Initially, Leonardo had thought to cast the horse with the mould placed in a upside-down position inside the casting pit. Using this method, it would have been easier to fill the mould by placing a series of runners in a line along the middle in the highest places of the horse body profile and using the legs as chimneys for gases produced during the pouring.
Vertical casting method
Folio 149v of Madrid Codex II depicts a foundry system scheme with four reverberating furnaces with a round barrel-shaped casting chamber, burners and fireplaces. To the upper right, there is a schematic plan of the horse where there is an aperture on its back for accessing the inside. In a note Leonardo refers to the pivot for the door that, as he intended, should have been directly made through casting. On the bottom there is a vertical section of the casting pit where the horse is seen upside-down as well as lines that indicate the casting canals.
Horizontal casting method
In Leonardo’s conclusions dated “A dì 20 di dicienbre 1493” [20 December 1493] he maintains that the only way to cast the horse is by performing the casting horizontally. Leonardo realised that the level of the water table in the Milan, at about five metres, would have prevented the complete burial of the vertically placed mould, without it coming into contact with water and endangering the success of the casting. There is also a drawing of a horse spread out on its side, around which a first hypothesis is shown along with the number of furnaces to be used and their placement along the edge of the pit.