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.
Buonarroto, — Learn that we have cast my statue, and that I was not over fortunate with it, the reason being that Maestro Bernardino, either through ignorance or misfortune, failed to melt the metal sufficiently. It would take too long to explain how it happened : enough that my figure has come out up to the waist, the remainder of the metal — half the p. 47 bronze, that is to say — having caked in the furnace, as it had not melted ; and to get it out the furnace must be taken to pieces. I am having this done, and! this week I shall have it built up again. Next week I shall recast the upper portion and finish filling the mould, and I believe it will turn out tolerably well after so bad a beginning, though only as the result of the greatest labour, worry and expense. I was ready to believe that Maestro Bernardino could melt his metal without fire, so great was my confidence in him : but all the same it is not that he is not a skilled master, or that he did not work with a will. But he who tries may fail. His failure has been costly to him as well as to me, for he has disgraced himself to such an extent that he dare not raise his eyes in Bologna.
If thou shouldst meet Baccio d'Agnolo, read this letter to him and beg him to inform San Gallo in Rome, and commend me to him. Commend me also to Giovanni da Ricasoli and to Granaccio. If this turns out satisfactorily I hope to be finished with it in from fifteen to twenty days, when I will return to you. If it is not successful I should perhaps have to do it again, but I will keep you informed.
Let me know how Giovansimone is.
On the 6th day of July.
P.S. With this I shall enclose a letter for Giuliano da San Gallo in Rome. Send it as securely and as quickly as thou canst : if he should be in Florence, give it into his hands.