The ancient metalsmiths worked with different tumbagas, which were alloys of silver - bearing gold and copper. 

With just as little as 5% gold content and 95% copper they managed to produce objects with gilded surfaces. 

Fire oxidizes copper,; the oxides migrate to the surface and cover it. 

The metalsmiths ground an herb to obtain an acid, which they mixed with boiling water and salt. 

When the object was submerged in this hot solution, the action of the acid removed the copper oxides. 

The final thickness of the gilding depended on how many times the process was repeated.