Filling the mould slowly so the bronze can shrink and feed
Advice to keep the pouring gates thin and frequent so the metal shrinks gradually and feeds the mould from below.
Folio 152r continues the founding instructions: the mould must not be filled too quickly. Leonardo advises making the pouring mouths (gates) frequent and thin, so that the bronze makes its shrinkage in the mould little by little; filling too fast would leave the metal liquid throughout, and on cooling the already-cold upper metal could not sink to relieve the deficiency below. A faint red-chalk offset of the facing horse study bleeds through at the left, and the oval Biblioteca Nacional stamp is visible on the sheet.
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Thin, frequent pouring gates
Leonardo warns against letting the mould fill too quickly. The mouths, or gates, should be made frequent and thin so that the bronze makes its shrinkage within the mould gradually rather than all at once.
Feeding the shrinkage from below
If the mould were filled too quickly, the metal would be liquid below as above. Then, drawing back as it cooled, the metal would be almost cold above and could not sink to supply the shortfall at the bottom.