Casting trials: mould material and why the horse stands upright
Plaster tempered with vinegar and lye; the differential shrinkage of legs cast lying down
Leonardo records experiments toward casting the bronze horse. He proposes testing whether fresh plaster tempered with vinegar, wine, aquavite or lye can withstand the pour without re-firing the mould, which would spare the flock-binder. He then argues from cooling and shrinkage that the horse must not be cast lying down, since its solid legs would still be liquid after the feeding gates had congealed, leaving the legs starved and imperfect. Further notes cover smoothing the ash coats before the outer mantle and sizing each furnace to the part of the body it feeds. A small lettered figure (n, m) is drawn in the margin.
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Testing plaster tempered with vinegar, wine, aquavite or lye
He wants to see whether fresh plaster tempered with vinegar, wine, aquavite or lye will withstand the casting. If the mould need not be re-fired, the flock-binder and other binding agents would not be burned away.
Why the horse must not be cast lying down
Cast lying, the massive legs hold far more molten bronze than the mouths feeding them. The mouths would congeal and finish their shrinkage while the legs were still liquid, so nothing could supply the metal the legs lose in contracting, and the legs would remain imperfect along their whole length.
Smoothing the dry ash before the outer mantle
After two coats of ash, before beginning the mantle, rub the hand lightly over the dry ash so that the mantle does not take an impression of the brush-strokes left in the ash.
Sizing each furnace to the body-part it feeds
Each furnace feeding the horse's body carries its own quantity of bronze, the largest charge going to fill the neck, head, chest and legs together. This follows from casting with the feet up.