How to do an Assay for Gold

How to do an assay for gold is something that basically hasn't changed since the 1400s and is probably even older. There is evidence that the knowledge for determining the amount of gold in an ore even dates to the days of the Romans. The process is explained in detail by Georgeius Agricola in his book about mining, "De Ra Metallica," that was published in the 16th Century. The process will give you an idea of the gold content is in your ore. This is called a "fire assay." In our day much of the ore analysis is performed with an Atomic Spectrograph by analytical chemists, but a fire-assay is good enough for the buyers of gold to accept if it is conducted by a certified assayer.

To perform an assay the first thing that has to happen is the ore is ground to a fine powder usually in an iron mortar and pestle, although others will work as well. However, don't use one made from marble as it is to soft, and the quartz in the ore will destroy it. Once the ore is ground it has borax added it as a flux with more borax then ore.

Once the two parts are mixed thoroughly they are added to a suitable crucible and placed in a furnace that is hot enough to fuse the parts of ore and borax into a liquid. The gold and other metals in the ore settle to the bottom of the crucible. The other minerals present in the ore float to the surface chemically combined with the borax as slag.

After the crucible and its contents has cooled it is broken open and its contents are removed. The gold and other metals present form a "button" that is found at the bottom of the crucible. The button is removed and weighed on an accurate scale capable of weighing in milligrams. This weight will tell how much total metal there is in the ore.

After the button has been weighed it still contains all of the other metals that it was associated with in the ore. To accurately measure how much gold is actually present in the ore the button is broken up, and dissolved in dilute nitric acid. All of the metals present in the button will dissolve in nitric acid except gold. After all the metals present have dissolved there should be a residue that hasn't dissolved in the bottom of the beaker you dissolved the button in. The remainder is gold.

The fluid is then filtered in filterpaper and weighed again after the filter paper has dried. Weigh an extra sheet of filterpaper to determine the weight of the filterpaper you have used. The difference is the actual weight of gold in the ore. As an alternative way of weighing is weigh the filterpaper before you use it to filter the fluid from dissolving the button.