Iron
The Metal of War
Copper plating out on iron in blue vitriol was taken as iron turned into copper. Redgrove pointed out in 1911 that the two metals had merely exchanged places.
H. Stanley Redgrove, Alchemy: Ancient and Modern (1911), section 14, citing The Golden Tract in The Hermetic Museum, vol. i, p. 25.
Iron in alchemy: the metal of Mars, written with the planet's sign, ♂.
More context
Iron takes the sign of Mars unchanged. Of the seven correspondences it is the least in need of explanation: iron is what weapons were made of, and Mars is the god of war. The alchemists held that each metal grew under the influence of a planet, and so called metals by their planets' names and signs. Iron was an imperfect metal, rusting where copper greens and gold does not change at all. Peter Bonus, whose fourteenth-century work was printed in 1546, made it the lowest stage of metallic development, and gold the highest.
Light
Bonus held that the imperfect metals serve nature's plan: without copper, iron, tin or lead, no useful tools could be made.
Shadow
On Bonus's scale iron is the lowest of the metals, furthest from the perfection of gold.