Salt and Silver

Summer 1990

Common salt, sodium chloride, was perhaps the first nonmetallic mineral to be mined in Nevada. Great quantities of salt were used in the processing of ores from the various silver mines around the state, and most of this salt was recovered from playa lakebeds, or "marshes" in western Nevada. Probably the first salt to be mined for this purpose was shipped from Dixie Marsh where production began in 1861. Discoveries were made at other marshes and, in 1862, a herd of camels was imported to haul salt from Rhodes Marsh, in what is now southern Mineral County, to Virginia City. Salt was discovered at Sand Springs Marsh in 1863 and, in 1870, salt-rich brine was discovered at Eagle Marsh (Leete Marsh). This location was close to the Central Pacific Railroad and a salt works was built there which supplied almost all of the needs of the Comstock and many other Nevada silver camps. The exact role that salt played in the recovery of the silver ores, however, was controversial in its day and remains somewhat unclear today.

Silver in the bonanza ores from Nevada's Comstock mines occurred mainly in the form of silver sulfide, the mineral argentite, accompanied by minor amounts of free gold and silver. Only two, rather crude, silver recovery methods were in general use at the time of the Comstock discoveries, direct smelting and amalgamation using mercury. Direct smelting worked well but it was expensive and therefore was used only on the highest grade ores. Amalgamation, which used mercury to collect particles of gold and silver, worked only on free-milling ores-ores which contained metallic gold or silver. In this process, mercury was mixed with crushed ore where it combined or "amalgamated" with the metallic particles forming a gold-silver-mercury alloy known as amalgam. The amalgam was then roasted, driving off the mercury and leaving behind a sponge of gold and silver which then could be melted into bullion. Neither of these two processes proved to be very successful in treating Comstock silver ores; the expense and sheer volume of the ores defied direct smelting and silver sulfides simply did not amalgamate with mercury. The milling problem was solved by calling on a method developed centuries earlier in Pachuca, Hildago, Mexico, to treat similar silver ores from the Pachuca-Real del Monte mines. At Pachuca, in 1557, a process was perfected whereby silver ores were crushed in arrastras to a fine slime which was then mixed with salt, water, copper sulfate, and mercury, and spread in a shallow-walled, open enclosure or patio. Horses were driven around on the patio to accomplish additional grinding and mixing and, after four or five weeks of mixing and soaking in the sun, the silver sulfides oxidized and the resulting silver could then amalgamate with the mercury and be recovered. By the time this process arrived in Virginia City in the late 1850s, stamp mills instead of arrastras were used for crushing, horses had been replaced by iron shoes, or mullets, and the patio had been replaced by small, enclosed iron tanks (pans); the chemistry, however, remained basically the same. Innovators on the Comstock altered shapes of the pans, added steam to heat the slime-salt-copper sulfate-mercury mixture, and fussed with the size and weight of the rotating iron shoes thereby reducing treatment time to a matter of a few hours compared with the several weeks required for the original Mexican patio process. All of this resulted in a technique thereafter known as the Washoe Pan Process. What role did the salt play in this process? There seems to be some question as to what any of the "required" chemicals accomplished, and the process always seemed to call to mind the workings of medieval alchemy rather than state-of-the-art metallurgy. Silver sulfide ore was, of course, a required ingredient, as was the mercury to accomplish the end result of amalgamation. Controversy existed, however, over the role of the salt and copper sulfate. Amounts of these substances used from mill to mill varied from one-quarter to ten pounds of one or the other, or both, per ton of ore treated. At times other substances, such as tobacco juice and sagebrush, were also deemed to be necessary to the success of the process. Some said the salt formed chlorides of copper and mercury which, in turn, brought about the formation of silver chloride and free silver which would then amalgamate with the remainder of the mercury. Others maintained that the chemicals did nothing and that agitation and heat were the only important considerations. Mercury loss during the process was quite high, about 1 to 1-1/2 pounds per ton of ore treated; some of it may have been as mercury chloride but most was as fine, floured mercury formed during the pan agitation stage. This mercury now resides in the Carson River drainage below the old mill sites. The pan process appears to have fallen into disuse toward the last few years of the 1890s and, with the invention of the cyanide process at that same time, use of the process disappeared altogether.

---J. V. Tingley, Economic Geologist