|Gold and silver are similar on a chemical level and are often found together forming an alloy known as electrum. Electrum wasn't always desirable for trade. |
When coinage started gaining popularity a means to standardize the purity of the gold and silver was needed. The first technique of gold parting was invented: salt cementation. Salt cementation involves adding gold/silver alloy, some burnt clay or old brick dust, salt, and urine to moisten it. The mixture is sealed and then heated, but not hot enough to melt the gold – less than 1000°C.
| In about 24 hours, the gold will be nearly silver-free at around 90% purity or greater. When heated in the presence of silica and alumina (found in the clay/brick dust), salt breaks down to form hydrochloric acid and chlorine. The acidity in urine helps decomposition. The hydrochloric acid from this reaction interacts with the silver to create silver chloride, which separates from the gold. When that occurs, the reaction is volatile – which is why it's sealed. |
After removing the gold, one can convert the silver chloride back into silver, giving you two separate, purified samples of precious metals for coins.