I'm getting ready to take the plunge into bullet casting, and was pulling out some ingots I melted down a few years ago. I noticed these dark spots on the surface of a number of my ingots, and was wondering if it's a possible sign of zinc contamination. My ingots are melted from a combination of wheel weights that I melted myself (and I might have missed a few zinc ones, especially as I was melting on full heat) and ingots that I bought off eBay that purported to be from wheel weights. Are these spots anything to worry about, or is it just normal oxidation?