In May 1491, Leonardo began to gather his notes and reflections about the casting process for the monument in the notebook appended to the Madrid Codex II. One can presume that by this time Leonardo already had in mind the type of casting he wanted to use.
Leonardo's idea to cast a gigantic monument, more than sixty tons of bronze, in a single pouring led him to develop a moulding method that would produce a casting of uniform thickness and lighter weight. In order to carry out the casting in a single session, it became necessary to develop an alternative, indirect method, which involved the mould being created in several sections to be assembled inside the casting pit.
Armature of the maschio (casting core) and of the cappa esterna (counter mould). Leonardo thought to fix the casting core to the counter mould with two concentric metal circles that would come together tightly using pins.
We find several very detailed drawings representing vertical cross section of the casting mould. The bronze spacers on the right (f. 142v) would have kept the counter mould in place on the casting core, on the left (f. 156r) the cavity is outlined and the chimney would have connected the gas-gathering chamber inside the core with the outside.
The second drawing on the left (f.149r) is the transversal section of the casting mould. In this last drawing Leonardo highlights the closure hinges that would have kept the two sections of the counter mould closed and in place on the casting core thanks to huge bolts.
It is important to point out that Leonardo intervened on the drawing a second time increasing the thickness of the lower part of the mould as a visual reminder along with the note: “onde io debo afforsificare il lato di sotto della cappa acciò non si sfondi” (so I have to strengthen the side under the mould so that it does not cave in) that stressed his concern about the melted metal pressure.
These are notes on the construction of the casting mould for the horse tail. Leonardo concluded that the complexity of the model in this case made it appropriate to cast the tail separately from the horse, to which it would then be attached.
This lever machine is composed of two parts with a base and a supporting wall for the valve hinged together. This junction point would be the fulcrum of a lever for helping the valve detach from the horse mould. Once the valve was detached, the machine would rotate until it settled horizontally on the base.