Casting the Sforza horse: seams, position, and firing pit
Caulking the mould, casting it upside-down, and re-baking it in the pit
Folio 141v continues Leonardo's plan for casting the colossal bronze horse. He describes caulking the seams under the plate with aquavite-moistened brick-dust and then gypsum and hemp, and argues for casting the mould upside-down: laid flat, the thick solid legs fill slowly and choke the rising bronze of the back. He weighs the danger that the weight of bronze filling the legs may break off the pieces beneath the body, advises clearing a blocked furnace with chestnut rods rather than iron, and, because the mould is of marvelous weight, would re-bake it in the pit.
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Caulking the seams of the mould
The seams covered by the plate are to be caulked along the edge with brick-dust moistened with aquavite, and then sealed on the outside with gypsum and hemp, to keep the joints tight against the molten bronze.
Why to cast the horse upside-down
If the mould lies flat, once the bronze fills to the legs the metal poured for the croup cannot rise, because the thick solid legs fill slowly and hold back the back. Leonardo therefore judges it useful to cast the horse upside-down, though warning that the weight of bronze in the legs may then break off and lift the pieces beneath the body.
Unstopping the furnace and firing the mould
Should a piece of bronze block the exit, clear the furnace with wood or chestnut rods, not iron, lest the bronze weld to the iron. And because the mould is of marvelous weight, it should be re-baked in the pit.