Molten bronze lifting the core; melting, alloying and charging the furnace
The buoyant force on the core, the boiling of bronze, adding tin to copper, and heating the furnace bottom
Leonardo reasons that molten bronze at the bottom of the mold exerts a great upward force on the lighter core, likening it to a man who braces his feet on a bench and pushes a beam with his shoulders until the bench breaks; he concludes the lower cap must be strengthened. Further notes treat the boiling of bronze as it drives off moisture like oil in a pan, where in the furnace it melts best, when to add tin to the coppery mush to make the alloy, and how to charge and heat the furnace bottom. A small figure at upper right is labelled b, c, a.
On this page
Bronze's upward force on the core, and its remedy
Molten bronze beneath the lighter core forces it upward, both because the core is lighter than bronze and because the bronze all around pushes on that below. If the core resists, the force must break through the lower cap, as a man bracing his feet on a bench and pushing a beam with his shoulders breaks the bench. Therefore the lower side of the cap must be strengthened.
Boiling bronze and where it melts best
When bronze is molten and boils it fights its opposite, and when it settles it has won, like oil that cries out in the pan until it has driven off all moisture. It is found to melt better at the mouths where the flames issue than in any other part of the furnace.
Adding tin to the copper for the alloy
When you see that the copper has been brought to a mush, give it its tin, and at once you will see them melt together. This is the moment to make the alloy.
Charging and heating the furnace bottom
In charging the furnace, the larger pieces should be put at the bottom over some wood so the fire heats the bottom. This prevents thin pieces of bronze resting on a bottom that is not well heated from failing to melt.