On August 11, 1860, the first successful silver mill, in Virginia City, Nevada, began operation. An ore mill consisted of machinery and materials that were set up in order to recover the profitable metals in the ores. These mills were usually enclosed in buildings with their equipment arranged in levels with the lower level last, so that gravity moved the product downward. Between the 1860s and 1880s, many different processes for separating ores were tried and abandoned before successful methods were hit upon. Each ore had its own peculiarities to contend with, and methods that worked with one mineral could be a waste of time and money with others.
Whatever the method, water was the key liquid to carry the crushed “pulp” to the next stage of the milling process. The ores were crushed by stamps, often weighing up to 750 pounds. This was akin to a mortar and pestle, with the pestle being a long metal rod with a heavy iron “shoe” at the crushing end. At one time, the Vulture Mill in Arizona ran 80 different stamps and on a clear day the clamor could be heard fifteen miles away!
While the stamp mills were all similar in the way they crushed ore, the second stage—separation of metals from the crushed ore—varied widely based on several factors. Basic ores (sometimes called “free milling ore”) could be separated by relatively simple processes like amalgamation. Complex ores required more elaborate methods, and as technology advanced, many new processes were introduced in western stamp mills.
The process of separating valuable metals from waste rock is called concentration. Stamp mills would either ship the bullion produced from free-milling ores, or it would ship a concentrate, which was the product of a more complex ore, that would need further processing at a smelter. Amalgamation involved combining the gold or silver with mercury. A time-consuming and dangerous process, as mercury (particularly the vapor) is poisonous. Amalgamation “plates,” long plates of mercury-coated copper, were placed directly underneath the discharge of the stamp mills. The finely-crushed rock, along with water, was slowly distributed over these plates, where the mercury combined with the metal ore. Cleanups (scraping the plates to recover the amalgam) were frequent. This was strained to try to recover a little of the liquid mercury, while the bulk of the “glob” was sent to a retort and heated to vaporize the mercury, which was then condensed for reuse. What remained in the retort or large crucible was called “sponge” as it was not heated enough to melt the metals.
The sponge was then melted in small, brick furnaces, often with the addition of a flux to speed up the process. The end result was a “dore,” which was a non-pure bar of gold and/or silver. These bars were further refined at the mill, or sent to a smelter. Some of the different amalgamation processes were called the Patio Process, the Washoe Process, the Sagebrush Process, and Reese River Process. For the silver ores of the Comstock, both iron and salt could promote a chemical reaction with the silver. The Washoe Process was especially effective in liberating silver from sulfides.
Here’s what the famous Mark Twain had to say about the milling process at the time: “From our bricks a little corner was chipped off for the “fire assay”—a method used to determine the proportions of gold, silver, and base metals in the mass. This is an interesting process. The chip is hammered out as thin as paper and weighed on scales so fine and sensitive that if you were to weigh a two-inch scrap of paper on them and then write your name on the paper with a coarse, soft pencil and weigh it again, the scales will take marked notice of the addition. Then a little lead (also weighed) is rolled up with the flake of silver, and the two are melted at a great heat in a small vessel called a cupel, made by compressing bone ashes into a cup-shape in a steel mold. The base metals oxydize and are absorbed with the lead into the pores of the cupel. A button or globule of perfectly pure gold and silver is left behind, and by weighing it and noting the loss, the assayer knows the proportion of base metal the brick contains. He has to separate the gold from the silver now. The button is hammered out flat and thin, put in the furnace and and kept some time at a red heat; after cooling it off it is rolled up like a quill and heated in a glass vessel containing nitric acid; the acid dissolves the silver and leaves the gold pure and ready to be weighed on its own merits. Then salt-water is poured into the vessel containing the dissolved silver, and the silver returns to palpable form again and sinks to the bottom. Nothing now remains but to weigh it; then the proportions of the several metals contained in the brick are knows, and the assayer stamps the value of the brick upon its surface.”
There are also mills famous for being spectacular failures. One such was the Gould & Curry Mill of Virginia City, Nevada. Built in 1863 at a cost of over $900,000, then rebuilt just a few years later at an additional cost of $560,000, the mill was “the most conspicuous monument of inexperience and extravagance ever erected in a mining district,” according to Eliot Lord (Comstock Mining and Miners, 1883).
If your character had a silver claim, they could have gone to an assayer, who would have sent their silver to a mill for processing. Even if they didn’t work with silver themselves, they may have lived near a noisy silver or gold mill.