Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

PURE SILVER FROM COIN METAL. 27

cupel is made. This process is made use of technically for obtaining pure

silver from the impure metal, and for working up argentiferous lead which is

obtained in the metallurgy of certain lead ores; the process is likewise used

on a small scale in the rapid and accurate quantitative estimation of silver

and gold in ores or metallurgical products (fire-assay).

Melt together, by means of a slightly luminous flame from a

blast lamp, about 0.3 g. of silver coin and 1 g. of pure lead in a flat

cavity, made as smooth as possible, in a piece of blowpipe char-

coal. Press some bone-ash firmly into a small porcelain crucible

with a pestle so that at the top there is a slight hollow with a

coherent, perfectly smooth surface. Place the metallic button

by means of pincers upon the bone-ash which is to serve as a

cupel. Support the porcelain crucible on a clay triangle, or

embed it in sand, with its top slightly inclined towards the blast

lamp; and direct the point of the oxidizing flame toward the

vicinity of the button, and some of the time directly upon it.

At first keep the metal but barely melted, as otherwise it is likely

to spirt. The lead oxide, as fast as it is formed, runs off and

is absorbed by the bone-ash; in order that it may not all run

into the same place change the inclination of the crucible from

time to time. Continue the heating until the size of the button

no longer diminishes; towards the end apply a higher heat,

whereby films of metallic oxide run across the button in the

direction of the oxidizing draft, and lead oxide separates out on

the opposite side in the form of dark-brown crystals containing

copper. The end of the process is reached upon the disappear-

ance of the oxide film, which at the last shows for an instant a

rainbow-like play of colors. This is known as the "blick." After

this point is reached, further heating causes a loss of silver by

volatilization. During the process a part of the lead is vo-

latilized.

The metal still contains a small amount of copper. Melt it

again with 1 g. lead, and cupel the greater part of this lead away

as before, using a fresh cupel. Free the button from bone-ash

by hammering it into a cube (holding it in pincers), and remove
the last traces of lead by heating it on a third cupel. Weigh the

silver button that is finally obtained. Yield, 96 to 97% of the

actual silver content of the coin.