Casting the bronze: fuel, smoke, and the simultaneous pour
Notes on dry willow fuel, avoiding smoke, and matching several furnaces that feed a single bronze pour.
Folio 151r carries a continuous passage of mirror-script notes on the founding of bronze. Leonardo observes that alkali salt promotes melting, that the firewood must be dry, sound and split so it gives off little smoke (willow serving well), and that a smoky, cooler fire causes molten bronze to congeal. A final paragraph warns that when the whole charge is melted at one stroke across several furnaces feeding a single pour, the quantities of bronze, the furnaces, the fire and the alloy must all be of equal quality. The facing verso (150v) is essentially blank apart from faint show-through.
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Fuel and smoke in the furnace
Leonardo notes that alkali salt aids the melting and that the wood used must be dry, unrotten and split so that it produces no smoke, willow being suitable. He warns that a fire giving off more smoke burns less hot, and that this is often the cause that the molten bronze congeals.
Several furnaces feeding one pour
Keeping the bronze molten is called a little dangerous, so the whole melt should be done at once in several furnaces that converge on a single cast. For this to work the quantities of bronze, the furnaces and the fire must be of equal quality, and the material melted of equal alloy.