Testing gold's purity using the fire assay method is not only the oldest, but also the most reliable gold test. This guide on how to test gold with fire will introduce you to the equipment you will need to perform a fire assay test.

To test the purity of gold using the fire assay method, you will need several pieces of highly specialized and potentially expensive equipment. Be aware that you will be melting down the item you are testing to produce a piece of pure gold. Therefore, this is likely not the testing method you will want to use on a valuable piece of jewelry. When performing the fire allay test, you will be working with extremely hot materials and acids, so observe any and all necessary safety precautions.

• A sample of gold to be tested

• Scale

• Lead foil

• Furnace or laboratory oven capable of reaching 2000 degrees Fahrenheit

• A small piece or a few scrapings of pure silver

• Cupel (a small bowl made of bone ash or another porous material)

• Nitric acid (35%)

1. 

   (Creative Commons photo by Tator1982)

2. Weigh the gold to be tested and record the weight

3. Wrap the gold and silver in lead foil

4. Place the foil-wrapped metal in the cupel

5. Place the cupel in the heated furnace and allow the metals to melt

6. Remove the cupel from the furnace and allow the melted metals to cool and solidify. The cupel will have absorbed the lead and any other non-precious metals in the gold, leaving behind a "button" comprised of pure silver and gold

7. Place metal button in the nitric acid. This will separate the silver from the gold

8. Remove the gold from the acid, rinse it in water and dry it off

9. Weigh the gold again

10. Divide the weight of your pure sample by the sample's original weight to calculate the percentage. For example, if you started with a sample that weighed four ounces and your resulting piece of pure gold weighed one ounce, you would divide one by four. Your result would be 0.25, meaning that your original sample was 25% pure gold

Testing gold for purity using the fire allay process is not often used by non-professionals, due in part to the specialized equipment used. You can buy kits and equipment for allaying gold, but will have to spend several thousand dollars to get all the supplies. Although allaying is the oldest and most reliable way to test gold, several other less labor intensive methods have since been discovered, including electronic, acid and touch stone testing.