Casting the horse's tail: waterproofing and sealing the mold
Testing pitch and oil on the outer mold, sealing its joints, and the runner-cluster for the tail
Foundry notes for the bronze Sforza monument. Leonardo describes an experiment to see whether pitch or oil soaked into the fired clay outer mold resists moisture and takes the bronze, and how to seal the joints of the cap with soft soap, sifted ash and brick powder. A central knotted diagram, labelled 'forma de la coda / Coda,' works out the branching mouths needed to cast the tail so the bronze does not splash, chill and block itself.
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Testing pitch and oil to waterproof the fired mold
An experiment to see whether pitch and oil soaked into the cap resist moisture and receive the bronze. First make a piece of clay shaped as a mold, fire it, coat it with Greek pitch or oil, leave it three days underground, then pour bronze into it; the same test may be tried with plaster.
Sealing the joints of the cap
Before joining the parts of the cap for the last time, smear the lips of the seam with soft soap mixed with sifted ash or brick powder so it seals well and restores the clay lost as sawdust. It should be worked soft, like shearing-clay, before closing.
The tail (Coda) and its casting mouths
If the tail were cast at its full thickness there would be danger it might fail, for bronze splashing up in threads would stop and cool, setting the liquid it touched and blocking the passage of the rest. Therefore the mouths are made for the core as figured, shown in the central knot of runners labelled 'form of the tail, Tail.'