According to a tradition mentioned by the medieval Florentine chronicler Giovanni Villani, the venerable ceremony called the "scoppio del carro" (explosion of the cart) is related to the First Crusade, preached in Florence by the then bishop, Ranieri. Two thousand five hundred Florentine citizens enrolled under the command of Pazzino di Ranieri de'Pazzi.
On July 15, 1099, the crusader army took Jerusalem after a lengthy siege. Pazzino is said to have been the first to hoist the Christian banner on the walls of the holy city, and to have received from the 'commander-in-chief', Godfrey IV de Buillon (whom the Italians called 'Buglione'), a reward in the form of three chips of stone from the Holy Sepulcher of Christ. These were carefully set aside and brought to Florence in 1101.
Originally kept by the Pazzi family, these stones were used to kindle the first spark of the 'new fire', a symbol of the new life that began with Easter. After it was blessed, this fire was distributed to other families for their household needs (all lights and hearth fires were extinguished on Good Friday, in commemoration of Christ's death, and relit on Easter).
Florence thus adopted a custom known to have been developed at Jerusalem during the crusades: the distribution to clergy and laity of 'holy fire' in sign of Christ's resurrection; in Jerusalem this rite was performed in the Basilica of the Resurrection ('Anastasis') itself.

Medieval Florence in a miniature conserved in the Vatican Library.

The stone chips were entrusted to the Church of Santa Maria Sopra Porta, later called San Biagio. When that church was suppressed in 1785, the Holy Sepulcher stones were transferred to the neighboring church of Santi Apostoli, the parish priest of which still has them in his charge.
For centuries the fire kindled from them, once blessed and carried to the Cathedral, was used on Holy Saturday to light the Paschal candle, the candles of the clergy and people, and the lamps of the church.
On this occasion, a cart brought the new fire to private houses as well: first of all to the dwellings of the Pazzi family, which long kept this privilege (along with the burden of organizing the entire ceremony).
The cart gradually became more and more splendid, and the custom developed of 'charging' it with explosive powder, which - at least as early as 1494 - was set off first in front of the Baptistery (where the ceremony takes place today), and then a second time at the "Canto dei Pazzi" (the street corner where the Pazzi family had their houses). This second 'explosion of the cart' ended in 1900.
During the pontificate of Leo X (Giovanni de'Medici, 1513-1521), the 'colombina' - a rocket shaped like a dove, with an olive branch in its beak - was used for the first time. The dove is an evident reminder of the Holy Spirit, "Lord of life" and symbol of Easter peace). At the Gloria of the Easter morning Mass, the deacon uses the holy fire kindled from the stone chips from the Holy Sepulcher to light a fuse attached to the dove, which shoots down a wire extended from the Cathedral choir to the cart, brought early on Easter morning into the small square in front of the Cathedral. The cart, laden with fireworks, explodes, the dove-rocket igniting pyrotechnic devices of different kinds.

With the liturgical reform of the Easter Vigil in 1957, this ceremony was moved from noon of Holy Saturday to Easter Sunday morning. On that occasion, the Archbishop and Cathedral clergy go to the Baptistery to receive the holy fire brought from Santi Apostoli. The fire is accompanied by a cortège in historical costumes, with representatives of the City and people bearing the heraldic insignia of the Pazzi family.

The Archbishop then blesses the fire and sprinkles those present, and the crowd assembled in the piazza outside the Baptistery, with holy water blessed the night before during the Easter Vigil liturgy. Then the clergy and above-mentioned official representatives return to the Cathedral and, as the Archbishop intones the exultant tones of the Gloria, the centuries-old tradition of the 'scoppio del carro' is celebrated.