Brass coins were first minted in Bithynia and Phrygia, in modern Turkey, around 100BC, and by the time of the first minting of dupondii brass coins were being minted in large numbers in Rome and quickly spread throughout the Roman Empire.
Early brass, including that used to mint dupondii, was made using a cementation process that involved heating metallic copper with calamine ore, which has a high zinc content. As the zinc vapourized it would be absorbed by the copper, creating brass alloys with between 15-30% zinc content.
Read more on the History of Brass.
Image: Dupondius of Vespasian, struck in Lyon around 72AD. 29mm in diameter. Author: Bedoyere. Public Domain.