California Stamp Mill

California Stamp Mill. . Soon after miners discovered gold in underground quartz veins, mills were brought in to process ore. Dating back to at least the 1500’s, the mills used heavy iron pounders called stamps to crush the rock. Each mill had a series of stamps equipped with iron “shoes.” Partially broken quartz was fed into bins behind the stamps and then into mortars holding dies on which the shoes fell. Each stamp rose and dropped separately. This example is an early, two stamp mill., California quartz miners improved stamp mills by changing from soft iron to cast-iron shoes, dies and mortars. They also introduced stamps that rotated on their vertical axis with each rise and fall. The result was the “California Stamp,” a machine so efficient it was used in gold mining throughout the world., Stamp mills often ran around the clock making a tremendous amount of noise; in some mining areas, the sound of the stamp mills could be heard and the rumble felt for miles. Many workers lost their hearing, or were exposed to poisonous mercury while tending the California stamps., The stamps were arranged in mortar boxes which generally contained 5 stamps. The photo above shows an array of the California Improved Gold Stamp Mill, whose stamp fell in a way to mix the ore and water, speeding up the process., Gold-bearing quartz was brought from the mine in ore carts, dumped down a chute or shoveled into the mortar where the rocks fed under the stamper shoes. Ore was crushed to a particle size small enough to go through a 40 mesh screen (1600 holes/square inch) at which point it flowed through the screen across the amalgamation table, where mercury was added. Any particle of gold that touched the mercury created an amalgam (mercury and other metals combined). The amalgam was then boiled to free the mercury (boiling point, 670°f) from the gold (melting point, 2000°f)., Marshall Gold Discovery State Historic Park