Koban and Bukin were made of mixed metal of pure gold and pure silver, and the amount of contained copper was at impurity level; whereas, about 3% of copper were intentionally added to Oban in order to have the color of gold to make it more aesthetic.
小判および分金が生粋金（純金）および花降銀（純銀）の合金で銅は不純物程度でしか含まれないのに対し、大判では3%程度の銅が意図的に加えられ、黄金色を演出させ審美性を持たせているとされる。