Instructions for lost-wax bronze casting
Building the plaster mould over a wax model, setting vents and pour, melting out the wax to cast hollow
This fragment gives step-by-step instructions for casting by the lost-wax method. Once the plaster is cold and of sufficient thickness, Leonardo pours the rest and fills with plaster, then breaks open the form, sets iron pins through the wax and plaster, cleans up the wax, and places it in a box over which a plaster mould is built, leaving the air-vents and the pouring mouth. The form is inverted and heated so the enclosed wax can run out; the void it leaves is filled with molten metal, and the cast object comes out hollow. He adds that, so the plaster does not disintegrate on re-firing, one should mix into it 'what you know'. The lower half of the sheet bears only faint compass-drawn arcs.
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Building the mould and casting the piece hollow
Leonardo directs that when the plaster is cold and thick enough, the rest is poured and filled with plaster; the form is then broken, iron pins set across through the wax and plaster, and the wax cleaned up. The whole is placed in a box and a plaster mould built over it, leaving the vents and the pouring mouth; inverting and firing the mould melts out the enclosed wax, and the void is filled with molten metal so the cast comes out hollow. To keep the plaster from disintegrating on re-firing, an additive is to be mixed in.