Acid testing is a procedure in which (nitric) acid is used to check the different purities of silver/if the metal is silver. Different purities of silver used within a metal will react to the acid in different colours, which can then be colour matched to identify the silver purity.
- 100% Solid Silver is the purest form and is not commonly found.
- 92.5% (925/1000) is prominently used in English silver and can be referred to as "Sterling Silver" or 925 Silver.
- 80% (800/1000) is most commonly found in German Silver with a slightly lower grade than the alternative silver purities.
Alternative acid testing can be used for gold karat testing - a mixture of nitric and hydrochloric acid (in different concentrations) are utilised to identify different gold karats. The difference in reaction in accordance with the different acid concentrations can give indications to the different levels of gold.
Precious metals are often mixed with other alloy base metals to increase hardness/durability/mass and therefore requires a metric standard to identify what percentage of the "gold" is actually solid gold.
Karats are a unit of expressing the fineness of gold out of 24 parts: 18/24 = 75% Alternative notations may include parts per 1000 such as (750/1000).
Below are some of the most common gold karat classifications:
- 9K - 37.5% (375/1000)
- 12K - 50% (500/1000)
- 14K - 58.3% (583/1000)
- 18K - 75% (750/100)
- 22K - 91.6% (913/1000)
- 24K - 100% (1000/1000)