20 December 1493: to cast the horse lying down and tailless
A dated resolution to cast the twelve-braccia horse on its side, with a red-chalk sketch of the horse in its frame.
On folio 151v Leonardo records, dated 20 December 1493, his decision to cast the great horse without its tail and lying on its side. Because the horse stands twelve braccia, casting it upright would bring it within a braccio of the water table, and the prolonged damp of the buried mould would spoil the pour. A second note proposes an opening (a 'window') in the horse's back so that the bronze finds an even depth and fills without rushing from the hind foot into the forehead. A red-chalk sketch shows the horse enclosed within a rectangular casting frame, and further faint drawing appears lower on the sheet.
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Resolution to cast the horse lying down
Dated 20 December 1493, Leonardo concludes to cast the horse tailless and recumbent. At twelve braccia, an upright cast would reach within a braccio of the water, the ground cannot be raised, and the mould, buried for many hours, would be soaked with damp so that the cast would fail.
A window in the horse's back
Leonardo proposes that the opening be made in the back. The bronze will then find almost equal depth and fill evenly, without the metal of the hind foot running down into the forehead, as would happen if the horse were cast upside down.
The horse within its casting frame
A vigorous red-chalk sketch shows a horse set within a rectangular armature or casting frame of straight ruled lines. The animal is drawn with raised foreleg and hindquarters, enclosed by the geometry of the mould that surrounds it.