Why does zinc turn dark?

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


[![enter image description here](https://i.stack.imgur.com/NxFs5.jpg)](https://i.stack.imgur.com/NxFs5.jpg)


The old ingot is on the left and the new one is on the right.


Here is another photo[![enter image description here](https://i.stack.imgur.com/Q8YTA.jpg)](https://i.stack.imgur.com/Q8YTA.jpg)


Is this oxidation possibly? What causes this? Any answers are appreciated.


