The industrial revolution and mechanization happened in between, but were there subtler improvements in technical ability to extract more gold out of the same sample?
I ask because I wonder about mining areas which were let go of in earlier centuries because they were deemed not viable or cost effective, but which had a resurgence in later times.
Were there specific, finer chemical processes that were developed in the intervening time for deriving more gold from rocks or was it all a matter of better machines for digging down to the ore?
From the 16th to the late 19th centuries, changes in precious metal mining occurred on two fronts. The first was mechanical, meaning that the late medieval approach outlined by Georgius Agricola (1494 – 1555) in De re metallica was replaced in the last half of the nineteenth century with "modern" industrial technology. Much of this was developed and/or perfected in Nevada's Comstock Lode, made famous by the wealthy mines of Virginia City and Gold Hill and then by the TV show "Bonanza" (1959-1973). These technological innovations were led by the use of steam power, but also involved such innovations as square-set timbers, flat wire cable, safety cages, dynamite, air-compressed drills, and other things that made digging safer and more efficient.
The second type of innovation was chemical. For centuries, gold and silver extraction depended on the use of mercury, which forms an amalgam with these precious metals. The mercury can then be cooked off, leaving the precious metal (and putting the toxic mercury into the air as a lethal particulate). In the late 1800s, chemists and mining engineers began working with a process that replaced mercury with cyanide and zinc, the method that is still used today.
The use of cyanide and zinc for the retrieval of precious metals was first conceived in Scotland in the final years of the nineteenth century. Limited testing in New Zealand and elsewhere proved the value of the process. In 1896, Professor Richard D. Jackson of the University of Nevada adapted the idea and perfected the process. He conducted tests on Comstock mill tailings, and then at the turn of the century, he set up a more formal operation at the industrial complex now known as the Donovan Mill. The success of his experiment opened the door for the industry to cease using mercury for processing gold and silver ore, arriving at an approach to milling that was more efficient and better for the environment.
The Donovan Mill was built in 1890 by F. S. Lacruts, with ten stamps crafted at the Virginia & Truckee Railroad Foundry in Carson City. Around the turn of the century, an additional twenty stamps (each weighing 1,100 pounds) completed an impressive bank of thirty of these industrial pistons, designed to pound ore into powder. When Jackson and his partner Dr. J. Warne Phillips outfitted the mill for cyanide processing, they added tanks and rooms to the south. Phillips eventually bought out Jackson and then sold the mill complex to William Donovan, Sr. The mill processed ore until 1959.
The cyanide/zinc method, also called heap leaching, has made lower-value ores profitable and this has inspired the reassessment of old mining districts and a resurgence of precious metal mining in areas previously abandoned because of the limits of mercury milling. Otis Young discuses this in his Western Mining. I also discuss the transformation of mining in my book The Roar and the Silence: A History of Virginia City and the Comstock Lode.
The use of steam powered machinery allowed shipbuilders to design and build gold and tin dredgers. These came up around the turn of the nineteenth century. The Dutch used these to dredge coastal areas of Indonesia for tin - sadly, that is where my knowledge on it ends. The Dutch did look into the viability of dredging for gold back home, but the cost benefit projections shot those plans down. This would be around 1905. Source: Ingenieur, a weekly magazine for engineering.
It might bot be what youre looking for, but I figured Id post it anyways.