Discovered around 800 A.D. by Iranian alchemist Abu Musa Jabir ebn Hayyan, aqua regia is a corrosive mixture of acids used to refine precious metals. "Royal water" in Latin, aqua regia is also used in etching and in cleaning glassware in certain applications. A foaming yellow or red combination of nitric and hydrochloric acids, aqua regia dissolves gold and platinum through oxidization. Certain other "noble" metals --- tantalum and iridium, for instance --- are impervious to aqua regia.
Dissolve Gold in Aqua Regia
Place one thick plastic bucket on an outside table. Ensure good ventilation in the area and wear protective clothing.
Weigh gold on a scale and record the total weight. This figure will determine the amount of constituent acids required for the aqua regia.
Put small pieces of gold into the bucket. Large pieces will not dissolve efficiently and need to be melted down into smaller pieces before immersion in aqua regia. Rings and most jewelry do not need to be reduced in size prior to dissolving.
Pour 2 tablespoons of nitric acid into the bucket for each ounce of gold. Let the nitric acid react with the gold for at least 30 minutes. Brown fumes may be present at this time. If you are using a commercial nitric acid substitute, such as subzero, it is not necessary to wait for 30 minutes. Note that subzero does not typically produce fumes of its own and will not react with the metal until hydrochloric acid is introduced.
Add 8 tablespoons of hydrochloric acid per ounce of gold. Aqua regia may not react quickly at first, but the mixture will eventually turn hot and brown as the gold is dissolved. Limit your exposure to toxic and corrosive fumes by leaving the area.
If further refining the gold, wait at least one hour, and preferably overnight, to allow fumes time to dissipate.