Page:Popular Science Monthly Volume 67.djvu/696

690 consisted of a small stone mortar with a stone pestle. But there was a development in milling methods, and the next consisted of a block of rock, with a slight depression in the top for the ore, which was pulverized by means of a heavy rubbing stone. A still later development, possibly belonging to the time of the Ptolemies, 330-322 was a mill somewhat like the arastra which may still be seen in California and Mexico. It consisted of a block of rock several feet in diameter and circular in outline, in the top of which was a shallow depression to receive the ore. The pestle consisted of a large spherical or cylindrical block of the same kind of rock. The finely broken ore was placed in the depression, and the movable block was rolled around over it until it was reduced to the desired fineness. The mill was essentially a huge shallow stone mortar in which the pestle was rolled by slaves.

The powdered ore from the mortars was sprinkled upon the tops of sloping stone tables over which water was kept, running. By this means the lighter rock powder was carried away and the gold remained behind. The supply of water was secured by the construction of large cisterns or reservoirs on the slopes of the mountain. In the upper ends of the tables there were tanks for the immediate supply, and the operator dipped the water out and poured it over the sloping surface. When a quantity of gold had accumulated on the table it was removed, dried and placed in a covered clay crucible and heated for five days in a charcoal fire blown constantly by a rude bellows, or by mouth blowpipes. No fluxes were used in the earliest times, but it was not long before these primitive metallurgists learnt the use of lead and common desert salts in refining their gold. Diodorus Siculus—second century B.C.—mentions the use of lead by the Egyptians in the refining of gold, but it is believed that it had been in use many centuries before his time.

Pictures and sculptures of very early date show the refiners of gold (and silver) sitting before the fire with blowpipes of very large size, but otherwise not unlike those in use at the present time. The main part of the pipe was probably of wood, but a metal tip concentrated and directed the current of air. Others show the workman blowing the fire with a bellows consisting of two leather bags furnished with metal nozzles. The bellows was distended by pulling a cord attached to the upper side of the bag. When at work, the operator stood with one foot on each bag and held the cords in his hands. Then by a swaying movement he threw his weight first on one foot and then on the other and at the same time pulled the cord attached to the bag from which the pressure was removed, raising its upper wall and causing it to take in air and become distended. The pressure of the foot forced the air through the nozzle of the bellows into the fire. When a large amount of metal was to be worked, an open fire of charcoal was used and a