eonardo da Vinci applied for the position of court engineer and court artist at the court of Milan in 1482 with his famous letter to Ludovico Sforza. In it, he recommended himself almost casually and only at the end of the letter for the realisation of an equestrian statue.
"I also deal with sculptures in marble, in bronze and in earth [terracotta]; likewise I make paintings, anything you want. I would also be able to work on a statue of an equestrian in bronze, which is to be erected for immortal glory and eternal honour, thus also for the happy memory of Your Glory Father and the princely House of Sforza."
The doubts of the Duke
The plans for the statue became more and more concrete and the Duke commissioned Leonardo around 1488. He began to have doubts about Leonardo's qualifications shortly afterwards, however, and in July 1489 he had a secret letter drawn up for Lorenzo de' Medici, Leonardo's former patron in Florence.
"[...] Mr Ludovico wants to prepare a worthy tomb for his father and in the meantime has commissioned Leonardo da Vinci to make a model. It is an oversized bronze horse on which Duke Francesco sits in full armour. And since His Serene Highness the Duke wishes to execute a thing of the highest order, he has instructed me to write to you on his behalf that he wishes you to send one or two masters who would be suitable for this work. And although His Serene Highness has already given the commission to Leonardo da Vinci, it does not seem to me that he is particularly recommended for the execution [...]".
Implementation of the project
But no one else was found for the project, at least Leonardo kept the commission. Due to its enormous height of 7m, the statue posed a challenge simply because of the supporting structure. Leonardo produced several designs. A first version with a rearing horse was discarded because of the difficult balancing. Presumably this change to the design was the moment when Ludovico Sforza began to have doubts about Leonardo's suitability for the project.
A second version then showed only a trotting horse, which was easier to realise. Typical of Leonardo, this version nevertheless shows a challenging peculiarity. The horse does not stand on three legs, as in contemporary models, but only on two. This increases the realism of the representation enormously and gives the sculpture a more dynamic appearance, but also makes it more difficult to balance the sculpture so that it does not topple over. This, and the size of the statue, show that Leonardo wanted to create something unprecedented.
Leonardo subsequently drew up concrete plans for carrying out the bronze casting process. Finally, in 1494, a preparatory full-size plaster model was made and put on public display. The 7m plaster model was absolutely necessary for the casting. Numerous YouTube documentations show the individual steps of the complex bronze casting process using modern examples.