How to Assay Gold

Assaying gold is the process of determining its purity or fineness. This is important for many reasons, including valuation, trading, and manufacturing purposes. There are several methods for assaying gold, including fire assay, aqua regia assay, and X-ray fluorescence assay. Here are the steps involved in each method:

1. Fire assay:

- Take a small sample of the gold and melt it in a crucible with a flux (usually a mixture of borax and soda ash). - Pour the molten gold into a mold and let it cool into a solid button. - Weigh the button and record the weight. - Grind the button into a fine powder and mix it with a known amount of lead oxide. - Put the mixture into a cupel (a small, shallow dish made of bone ash) and place it in a furnace. - Heat the cupel in the furnace until the lead and other impurities in the mixture are oxidized and absorbed into the cupel, leaving behind only the pure gold. - Weigh the resulting bead of pure gold and calculate the percentage of gold in the original sample.

2. Aqua regia assay:

- Take a small sample of the gold and dissolve it in a mixture of nitric and hydrochloric acids (known as aqua regia). - Neutralize the acid with a base (such as sodium bicarbonate) and filter out any insoluble material. - Add a known amount of a silver nitrate solution to the gold solution. - The gold will react with the silver to form a precipitate. - Filter out the precipitate, wash it, and dry it. - Weigh the precipitate and calculate the percentage of gold in the original sample.

3. X-ray fluorescence assay:

- Take a small sample of the gold and place it in a sample cup. - Insert the sample cup into an X-ray fluorescence machine. - The machine will bombard the sample with X-rays, causing the gold atoms to emit characteristic X-rays of their own. - The machine will measure the energy and intensity of these X-rays, which can be used to determine the amount of gold in the sample.

Note that each method has its own advantages and disadvantages, and the choice of method depends on the specific requirements of the application. It is also important to note that assaying gold requires specialized equipment and expertise, so it is usually performed by professionals in a laboratory setting.