Conservation Science

the end products being blooms. These were solid mixtures of iron and slag
from which the slag was removed by hammering. It was not until the beginning
of the fourteenth century that furnace designs allowed the temperature to reach
1000°C and only in the eighteenth century when coal began to be used as fuel,
could temperatures be attained to near the melting point of iron.
Lead was one of the first metals to be extracted as it melts at 327°C and is
reduced by carbon just below 800°C. The main ore is galena (PbS), and may be
readily smelted in a charcoal or dry wood furnace. At the top of the furnace
where there is an abundant supply of oxygen (oxidizing), the sulfide is roasted
to the oxide:

(3)

The conditions within the furnace change from oxidising to reducing (no oxy-
gen) as the ore moves down the furnace. The remaining unreacted lead sulfide
mixed with the oxide, acts as a strong reducing agent and converts the oxide to
metallic lead as shown in Equation (4):

(4)

The molten lead is run off at the bottom of the furnace into clay moulds.
The chief copper containing ores are malachite CuCO 3 · Cu(OH) 2 , chalcocite
(Cu 2 S ) and chalcopyrite (CuFeS 2 ). The production of copper from its ores is far
easier from the oxide than the sulfide-bearing ones. The malachite is mixed with
charcoal and placed in a furnace with some type of bellows arrangement to
increase the temperature in the furnace to 700–800°C. Under reducing condi-
tions in the furnace, the charcoal is converted into carbon monoxide gas, which
is the reducing agent at these temperatures. The ore is reduced to metal accord-
ing to Equation (5):

(5)

The copper usually remained in the furnace as a solid ingot if the temper-
ature could not exceed the melting point of pure copper (1083°C). As furnace
design improved and temperatures above this value were achievable, the metal
could be tapped off as a liquid.
Sulfide-bearing ores were first roasted by placing the chalcocite or mixed
copper/iron sulfide ore over burning wood in shallow cavities for up to 30 days
to convert the sulfides to oxides of copper according to Equation (6):

Cu S 2  2O 222 CuO SO (6)

COCuCO 322 CO Cu

23 PbOPbS Pb SO 2

232 2PbS O 22 PbO SO

Metals 125