From Erasmus’ Adagia (2.6).  My own hasty rendering.

IGNEM NE GLADIO FODITO

‘Do not stir the fire with a sword’, that is to say, do not provoke someone already stirred to anger.  It is far better to yield and calm his enraged spirit with kind words.  This is the opinion of Saint Jerome and of Demetrius of Byzantium, cited by Athenaeus.
Diogenes Laertius explains that the choleric temperament of violent and wrathful men ought not to be stirred up with reproaches, because the more a flame is stirred up, the stronger it grows.
Plutarch does not judge any differently.
Plato, however, in Book 6 of The Laws, has used this saying of men who strive in vain for what can be in no way accomplished, showing this to have been a type of game – that they would cut up a fire with a sword.
Saint Basil mentions a nearly identical sense in his letter to his nephews – how they wish to cut fire with a sword and draw water with a sieve.
And it is surely to that definition that Lucian refers in book 2 of his True History.  He tells that that at his departure from the Isles of the Blessed, Rhadamanthus ordered him to follow three rules when he came back to our earth: not to stir the fire with a sword, not to eat beans, and not to bed a boy more than 18 years old.  If he kept these in mind, he would one day return to the isle.
It seems that Horace, by this saying, points out cruelty mixed with madness.  For love is in itself mad and if it breaks forth into fighting and murder, the fire is pierced by a sword.  Satire 2.3: “Add bloodshed to these and stire the fire with a sword.”

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Πῦρ σιδήρῳ μὴ σκαλεύειν, id est• Ignem gladio ne fodito, hoc est ira percitum ne
lacessas. Quin magis concedere conuenit et blandis verbis tumidum animum
placare. Ita diuus Hieronymus et apud Athenaeum Demetrius Byzantius.
Diogenes Laertius exponit potentium et ferocium iracundiam non esse
conuitiis exagitandam, propterea quod flamma quo magis exagitatur, hoc
magis atque magis inualescit. Neque dissentit ab hoc interpretamento Plutarchus.
Quanquam Plato libro De legibus sexto sic vsurpauit, vt de iis dici
solitum videatur, qui frustra moliuntur quod effici nullo pacto queat, osten-
dens id lusus genus quoddam fuisse, vt ignem gladio dissecarent. Ad eundem
ferme sensum retulit diuus Basilius in Epistola ad nepotes, vt idem sibi velint
ignem gladio dissecare et cribro haurire aquam. Huc nimirum allusit Lucianus
in secundo Verarum narrationum libro, cum ex insulis fortunatis dimitteretur,
fingens se a Rhadamantho admonitum, vt si quando rediret in hunc nostrum
orbem, tria quaedam obseruaret, Μὴ πῦρ μαχαίρᾳ σκαλεύειν, μήτε θερμούς
ἐσθίειν, μήτε παιδὶ ὑπὲρ τὰ ὀκτωκαίδεκα• ἔτη πλησιάζειν, id est Ne gladio ignem
diuerberaret, ne lupinis vesceretur, ne se puero decimumoctauum annum egresso adiunge-
ret. Si quidem horum meminisset, futurum vt aliquando ad eam insulam
reuerteretur. Horatius hoc dicto videtur indicare crudelitatem cum insania
coniunctam. Amor enim per se furor est, qui si erumpat in pugnas ac caedes,
ignis gladio perfoditur. Libro Sermonum secundo, satyra iii.: His adde cruorem
/ atque ignem gladio scrutare.