Collection: Books and Periodicals > Nevada County Historical Society Bulletins

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Title: Volume 045-4 - October 1991 Collection Type: Books and Periodicals Collection Name: Nevada County Historical Society Bulletins # of Pages: 8 Download PDF: User Login Required Download OCR Text: User Login Required

Date Scanned: 2014-02-19 07:59:22 OCR Software Version #: tesseract 5.4.1 Item Sourced From: PDF First Created: 2025-03-17 04:05:59 Last Updated: 2025-03-17 04:05:59

Original Held At: Searls Historical Library Location: 161 Nevada City Hwy, Nevada City, CA 95959

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From these bins, the ore was transported to the stamp mill and fed into the Hendy feeder, from which it was admitted to the mortar box of the stamp mill. The mortar box is the trough in which the stamps operated. The fm gradual feeding of the ore was controlled by the center » stamp. STAMP MILL WITH AMALGAM TABLE Inside the mortar box there was a curved piece of silver plated copper which was called a chuck block. Water and mercury were introduced into the mortar box. As each stamp dropped 90 times per minute, there was considerable movement of the water and the mercury. The mercury picked up the gold as it broke loose from the rock, forming an amalgam which attached itself to the chuck block. The amalgam was removed from the mortar box at regular intervals. The amalgam is actually a solution of gold in mercury, similar to the solution of sugar or salt in water. At the other side of the mortar box there was a screen. The ore was milled in the stamp mill until it had the size of fine sand, fine enough to wash through the screen. The mixture of finely milled ore and water flowed from the screen onto the amalgamation tables. These were covered with silver plated copper sheets which had been treated with mercury by rubbing it into the silver. As the pulp flowed over the table, free gold was picked up by the mercury. The person supervising this process could tell by watching or feeling the amalgam when more mercury should be added to the tables or to the mortar box. The more gold the amalgam absorbs, the firmer it gets. It should have the consistency of putty. If too little mercury was added, the amalgam turned hard and could not be 29 scraped off. In this case it was necessary to burn the plates off and resilver them, an expensive operation. The trick was to keep the amalgam pliable. At regular intervals, the plates were scraped with rubber squeegees. The recovered amalgam was brought to the Bullion Room. Another way to remove the gold from the ore was to pass the pulp over ashaker table. This table had wooden riffles from which the gold was recovered, just like in a Sluice Box. Especially rich ore and concentrates were ground in a Berdan pan. This Berdan pan was driven by a 24 inch diameter Pelton wheel, SHAKER TABLE operating at 192 RPM _ through action of water under a 30 foot head. The panitself rotated at20 RPM. The iron balls which performed the grinding weighed 100 pounds each. This pan ground much finer than the stamp mills. The mixture of water and finely ground ore washed over the rim of the pan to be treated further at another place in the mine. The amalgam, collected at various places was put into chamois skins and squeezed until all free mercury had been forced out. The amalgam was then placed in amalgamation boats, which were placed in heavy retorts. Two of such boats, one made of iron and the other made of graphite, can be seen in the museum. A pipe, which led to acondenser, emerged from the lids of these retorts. The amalgam which is after all nothing but a solution of gold in mercury was heated to 700 degrees F. or more. This caused the mercury to vaporize and pass to the condenser where it was condensed to liquid mercury again. This mercury could be used again in the recovery of gold. The gold left in the retorts was put into a large graphite crucible. Many such crucibles can be seen in the museum. After a flux was added, the crucibles were placed in a furnace and heated to about 2,000 degrees F. BERDAN PAN