The hieroglyphic for iron is which denotes “gold at the bottom” (O), only its upper part is too sharp, volatile, and half corrosive this being taken away, iron would become gold. Iron is called Mars.
Strike while the iron is hot. “Battre le fer pendant qu'il est chaud.” Make hay while the sun shines. To have many irons in the fire. To have many affairs in hand.
If you have too many irons in the fire, some will burn. If you have more affairs in hand than you can properly attend to, some of them will be neglected and turn out badly. Both these locutions refer to the “heaters” or irons employed in laundries. If the “heater” is too hot, it will scorch the linen.
To rule with a rod of iron. To rule tyrannically. “Gouverner avec une verge de fer.”
(See Pig Iron.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894