What Are The Symtoms Of Zinc Contamination?

[Shooter30-06]

I began casting last summer and successfully smelted my first batch of wheelweights at that time. Today I tried my second batch but after only producing a few ingots the contents of my pot turned a grainy yellow brown. After spooning it out the surface then turned a bright blue/purple. No amount of spooning would stop this. Finally I emptied the pot, cleaned and scraped the pot and all my dippers as thoroughly as possible and put everything away. Did I have a zinc contamination or some other problem? Is a thorough cleaning and scraping suffiecient to prevent problems the next time? Thanks for your help.

[arcticbreeze]

It doesn't sound like zinc. From My experience you would get a cottage cheese type consistency on top of the melt at normal smelting temps unless you run your pot really hot. It sounds like normal oxidation to me. The best way to keep from getting zinc in the melt is to get a good thermometer and keep your temperature just above the ww melting point than if you missed a zinc ww it will just float to the top and you can skim it off with the clips.

[zt77]

from what I hear, a purplish and bluish colored metal on top is oxidized lead

[geargnasher]

Use copius amounts of coarse sawdust, crushed charcoal briquets, used engine oil, whatever and get that frozen antimony and lead oxides to reduce back into the melt. You should have a bright, silver shiny surface under a layer of carbonizing organic material that you can see when you stir the melt. If you have an oatmeal mess forming on top, DON'T skim it! Thats the good stuff and you want it to melt back in. if you watched your temps initially and kept them under 700 when smelting and skimmed the weights that didn't melt in, you don't have zinc contamination and you can use lots of heat without worry to help get that antimony to re-mix. Just keep a layer of flux on the surface to keep the tin and lead from oxidizing out too fast at the higher temps. Zinc does different things, and the biggest issue is it alloys fairly well in small quantities (better than antimony) and you won't know you have a problem until you start casting boolits and they don't fill out and the spout keeps freezing.

Gear

[geargnasher]

from what I hear, a purplish and bluish colored metal on top is oxidized lead

It is. This is exacerbated at higher temps and can be prevented by keeping a generous layer of sawdust or wood chips (like PatMarlin's california flake flux) on top to reduce the oxides as soon as they form.

Gear

[lwknight]

Sounds like you have pure lead. If you think this might be the case, drop in a bit of tin to see if it stops doing that.