In 1557 Cosmo de' Medici begged him to come back to Florence "where honour and rest awaited him," but he answered firmly, "I can not leave here until I have carried the construction of St. Peter's so far that my plan can not be changed or destroyed and there will no longer be any possibility for thieves to begin their work again, as these scoundrels are only waiting for a chance to do. This is my resolve." That same year his friends, who were afraid that he would carry his great designs with him into the grave, for none of them were written down, urged and besought until they succeeded in persuading him to execute a model in wood of the dome of St. Peter's.[91] He was still working on this in 1560. The building went on, but not without many difficulties. It was necessary in 1557 to rebuild almost the entire vault of the Chapel of the King of France because Michelangelo had been ill and unable to watch the work closely enough. The attacks on him began again with fresh vigour at each mistake, and some of his best friends, like Cardinal Carpi, joined in them. Michelangelo heard from Francesco Bandini that the cardinal said everywhere that the work on St. Peter's could not be worse managed. Much hurt, he wrote to him at once that, "although he felt sure that the work was going on well, he feared that possibly his own enthusiasm and his age had blinded him and were, without his knowing it, a source of harm and danger to the building." In consequence he begged that they "would be so kind as to relieve him of the load which he had carried without pay for seventeen years under the orders of the popes." He offered his resignation. "A greater favour than its acceptance," he said, "could not be accorded him."[92] His resignation was not accepted, and Pius IV in a pontifical letter gave him full powers and forbade that his plans should be set aside. But Nanni di Baccio Bigio, indefatigable in his hate, moved heaven and earth to drive him away. In 1562 Nanni appealed to Cosmo de' Medici for his aid in securing the appointment of architect of St. Peter's. Cosmo answered that he would do nothing about it while Michelangelo lived.