Costanza was born around 1614, daughter of Lorenzo Piccolomini, member of a minor branch of the important Sienese family. The first time she appears in documents is in Rome during 1625, when she was 11 years old. The name of her mother is unknown because in this document, coming from the Stati d'Anime of the parish S. Lorenzo in Lucina, Costanza was resident in the street "behind the palace of the church" - the current via della Vite - with her father Leonardo and her step-mother Tiberia. Costanza did not mention her mother in her last will, signed around 23 January 1662.^[1] Even if Lorenzo was part of the "poor" branch of the Piccolomini family and he worked as a groom, his name and his association with the powerful clan are still today for the identification of Costanza, who in the documents is always named 'Signora' and, in the aforementioned will, she established that her inheritance could be inherited by all descendants, as long as their last name was Piccolomini.

On 15 August 1628, on the occasion of Solemnity of the Assumption, Costanza received a dowry of 45 scudi (the equivalent of an annual rent for a house of modest dimension) from the Confraternity of San Rocco, funded by Giambattista Borghese, brother of the late Pope Paolo V. In 1630, Costanza, named as 'la zitella da Viterbo' ('the spinster of Viterbo'), received the promise of a second dowry of 26 scudi and 44 baiocchi, this time from the Gonfalone Confraternity. Costanza then married the sculptor, restorer and art dealer Matteo Bonarelli (or Bonucelli) from Lucca, on 16 February 1632 in his parish of San Lorenzo in Lucina. On 28 February, the marriage contract was signed between Costanza, her father Leonardo and her husband Matteo.^[2] The dowry was fixed at 289 scudi. Costanza was 18 years old and Matteo 28: they settled in the current Scanderbeg alley, at the foothills of the Quirinale.

The first evidence of Matteo Bonucelli being employed as an assistant of Bernini is the payment, by the year 1636, for three putti of marble for the S. Pietro; the following year, Matteo assisted Bernini with the mausoleum dedicated to Countess Matilde, also in San Pietro. When they first met, Costanza was a married woman of 22 years old, Gian Lorenzo Bernini a 38-year-old bachelor. The portrait of Bargello, in which the sculptor of cardinals and popes immortalized his lover in marble, may have been started in 1636, but it was definitively finished in October 1637 when Fulvio Testi, a friend of Bernini, declared in a letter to the Count Francesco Fontana that it was the most beautiful portrait ever executed by the artist. With this last annotation, we can assume that the portrait was partly known to the public.^[3]

In the summer of 1638, a scandal broke out. When Gianlorenzo Bernini discovered that Costanza was having an affair with his younger brother Luigi, Gianlorenzo was blinded by jealousy, and his excesses were described by his mother, Angelica Galante Bernini, in a letter destined for the Cardinal Francesco Barberini, dated around autumn 1638.^[4] Costanza was punished by a groom of the Bernini family who slashed her face upon request of Bernini himself. Courtesans were frequently victims of the crime of being slashed in the face, which was described by the late Italian lawyer Prospero Farinacci as "atrox et grave delictum". Costanza was indeed described as a courtesan and punished (perhaps for adultery) with detention in the Domus Pia de Urbe, a place also known as the monastery of Casa Pia, while the groom who injured her was exiled. Gianlorenzo Bernini, however, was at first condemned to pay a penalty of three thousand scudi, and then pardoned, whilst his brother Luigi, who was less famous and considered less important by the Pope, was also exiled from Rome. It was not until 7 April 1639, and only after writing a heartbreaking plea to the Governor of the house,^[5] that Costanza was "given back to her husband".

During the following years, Costanza lived in her old house/study in harmony with her husband, and she continued the business of merchant and art dealer with fair success, including after the death of Matteo Bonacelli (18 January 1654) and during the pontificate of the Sienese Alessandro VII Chigi. In his will, signed in 1649, Matteo designated as sole heir "Signora Costanza Piccolomini mia dilettissima moglie".^[6] Payment orders intended for the Bonucelli widow refer to her as "Signora Costanza", or "Costanza Piccolomini" or also "Costanza scultora".^[7]

Costanza had a large collection of artworks that was exhibited in the main floor of her house, and in two rooms on the upper floor. One of the most famous works collected by her was the Plague of Ashdod by Nicolas Poussin, commissioned by the Sicilian nobleman Fabrizio Valguarnera in 1630. In 1665, when Bernini was in Paris, he saw the artwork in the palace of the Duke of Richelieu and claimed to know it, suggesting that the painting be hung lower to view it better. In the same year, Richelieu sold the painting (today located in the Louvre^[8]) to King Louis XIV of France.

During Easter 1657, Costanza Piccolomini became the mother of a child, Olimpia Caterina Piccolomini. Costanza made her will in February 1659, and three days before her death she added a codicil, on 30 November 1662. Costanza was not buried with her husband in the crypt of their parish of St. Vincenzo and Anastasio, but was buried in the church of Santa Maria Maggiore.

• Sarah McPhee, Bernini's Beloved. A portrait of Costanza Piccolomini, New Haven and London, Yale University Press, 2012.
• Domenico Bernini, Franco Mormando, The Life of Gian Lorenzo Bernini, Pennsylvania State University Press, 2011.