Assay Office
The assay office was the "control center" of a gold mining operation. The assayer most often had his laboratory close to the office of the mine superintendent so the two could confer easily. Throughout the mining day, samples of gold ore were brought to the assayer for testing. As a result of those tests, the superintendent decided which areas of the mine should continue operations and which should not. The ore had to be rich enough in gold to justify the expensive processing which would follow. During the more productive years, the Eureka Mountain mines generally yielded at least one ounce of gold per ton of ore. That is considered extremely rich by any standards.
Assaying, or testing for gold, involves a knowledge of chemistry and requires painstaking precision. The process began when samples were brought to the assayer. The ore samples usually ranged from 10 to 100 pounds and had to first be crushed. Then the material was split into identical portions until a tiny sample was placed in a crucible made of fired clay. Crucibles were purchased and could be reused many times. Lead oxide, called litharge, was added. In the fire assaying process it was the lead which "grabbed onto" the gold. Flux was also added in order to reduce the overall melting point of the mixture. Flux materials were most often sodium carbonate, borax, or calcium flouride. Ordinary flour was added which helped to reduce the lead oxide to metallic lead. The types and amounts of flux used depended on the minerals present in the ore.
Using tongs and asbestos gloves, the assayer placed the crucible into the oven. The mixture was "cooked" at 2,000 degrees for about 30 to 45 minutes until liquified. Again using tongs, the molten mixture was poured into iron molds. As it cooled, the heavier gold and lead settled to the bottom and the lighter material, called slag, rose to the top. When cooled and hardened, the slag could be chipped away leaving a gold/lead "button". Silver was usually recovered along with gold in most ores. If it was known or suspected that the ratio of gold to silver was not optimal for total separation of gold from silver, a specified amount of silver was added into the crucible before melting to aid the separation process which took place later.
The next step, called cupellation, began as many accumulated precious "buttons" were placed in tiny cups called cupels. Many "buttons" could be fired at once but careful records had to be kept to prevent confusion of samples. Cupels are made of pressed bone ash and are porous. When heated in the oven at 1,700 degrees for about 30 minutes, the cupel absorbed the litharge (lead oxide) leaving only the precious gold and silver in the bottom of the cup. Cupellation was more critical than the earlier firing and had to be carefully watched by the assayer. The point where only the precious metals remained was marked by a sudden flash of light. The cupels were then removed from the furnace and cooled.
The beads of precious metal were then weighed on a scale (balance) which had to be accurate to 1/1000 milligram. The bead was "parted" in nitric acid to remove the silver. Finally the tiny bead was weighed and compared with the weight of the sample from which it came and the report was given to the superintendent.
The Assay Office is open periodically for tours.