The Pinto Wye Arrastra is an example of nineteenth and twentieth century milling technology. The most interesting aspect of this particular arrastra is the use of a wooden wheel, as the pivot mechanism. Regionally this is the only wagon wheel arrastra that still has integrity of its original location, and construction.
The National Park Service was unaware of the arrastra until the mid-1960, when an employees wife “discovered” it while out on a hike. Locals claim to have known of its existence for sometime prior.
To give you a better understanding of the workings of an arrastra, I am providing you with an excerpt from the HISTORIC AMERICAN ENGINEERING RECORD, which was published by Joshua Tree National Park:
The design of the arrastra remained basically the same for more than a hundred years. Miners from Sonora, Mexico, brought arrastra technology northward with them into California in the very early days of the California gold rush. These simple mills were used from 1850 to the 1930s. An arrastra had a short circular retaining wall which ranged in diameter from 8′-20′.9 It was made of stone or wood, whichever was more convenient. An outlet at the low end was screened or had a gate. Within the wall was a circular floor made of stones which were flat, large and hard. They were set in either rammed clay or concrete which stopped an inch below the surface of the stones. Narrow channels were left between the stones.
There usually was a short stone or wood pillar in the center of the circle, on which a substantial wooden post was set. Between this pillar and the wall was a circular channel. The channel was about 2-1/2′ to 6′ wide or larger. One or two horizontal beams (making two or four arms) sat on the central pivot post and extended out over the floor. (If two beams were used, they would be at right angles to each other.) A rope or chain was attached to each arm above the channel. The other end of each tether was fastened to an eye bolt which was tightly wedged into a drilled hole in a drag stone. These stones weighed from 150 to 1,000 pounds each and were made of the same hard material as the floor. They were dragged around the circle, on top of the ore. The front edge of a drag stone was slightly beveled, so the ore particles went underneath the drag, instead of being pushed ahead. The drags were positioned so some were near the center and some were near the outer edge, to ensure complete coverage. Dragstones wore out in about six to eight weeks, then were replaced.
Ore, water and, in later years, mercury were the basic ingredients in the milling recipe. The ore and water were added in a certain routine. Sometimes the ore was first partially ground dry, and then water was introduced. Other mills used water from the beginning. The operator controlled the movement of the dragstones; if they started to jump he added water or ore. The stones rode on about 1″ to 1-1/2″ of ore. The ore was about an inch in diameter when it was put into the arrastra. Sometimes it was broken with a crusher to get it to this size.
When the arrastra was used as a simple grinder, the drag stones ground the ore to a thin pulp or slime, which filtered out the screen in the outlet. The gold was then separated from the pulp using a pan, sluice, or similar method.
A later modification incorporated the use of mercury. The ore was ground to the size of coarse or middling sands, which took about five to seven hours. Mercury was then added to this pulp. The mercury was in a little bottle with a cloth over the top; this was shaken like a salt shaker, all around the arrastra. Approximately one ounce of mercury (about a tablespoon) was used for each estimated three-fourths to one ounce of gold. Grinding then continued an additional two or three hours, until the ore was a slime, or fine sand.
The mercury formed an alloy, called amalgam, with the gold. Being heavy, it fell to the bottom. The operator panned samples of the pulp until he found no gold or amalgam, because it had dropped into the recesses. Then he added more water, until the pulp was slushy, and opened the discharge gate. The pulp drained out, leaving the amalgam in the arrastra. These tailings usually drained into sluices lined with copper plates, riffles or blankets, where gold residue was captured. The total time to mill a load of ore (about 2-1/2 tons) varied with the character of the ore, but it ranged from six to twelve hours.
Traditionally an arrastra was powered by mules and burros, the Pinto Wye Arrastra was however powered by a four horsepower
International upright gasoline engine.
It is a mystery to this day who constructed the arrastra, and sunk the nearby mine shafts. Archaeologist have placed the time period between 1920-1930.
The arrastra is located in the Pinto Wye vicinity of the park. There is no trail leading to the site, however there is an interpretive sign posted on location. If you manage to find it, respect it, it is a beautiful specimen.