One of the most important steps to exploring for minerals is assaying ore bodies in the area of interest. The word “assay” comes from the Anglo-French word assai, which means “a trial” or “a test of character.” And that is just what an assay is – a test of the quality and character of an ore.
The goal of assaying is to determine the relative proportions of different metals within an ore.
Different assaying methods are used depending on the type of ore being sought. In this article, we’ll talk about one of the most important methods of assaying – called fire assaying.
Fire assaying is most often used to determine the percentages of silver or gold in a rock ore by weight.
The first step of the fire assaying process is to prepare the samples. Unprocessed ore rocks need to be crushed into what is called a “pulp” – essentially a fine powder.
This can be accomplieshed through the use of rock crushing machines, buckboards, and rockers.
Once the ore is reduced to pulp, it is passed through a sieve to ensure a consistant grain size.
Next, the ore is loaded into a scorifier, a container capable of withstanding extreme temeratures. At the same time, lead oxide and flux agents are added.
When the loaded scorifier is placed in the oven, the lead oxide is reduced to lead, forming an alloy with any precious metals present in the sample. At the same time, the flux agents react with the rest of the ore, forming a slag.
When the samples are cooled, the metals, which settle to the bottom of the scorifying tray, are separated from the slag.
Next, the samples are placed in a cupel, a special container that absorbs lead and other impurities. When the metals in the cupel are remelted, the silver and gold are the only components that aren’t absorbed into the cupel.
After the silver and gold remaining in the cupel cool, their total weight is measured. The next step is to separate the silver from the gold.
This is done using water, nitric acid, and heat. The silver dissolves and can be washed away, leaving only the gold. Finally, the gold sample is weighed. Given the weight of the original cupel contents and the weight of the gold component, the weight of the silver component that was lost can be calculated.
The assay is now complete.
If you want to learn more, a humorous description of the process is related in chapter 36 of Mark Twain’s autobiographical book, “Roughing It”. It can be read at: http://www.classicreader.com/book/1407/37/