into the crucible, stirs it twice. Afterward he draws off the slags with a
rabble, which consists of an iron blade, wide and sharp, and of alder-wood ;
the blade is a digit and a half in width and three feet long ; the wooden handle
inserted in its hollow part is the same number of feet long, and the alder-wood
in which the blade is fixed must have the figure of a rhombus ; it must be
three palms and a digit long, a palm and two digits wide, and a palm thick.
Subsequently he takes a broom and sweeps the charcoal dust and small coal
over the whole of the crucible, lest the copper should cool before it flows
together ; then, with a third rabble, he cuts off the slags which may adhere
to the edge of the crucible. The blade of this rabble is two palms long and
a palm and one digit wide, the iron part of the handle is a foot and three palms
long, the wooden part six feet. Afterward he again draws off the slags
from the crucible, which the assistant does not quench by pouring water
upon them, as the other slags are usually quenched, but he sprinkles over
them a little water and allows them to cool. If the copper should bubble,
he presses down the bubbles with the rabble. Then he pours water on the wall
and the pipes, that it may flow down warm into the crucible, for, the
copper, if cold water were to be poured over it while still hot, would spatter
about. If a stone, or a piece of lute or wood, or a damp coal should then fall
into it, the crucible would vomit out all the copper with a loud noise like
thunder, and whatever it touches it injures and sets on fire. Subsequently he
lays a curved board with a notch in it over the front part of the crucible ; it
is two feet long, a palm and two digits wide, and a digit thick. Then
the copper in the crucible should be divided into cakes with an iron wedgeshaped bar ; this is three feet long, two digits wide, and steeled on the end
for the distance of two digits, and its wooden handle is three feet long. He
places this bar on the notched board, and, driving it into the copper, moves
slag from refining " dried " copper, showing it to contain from 51.7 to 67.4% lead oxide, 6.2
to 19.2% cuprous oxide, and 21.4 to 23.9 silica (from the furnace bottoms), with minor quantities
of iron, antimony, etc. The " bubbles " referred to by Agricola were apparently the
shower of copper globules which takes place upon the evolution of sulphur dioxide, due to the
reaction of the cuprous oxide upon any remaining sulphide of copper when the mass begins to
cool.
Historical Note.—It is impossible to say how the Ancients refined copper, beyond
the fact that they often re-smelted it. Such notes as we can find are set out in the note on
copper smelting (note 42, p. 402). The first authentic reference to poling is in Theophilus
(1150 to 1200 A.D., Hendrie's translation, p. 313), which shows a very good understanding
of this method of refining copper :—" Of the Purification of Copper. Take an iron dish of
" the size you wish, and line it inside and out with clay strongly beaten and mixed, and it is
" carefully dried. Then place it before a forge upon the coals, so that when the bellows act
" upon it the wind may issue partly within and partly above it, and not below it. And very
" small coals being placed round it, place the copper in it equally, and add over it a heap of
" coals. When by blowing a long time this has become melted, uncover it and cast immedi" ately fine ashes of coals over it, and stir it with a thin and dry piece of wood as if mixing it,
" and you will directly see the burnt lead adhere to these ashes like a glue, which being cast
" out again superpose coals, and blowing for a long time, as at first, again uncover it, and
" then do as you did before. You do this until at length by cooking it you can withdraw the
" lead entirely. Then pour it over the mould which you have prepared for this, and you will
" thus prove if it be pure. Hold it with the pincers, glowing as it is, before it has become
" cold, and strike it with a large hammer strongly over the anvil, and if it be broken or split
" you must liquefy it anew as before. If, however, it should remain sound, you will cool it in
" water, and you cook other (copper) in the same manner." Biringuccio (hi, 8) in 1540
describes the process briefly, but omits the poling, an essential in the production of malleable
copper.
