Casting the Horse: Furnaces, Mould Joints and Armature Irons
Running several melting furnaces at once, sealing the fired mould's seams, and setting the reinforcing irons
This page of Leonardo's casting notes for the colossal Sforza equestrian monument treats three practical problems of the great bronze pour. He argues for preparing five or six melting furnaces at once so the alloying goes faster and no single furnace is overworked, then gives recipes for sealing the joints of the fired mould with a turpentine, linseed-oil or egg-white paste closed under iron wedges, and finally explains how deep the fired clay penetrates so that the armature irons are set between the earth and its surface rather than buried in fired clay. A small drawn box in the lower right, captioned 'Pruova' (test), stands as a trial specimen.
On this page
Preparing several melting furnaces at once
Once one furnace proves good for alloying the metal, set up the number of furnaces the pour requires and alloy in each. This shortens the time, since five or six furnaces melt faster than one at a time, and spares any single furnace from wearing out; furnaces also perform better and more safely at their second and third melt than the first, which matters for the casting of the horse.
Sealing the joints of the fired mould
When the mould has been fired and you wish to reassemble it, smear the joints with a paste of turpentine, or linseed oil, or egg white, mixed with brick powder. Once the seams are plastered, force the mould shut with iron wedges.
Setting the armature irons in the fired mould
Take a slab of earth as thick as the intended mould (about one-third of a braccio), fire it, and observe how deep the firing penetrates. Set the irons only where the outer surface layer is protected from the fire; irons buried within fired earth do more harm than good, so they should lie between the earth and the surface layer.
Sketch of a test specimen
A small box-like block is drawn in the lower right margin and captioned as a trial piece. It illustrates a sample of the mould material to be tested before the full work.