Roman city dig: session 7, 2012: Roman metals, by Geoff Tindall
Metals in the Roman World Geoff Tindall Session 7 -2012
Metals of the ancient worldGold, Silver, Copper, Lead, Iron, Mercury, Tin, Zinc,Antimony and Arsenic.Of these only one was usually found native, the otherswere primarily made from ores.From these metals important alloys were made Bronze (80-90% Copper, 10-20% Tin) Brass (80-90% Copper, 10-2% Tin, 2% Zinc) Pewter (85-90% Tin, remainder Copper) Solder (70-90 % Lead with Tin or Antimony) Steel (Iron with 0.2 – 1.3 % carbon
Roasting and Smelting(Base Metals: Copper, Lead, Tin, Zinc, Mercury) Roasting is just heating up the ground rock and blowing air in to it through a tuyere (clay pipe), this gets the sulfur out of the rock PbS +3O2 -> PbO + SO2 Smelting is the next stage which is to heat the oxide with charcoal in a furnace and a chemical reaction makes metal. PbO + C -> Pb + CO2 The fire is hot so liquor metal flows to the bottom, unreacted rock (slag) floats to the top
Blooming and ForgingThe romans did not have the technology to make a fire hotenough to smelt or cast iron so when the iron wasreduced it formed a ‘bloom’. Which is a sponge likematerial.The bloom was then forged or wrought, that is heated upin a fire and hit with a hammer until it became one piece.By putting iron in a fire with coals and hitting it, carbonbecomes included into the metal structure as ‘pearlite’crystals and steel is formed. The more it is worked, themore carbon is included and better steel is made.Ancient steel was highly variable, and good smiths wereattributed magical abilities.
Did iron supersede bronzeThe ancients could not cast iron, but bronze casting iseasy, so bronze objects could be made in a way iron couldnot.Iron is soft and not ductile, so does not make good toolsor weapons. Steel superseded bronze for thesepurposes, not iron.
What about silver ?Silver is often associated withlead geologically, so a processcalled cupellation was used toseparate lead metal from silvermetal.Pb + Ag + O2 -> PbO2 + AgBy heating lead and silvermixture and blowing in air, thenlead can be converted back to arock like substance, and liquidsilver separated from it. Thelead was not recoverable fromthis process.