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The Museo dell'Opera, established in 1869, is housed in space that was created by bricking in the first three bays of the right aisle of the so-called "New Cathedral," on which building began in 1339 and was halted after the plague epidemic in 1348. Famous above all for its outstanding collection of marble statues of ancient prophets, sibyls, and philosophers carved by Giovanni Pisano between 1285 and 1297, and Duccio's great altarpiece of The Virgin and Child Enthroned in Majesty, known as the "Maestà," which is one of the supreme achievements of Sienese art, the museum contains numerous masterworks by artists like Ambrogio Lorenzetti, Taddeo di Bartolo, Sano di Pietro, Matteo di Giovanni, and Domenico Beccafumi. Other significant pieces in the collection are wooden sculptures by Francesco di Valdambrino, Jacopo della Quercia, and Francesco di Giorgio Martini, and an extraordinary display of gold and silver liturgical objects; especially noteworthy is the set of altar vessels made for the Chigi Chapel in the cathedral using embossed silver, enamels, gold, and rock crystal. The Hall of Vestments gives access to a staircase inside the wall that leads to the top of what would have been the façade of the New Cathedral, familiarly called the "Facciatone," which offers a magnificent view of the city and the surrounding countryside. The museum incorporates the church of San Niccolò, a rare example of late baroque art in Siena. The church was originally annexed to the Spedaletto di Monna Agnese, a charitable institution founded in the second half of the thirteenth century to aid needy mothers and girls. Works by Rutilio Manetti, Raffaello Vanni, and Astolfo Petrazzi adorn the side altars, while the high altar is graced by a beautiful altarpiece by Francesco Vanni showing The Virgin and Child Enthroned with Saints Lawrence, Gregory, Nicholas, and Agnes. The rich stucco decoration is the work of the noted master Ludovico Chiappini. The scenes painted inside the stucco frames constitute the final phase of decoration of the church and are the work of the painter Giambattista Giustammiani, known as "Francesino" ("the little Frenchman"), who was active in Siena from 1608 to 1643. The cycle of stories from the life of the Virgin Mary also includes pictures of sibyls, saints, and sacred allegories, in a sentimental, idealized style well suited to the devotional needs of the community of pious women who were its principal patrons.