So I casted a zinc ingot around a week ago and I also casted 2 zinc ingots yesterday. After observing the two side by side I noticed that the older zinc ingot was darker colored, then I went and looked at all of my old ingots containing zinc. They where all dark. Here is a picture of the old one and the new one side by side
The old ingot is on the left and the new one is on the right.
Is this oxidation possibly? What causes this? Any answers are appreciated.