1) None of them is actually bronze
2) The Ife bronze heads and the Obalufon mask are copper alloys
3) Four of the Ife heads, and the Obalufon mask are nearly pure copper a most difficult metal to manipulate .Producing these casts made of almost pure copper was “a feat that artists of Ancient Greece and Rome, the Italian Renaissance , and Chinese bronze casters never achieved”- Suzanne Preston Blier
4) All of the heads and the Obalufon mask ( 19 altogether) were made using the Lost Wax technique
Very informative