One of the most primitive, and easy to construct, devices for hard rock ore processing is the “arrastre” (or, variously, the “arrastra”). The typical arrastre consists of a tight stone basin, constructed on a flat, level surface. A sturdy pole is placed vertically in the center of the stone basin of the arrastre, forming a pivot point. A beam is secured to and extended from the pivot point over the rim of the basin; the beam is often counter-balanced. Attached to the beam, and hanging down into the basin, is a large, heavy stone. The heavy stone is used to crush ore placed in the basin. The portion of the beam hanging over the rim is attached to a source of biological power, such as a burro, mule, or unfortunate person or persons. The biological power source drives the beam around and around the basin, much like in the scene of the (original) movie Conan the Barbarian in which the main character attains great physical strength by monotonously driving a millstone to which he has been chained throughout his adolescence. Ore and water are continuously added to the basin, wherein the water acts as a lubricant.
Of extreme importance is the addition of mercury; also called “quicksilver” or “hydrargyrum” in antiquity. The mercury collects minute particles of precious metals ground out of the ore, and eventually forms an engorged “amalgam”, full of gold and (if present) platinum. If you research colonial Spanish mining law, you will find that any Spanish citizen could denounce (claim) a gold, silver, or copper mine, but the King owned rights to all Mercury mines. Hard to cheat the King if you had to account for your use of Mercury…
Once a sufficient amalgam has been formed, the mercury and precious metals are recovered by a process involving a “retort”, which is the evaporation and re-collection of the Mercury through the process of relatively low-temperature heating. The remaining precious metal content must then be recovered by typical means. As they say, “don’t try this at home” without sufficient research, education, and proficiency.
Depending on where you hike, you may encounter an arrastre if you are near an area of historical mining activity. Several examples are evident in the local area, if you know what to look for. One is only a few miles on the outside of the second largest town in the state, Las Cruces, and located only a few hundred feet from a busy interstate. The nearby ruins and debris suggest many periods of occupation at the site, with ranching, trade, and mining activities in evidence. Unfortunately, the adjacent mining districts are known to supply mostly lead, fluorite, and silver; though ancient treasure legends related to gold such as that of El Chato Nevarez abound in the region.
There are other structures used for mining and activities such as agriculture that have a similar appearance to an old arrastre. For example, a raised circular platform is more likely to have been the base of an old water cistern. Others, though not an arrastre, are still interesting from a mining perspective. Most structures related to mining are good to search, NON-DESTRUCTIVELY, for what few scraps of precious metal may remain. For one thing, the scraps may reveal hints about the nature and mineralization of the local deposits which may still be searched out and used. One example is an old structure discovered in the nearby Florida mountains, which, while certainly circular and possessed of a depressed basin, has other associated details that hint at another use. This particular structure has high walls, and is open on one side. Furthermore, the structure is situated below a flat dock strewn with crushed ore and, most tellingly, is surrounded by “coke” and “slag”. These details suggest a circular smelter or furnace rather than an arrastre.
One thing that an old arrastre may have that many other similar structures are unlikely to be associated with is the drag stone. The drag stone is the “business end” of the arrastre, used to constantly crush and grind the ore into a fine power for extraction of any free-milling gold that may be present. One of the most productive local mining districts in the state, Hillsboro, is just down the road, and the reports of early recorded activity in the district indicate that arrastres were constructed to mill ore after the first strike was made; a number of stamp mills and smelters were eventually constructed. There is a very nice, though relatively small, arrastre right near one of the better springs in that district, placed smack dab in a nice habitable zone covered with old foundations and signs of occupation. Supposedly, a then-youthful (now deceased) uncle of one of the elderly locals even found a “dore’” (do-ray) bar at the spring sufficient to yield a shiny new rifle for the finder, and who knows what else for the adults that excitedly rushed him into town to the assay office with the find. At this particular arrastre, there are the usual signs of ore brought from other locations, much of which shows obvious metallic mineralization and has been crushed into small fragments, but one other telling artifact as well: a fragment of a drag stone.
Who would leave a bar of dore’ gold just lying around? Well, the area has a long history of conflict going back to colonial Spanish times, and numerous battles and skirmishes are well documented between the modern settlers and the local native population. A fun and easy way to become familiar with the history of the area is to read James McKenna’s Black Range Tales. This particular arrastre appears ancient, but also has signs of more modern construction; the ideal setting near precious water and habitable terrain make it logical that the same arrastre may have been used both historically and in antiquity.
Exploring off of the beaten path is always a good idea, and just up over the Animas mountains from the spring location it appears that time has stood still. In addition to some great ore samples, including a cache of Galena nuggets that should not appear in this locality, another arrastre is found. This one appears to be of more modern construction, and is located below a huge mine dump.
Though ancient technology, arrastres were an important tool for centuries in the pursuit of precious metals such as gold and silver. It might be fun to find some in your area and share the pictures and tales.