Pouring the Bronze: Furnace Taps, Soaking and Heating the Mould
Keeping furnaces plugged with hot iron rods, impregnating the mould with pitch and oil, and its outer clay
These casting notes for the Sforza horse concern the moment of the pour itself. Leonardo directs each man to keep his furnace stopped with a red-hot iron rod and to have four spare rods ready in case one breaks, so that no lump of unmelted bronze blocks the tap. He then gives recipes for impregnating the hot mould with Greek pitch, linseed oil, turpentine or tallow, for keeping the mould warm during casting with a pitch coat that the rising flame reheats, and for the outer clay layer of powdered furnace earth moistened with vinegar. He repeatedly insists on testing every method first.
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Managing the furnace taps during the pour
In casting, let every man keep his furnace stopped with the red-hot iron rod. At the right moment he unstops it and takes hold of the thin rod so no piece of unmelted bronze blocks the opening. Keep four red-hot rods beyond the needed number, so that if one breaks others are ready.
Impregnating the inside of the mould
As soon as the mould is fired, while still hot, impregnate it with Greek pitch, or linseed oil, or turpentine, or tallow; but first test all of them and take the best.
Keeping the mould hot during casting
If you give the whole mould a coat of Greek pitch mixed with aqua vitae or turpentine, the flame that rises above the settled bronze will keep heating the mould above it; but make a test.
The outer clay that touches the mould
Let this be dry earth from the castellaccio, powdered and moistened with vinegar, or if you wish with linseed oil, then well rammed, and made two fingers thick.