The fine ore is then fed to the stamp mills
, which consist of batteries each having five stamp-heads dropping in a cast-iron mortar.
The ore was treated at a place called La Union, on the Rio del Quebrada Honde, by an arrastra and a stamp mill of primitive pattern, the ruins of which may yet be seen.
There remains the ruins of molinettes, arrastras and a 10- stamp mill of early pattern, the date of operation and production of which I have not learned.
A 20 stamp mill was operated without a profitable recovery.
One tour winds its way to a stamp mill
(the Standard Consolidated Mill); the other, called the Historic Mining District Tour, ends up on a ridge near a mine shaft (now closed for safety reasons).
Quartz ore is first crushed by double-jacks, then further size-reduced in stamp mills patterned after the ones in the mill at Portovelo.
Ball and rod mills gradually replaced stamp mills for finer size reduction of ore.
Stamp mills may sound outdated but in fact are ideal for small operations of this type.
That is to say the owners hold legal title, have the advantage of mains electricity, and have sufficient funds for mechanization: an Atlas Copco compressor, a single small Boart pneumatic drill, and a stamp mill.
The argument lost its force when stamp mills
were replaced by ball mills, but has lingered because it has a degree of validity for ores containing a large proportion of course gold.