Fiore described this weapon like so:
"This ax of mine is hollowed all around and the ax is full of powder, and this powder is such a strong corrosive that immediately as it touches the eye, the man can in no way open it, and perhaps will never see again. And I am a heavy and cruel and deadly axe; better blows I make than any other handheld weapon. And if I fail in the first blow that I come to make, the axe is damaging to me and worthless ever after. And if I wound with the first blow that I make, I overcome all other handheld weapons. And if I am accompanied by good arms for my defense, I can take the pulsative guards of the sword. Noble Lord, my Lord Marquis, many such malicious things are in this book that you would not do, but to know more, I ask that you see them."
He goes on to give a recipe for the powder: take sap from a spurge plant, bake it, and render it into powder; mix a pinch of this powder with one ounce of fior di preta powder. The sap of certain spurges causes blistering of the skin and blindness on contact with the eyes (even the fumes cause swelling); fiore de preta causes local swelling when applied to the skin and was an ingredient in some makeups. Together this mixture certainly seems like one of the last things you'd want deposited inside your visor, though Fiore goes on to note that really any fine, caustic powder will get the job done.
But was this actually a historical practice, or a fantasy that Fiore concocted to put one over on old Niccolo? Interestingly, we can find independent corroboration of this device in a treatise on duelling by Italian jurist Paride del Pozzo, originally printed in Naples in 1476 (depending on your definitions, this could be considered the oldest "HEMA book" in the world). Pozzo's book seems to have been popular, since it was translated into Italian and published in Venice in 1518, '21, '23, '25, '30, '36, '40, and '44, and also in 1544 translated into Spanish and published in Seville; the Latin text was reprinted as well, in Lyon in 1543. And, of course, that rogue Achille Marozzo lifted large tracts of it to compose Book 5 of his own treatise published in Modena in 1536.
This comes from "Book 8, Of the cases subsequent to particular Battles; of the accords and conventions of the combatants." (Roughly translated from the Spanish edition, p XLVIIv, since it's way easier to read than the others.)
"Chapter X, On two knights challenged to mortal combat with iron-shod maces, of which one brought the wood of the mace hollowed out and filled with stinking powder, with which powder he defeated his enemy.
"Two knights, having been challenged to mortal combat with iron-shod maces, entered into the lists. One brought a hollowed-out mace, which cavity he carried full of a stinking powder that, sending it above the visor of his enemy, suddenly deprived him of the sight of his eyes in a manner that he was forced to cushion(?) (amortecerse) them, and with this malign cunning he became victor, having deprived his enemy of sight and sense. For which it was wondered if he had justly fought, and if he merited the victory. It was said that he didn't follow the ancient Lombard Law: that which says that none of the combatants should enter inside the [dueling] field carrying with them poison or any stinking herb, nor other powder, which violence obviates the working to the offending of the enemy without operation of the personal virtue of the spirit, even though it does not obviate the working of the jury by the same commandment; not for nothing does the Lombard Law will that no hidden arms should be carried: that without combat offending the person of the combatant, that he should overcome with weapons of battle through force, and wariness, and ingenuity, and showing virtue of spirit, and not with poison or other false purposes..."
The text sort of meanders along in the same vein, mulling over the law and the importance of virtuous martial spirit, for another two dense pages, but you get the point.
Images from Getty Museum Ms. Ludwig XV 13, f 37v, and Novati (1902), carta 36a. Thanks to Piermarco Terminiello for pointing this chapter of Pozzo out to me.