Zinc contamination?

[pomofo]

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?

http://castboolits.gunloads.com/imagehosting/234034f7f8f4deae80.jpg

[JonB_in_Glencoe]

i wouldn't worry about spots,
but if you can't get the boolits to fill out
you may have some zinc in there.

[gbrown]

Get some swimming pool testing kits at Walmart or local hardware store. Small red capped bottle and yellow capped bottles--yellow is labelled OTO. Hydrochloric acid, very light concentration. When you put a drop on an ingot with zinc in it, it will show bubbles and the color (goldish) will disappear. I have cast several alloys that showed a goldish sheen on them and it dissolved the sheen, indicating zinc present. However, as I understand it (and there are far more knowledgeable people than me on this forum), you have to have 1%-2% zinc before it impacts the alloy. The spots are probably from your ingot molds.

[runfiverun]

i'd say some oil got on them.
some bathroom cleaners have muratic acid in them, it will fizz and bubble on zinc but not on lead.

[pomofo]

Thanks for the advice. I think we may still have some muriatic acid around from clearing our pipes out a few years ago. I cast a few more ingots today and definitely noticed some marks on the ingots that came from something in the molds. Dirt, oil, who knows.