ORES METALS AND ALLOYS - Archaeological Chemistry - page 160

recognizable because of the color, shape, and size or other characteristics of
the rocks, boulders, or pebbles from which the metal could be recovered by
relatively simple smelting procedures. This confined mining centers to the
relatively few geographic regions where such ores were abundant. On the
island of Cyprus, for example, which served as an important copper-mining
center in antiquity, many of the large copper ore deposits are easily recog-
nizable by their mostly greenish color. Also in Europe much tin was mined,
for many centuries, from easily recognizable ore deposits in Britain, which
was therefore referred to as the “island of tin.”

5.4. ORE DRESSING

Metalliferous ores are generally mixtures of metal-rich minerals and varying
amounts of worthless other minerals. After mining, ores therefore undergo
a process known as ore dressing , intended to separate and concentrate the
metal-rich minerals from the useless gangue , as the worthless minerals are
called. Ancient ore dressing most probably entailed such operations as hand
picking, pounding, screening, or washing the raw ore with running water,
so as to omit or remove the generally less dense gangue. Ores containing as
little as 10% of metal-rich mineral for example, could be separated during
dressing operations into two fractions: the ore concentrate , containing as much
as 60% of metal-rich mineral, and the gangue , waste material with little, if
any, metal (Craddock 1980; Evans 1987). Some ore concentrates, those in
which the relative amount of metal-rich mineral consisted of metal sulfides
or metal carbonates, were then either roasted or calcinated so as to convert
them into metal oxides (see Textbox 33).

5.5. SMELTING

To recover metals from dressed mineral ores, the latter are smelted (processed
at high temperatures to convert them to metals) (see Textbox 37). Early smelt-
ing was probably initially accomplished in open hearth furnaces, as found
in many ancient archaeological sites, consisting of a hole dug in the ground,
which may have been lined with a layer of fire-resistant clay or stone. In
most furnaces, charcoal was layered on top of the clay floor or stone floor,
and then covered with a layer of metal ore. After charcoal is ignited, it burns,
releasing reducing gases that react with the mineral and reduce the combined
metal to free metal. The burning temperature in such a furnace is sufficiently
high to melt the metal, thus creating a molten mass of dense metal topped
by a lighter layer of waste products, known as slag . When the hearth is finally