Or someone melted down a glass and gold object and noticed the gold that floated (precipitated?) out was purer than that which went in.
Even today various artists playing with fire rediscover that while gold doesn't naturally work into or onto glass it's still possible to adhere gold to glass if the timings and tempreptures are "just right".
Or someone melted down a glass and gold object and noticed the gold that floated (precipitated?) out was purer than that which went in.