On August 14th, 1730, during devotions for the vigil of the feast of the Assumption of the Blessed Virgin Mary, thieves broke into the Church of Saint Francis (Siena), broke the lock to the tabernacle and stole the the golden ciborium containing consecrated Hosts. The friars discovered the break in the following morning and a parishioner found the lid of the ciborium discarded in the streets.
The archbishop immediately called for prayers and reparations as the celebrations for the Feast were cancelled. On the morning of the 17th, while praying in the Church of Saint Mary of Provenzano, a parishioner alerted the priest that he could see glowing light emanating from the alms (collection) box. In the presence of the Archbishop, the collections box was opened with the priest and friars; inside were all 351 hosts in the box, suspended by cobwebs. When examined, all 351 matched exactly the unconsecrated hosts that had been baked for the 14th and were immediately cleaned.
After placing them in a new ciborium the townspeople and Archbishop Alessandro Zondadari carried in the Sacred Hosts in a procession to the Church of Saint Francis. Over the next nearly 50 years, the Hosts showed no signs of decomposition. On April 14th, 1780, Father Carlo (the then Superior General of the Franciscan Order) consumed one of the un-corrupted hosts, finding it fresh and as if it had been baked the night before. Since some of the host had been distributed during those 50 years, Father Carlo ordered the remaining 230 pieces be sealed in a new ciborium and forbade any further distribution or consumption.
In 1789, Archbishop Tiberio Borghese of Siena ordered a further, more meticulous investigation. The three Franciscian friars present at the 1780 investigation were questioned under oath and the hosts inspected under a microscope. The investigation found that the Hosts were the same from 1730 and that no corruption or deterioration had occurred. Furthermore, the Archbishop placed a group of unconsecrated hosts into a similar box in his chancery office. When the box was opened, it was found they had all deteriorated. They were again placed in a sealed box and reopened in 1850. The hosts were by then fully disfigured with a dark yellow tint - the 1730 Consecrated Hosts still appeared fully fresh and with pleasant smells.
In 1914, Saint Pope Pius X ordered a scientific investigation involving scientists, doctors and theologians. The team concluded that the Hosts had not been prepared in any special way and had been stored in common conditions of humidity and light which should have caused expected deterioration. Professor Sir Grimaldi, the professor of chemistry at the University of Siena and director of the Municipal Chemical Laboratory was the chief chemical examiner during this investigation. He would later write in his book Uno Scienziato Adora:
“grain flour is the best breeding ground for microorganisms, animal and plant pests, and lactic fermentation. Siena particles are in perfect condition, against the physical and chemical laws, despite all unfavorable conditions in which they were found and preserved.”
In 1922, Cardinal Giovanni Tacci, accompanied by the Archbishop of Siena and Bishops of Montepulciano, Foligno, and Grosseto consumed a piece of the host and declared:
the Hosts tasted like unleavened bread, were starchy in composition and were completely preserved
In 1950, the Hosts were placed in an elaborate new ciborium only to be stolen on the night of August 5th, 1951. The Clergy located the Hosts in a corner of the tabernacle, apparently left by the thief. The Archbishop counted 133 Hosts and sealed them in a silver ciborium. On the 17th of every month, the Hosts are now publicly displayed and on the Feast of Corpus Christi they are placed in the processional monstrance and carried in procession through the town with townspeople participating.