, too, their brick-faced concrete structure was originally covered with stucco painted to imitate marble; less is known about the fate of their marble and mosaic decoration, and less has been found here than in Caracalla's baths.
During the Middle Ages these were the most celebrated of all Roman baths. A late version of the Mirabilia used them to illustrate the magnificence of baths in general--'as may be seen in the thermae of Diocletian before Saint Susanna' and added: 'In the palace [baths] of Diocletian were four temples, of Aesculapius and Saturn and Mars and Apollo, which are now called the Bushels.' These were the round buildings at the corners of the enclosure walls.
In the fourteenth century, Petrarch described the ruins to his friend Giovanni Colonna, writing of the pleasures they had shared during the poet's early visit to Rome: 'We used, after the fatigue of wandering about the immense city, often to make a halt at the Baths of Diocletian, and sometimes to ascend to the vaulted roof of that once most magnificent edifice; for nowhere is there sweeter air, a wider prospect, more silence and desirable solitude. . . . And wandering among the crumbling walls, or sitting on the roof, the fragments of the ruins beneath our eyes, we used to have much talk on history; I being allowed to be the better versed in ancient, you in modern story.' Du Pérac's etching, more than two hundred years later, suggests that there was among these ruins then something of the same luxuriant growth as that which delighted visitors centuries later in the Baths of Caracalla.
The ruins changed owners several times during the Middle Ages and early Renaissance. In 1091 Pope Urban II granted the baths to Saint Bruno for a monastery of Carthusian monks; again, in the fourteenth century, they were granted by the papal Curia of Avignon for the same purpose. The representation of the baths in fourteenth and fifteenthcentury views of Rome, which show eight or ten monastic cells, suggests that a Certosa was actually installed.
In the 1560's Pius IV renewed the idea of a Carthusian convent, and turned the great central hall of the baths into a church. Michelangelo was put in charge of the enterprise, which he carried out with magnificent self-restraint, respecting the ancient structure wherever possible and giving it, in its Christian guise, a feeling essentially harmonious with that of ancient Rome. Diocletian might feel at home today in the church occupying the central hall. Vanvitelli, who reoriented the interior in the eighteenth century, did more damage to the old structure, but could not spoil its essential harmony.
Hawthorne's description in the French and Italian Notebooks of the ruinous pile, as he saw it in 1858, stressed the church rather than the ancient baths:
'Today, which was bright and cool,' he wrote, 'my wife and I set forth immediately after breakfast, in search of the Baths of Diocletian, and the church of Santa Maria degli Angeli. . . .
'We turned into the Piazza di Termini, the entrance of which is at this fountain [the fountain of the Esedra]; and after some inquiry . . . we found our way to the portal of Santa Maria degli Angeli. The exterior of this church has no pretensions to beauty or majesty, or, indeed, to any architecture whatever--for it looks like a confused pile of ruined brickwork, with a façade resembling half the inner curve of a large oven. No one would imagine that there was a church under that enormous heap of ancient rubbish. But the door admits you into a circular vestibule, once an apartment of Diocletian's Baths, but now a portion of the nave of the church. . . . now, with little change, except of detail and ornament, transformed into the body of the church. This space is so lofty, broad, and airy, that the soul forthwith swells out and magnifies itself. . . . It was Michel Angelo who contrived this miracle; and I feel even more grateful to him for rescuing such a noble interior from destruction than if he had originally built it himself.
In Hawthorne's time the ruins housed not only the churches of Santa Maria degli Angeli and San Bernardo and the Carthusian convent and cloister, but granaries, charitable institutions, and prisons as well. In the last quarter of the century an American sculptor's studio was added to the miscellany which the Italian Government gradually took over for public purposes. Moses Ezekiel's studio, nestled in the southwest angle of the ruin at the right of the present entrance from the Piazza dell' Esedra to the Musco delle Terme, was a gathering place for artists, musicians, and the cosmopolitan society of Rome. It was taken over in 1910 as part of the new National Museum which had been begun in the Carthusian cloisters in 1889. In Ezekiel's time this small chamber near the entrance was divided into two stories; one a lower studio or work room, the other an upper studio for living and entertaining, reached by an inclined plane leading up the outside wall to a balcony overhung by wistaria and white roses. Ezekiel had originally selected his quarters partly from motives of economy and partly from the romantic desire to live in a ruin. When he first established himself there, the neighborhood of the baths, so busy now, was a 'wide and empty space crossed by deserted roads leading past the vast and solemn ruins'.
Today the circular Piazza dell'Esedra in front of the church still preserves the shape and name of Diocletian's immense exedra in the park south of the calidarium. In the semi-circular modern colonnades facing the church, visitors may sit now to eat ices or sip cool drinks and to watch the great jet of water splashing into the modern Fountain of the Naiads from the Aqua Marcia which supplied the baths in Diocletian's time.