Leonardo's work on the cartoon of the Battle of Anghiari was suddenly interrupted in the last months of 1504, and he went to Piombino on a somewhat official mission as a military architect. (During his absence Michelangelo was to receive the commission for the Battle of Cascina.) His mission was preceded by a diplomatic action conducted by Machiavelli himself. It can be inferred, therefore, that it was Machiavelli who suggested Leonardo's name for the programme of fortification projects suggested by Jacopo IV Appiano, Lord of Piombino, an ally of the Florentines at the time of the Pisian war, 1503-4, when Leonardo had already been consulted on the project of diverting the Arno River for strategic reasons, and when Antonio da Sangallo the Elder was the chief military architect of the Florentine Republic. Leonardo's activity at Piombino, revealed by newly discovered evidence, included the study of the city walls, the citadel and the main gate.
Source:Leonardo: A Study in Chronology and Style, p. 95.
in Piazza della Signoria, Carrara and Rome
The Genoese enter into negotiations for a colossal statue of Andrea Doria, which they desired to obtain from the hand of Michelangelo. Its execution must have been seriously contemplated, for the Senate of Genoa banked 300 ducats for the purpose.
Michelangelo works on a sculpture called Dusk over the course of ten years until 1534.
Pope Clement VII offers Michelangelo a pension in order to retain his services. It appears that Michelangelo only asked for fifteen ducats a month, and that his friend Pietro Gondi had proposed twenty-five ducats. Fattucci rebuked him in affectionate terms for his want of pluck, informing him that "Jacopo Salviati has given orders that Spina should be instructed to pay you a monthly provision of fifty ducats." Moreover, all the disbursements made for the work at S. Lorenzo were to be provided by the same agent in Florence, and to pass through Michelangelo's hands. A house was assigned him, free of rent, at S. Lorenzo, in order that he might be near his work. Henceforth he was in almost weekly correspondence with Giovanni Spina on affairs of business, sending in accounts and drawing money by means of his then trusted servant, Stefano, the miniaturist.
Two important letters from Michelangelo to Fattucci, written in October 1525 and April 1526, show that he had then abandoned the original scheme (for the Medici tombs), and adopted one which was all but carried into effect.
Pope Clement VII makes Fattucci write to Michelangelo that he wishes to erect a colossal statue on the piazza of S. Lorenzo, opposite the Stufa Palace. The giant is to surmount the roof of the Medicean Palace, with its face turned in that direction and its back to the house of Luigi della Stufa. Being so huge, it would have to be composed of separate pieces fitted together. Michelangelo speedily knocked this absurd plan on the head in a letter.
By a deliberation of the Signory, we are informed that the marble had been brought to Florence about three years earlier, and that Michelangelo now received instructions, couched in the highest terms of compliment, to proceed with a group of two figures until its accomplishment. If Vasari can be trusted, Michelangelo made numerous designs and models for the Cacus, but afterwards changed his mind, and thought that he would extract from the block a Samson triumphing over two prostrate Philistines. The evidence for this change of plan is not absolutely conclusive.
in Florence, Ferrara, Venice, France, Germany, Pisa, Arno and Arezzo
The War Office of Florence issues a patent recruiting Michelangelo's services as a military engineer, conferring on him full authority over the work of fortification.
Michelangelo had scarcely been chosen to control the general scheme of fortifying Florence, when the Signory began to consider the advisability of strengthening the citadels of Pisa and Livorno, and erecting lines along the Arno. Their commissary at Pisa wrote urging the necessity of Buonarroti's presence on the spot. In addition to other pressing needs, the Arno, when in flood, threatened the ancient fortress of the city. Accordingly we find that Michelangelo went to Pisa on the 5th of June, and that he stayed there over the 13th, returning to Florence perhaps upon the 17th of the month.
Furnished with letters to the Duke, and with special missives from the Signory and the Ten to their envoy, Galeotto Giugni, Michelangelo left Florence for Ferrara after the 28th of July, and reached it on the 2nd of August. He refused, as Giugni writes with some regret, to abandon his inn, but was personally conducted with great honour by the Duke all round the walls and fortresses of Ferrara. On what day he quitted that city, and whither he went immediately after his departure, is uncertain.
The Ten write to Galeotto Giugni, saying that Michelangelo's presence is urgently required at Florence, since the work of fortification is going on apace, "a multitude of men being employed, and no respect being paid to feast-days and holidays." It would also seem that toward the close of the month, Michelangelo is expected at Arezzo, in order to survey and make suggestions on the defences of the city.
Michelangelo flees Florence, abandoning his post as military engineer in charge of the city's fortifications. A letter from Michelangelo to his friend Battista della Palla on the 25th of September 1529 makes clear his reasons:
Michelangelo gives the statues of the Rebellious Slave and the Dying Slave in the Louvre to Roberto Strozzi, for his generous hospitality in his Roman house during Michelangelo's periods of sickness in July 1544 and June 1546.
Giorgio Vasari publishes his 'Lives of the Painters, Sculptors and Architects', the first book of art history scholarship. It features a prominent chapter on Michelangelo, who is still alive at this point.