Another Zinc Question

[Irascible]

I sorted through a 5gal bucket of WW. I looked for a ZN or FE and then I touched them to a magnet and those that didn't stick I dropped on the floor (none rang)and then tried to cut with a pair of side cutters(they seemed slightly harder or softer, but not by a large amount. I only found 2 steel clip ons and 3 strips of steel stick ons. No zinc clip ons that I Know of and a few strips of zinc stick ons. I smelted over a plumbers pot at low temp (below 600 deg) fluxed and stirred, fluxed and stirred, fluxed and stirred. I melted about 25 lbs at a time and I'm now convinced that smelting that much at a time is a problem, because the weight of the top layers keeps the dirt from floating easily.
QUESTION 1 - would it be easier to clean a smaller amount of weights?
After getting most of the dirt out of the mix and pouring ingots, I later melted 10 lbs in my pot at 550 degrees and fluxed a few times. Then I added another 10 lbs and fluxed again, and again. Thinking That I had clean metal I started casting at 700 degrees. I was getting frosted, sunken in spots which I decided was zinc. Over the next two days, I warmed the pot to 550 degrees and fluxed and skimmed. I the unplugged the pot and repeated this many times. I started casting with acceptable, but not perfect results (50% rejection).
QUESTION 2 - Each time I return metal back to the pot, I flux and skim after melt. Within a few minutes, I get a top coating that looks like zinc coating. IS IT?, Or is it tin oxide of something else? I flux again and skim off the coating and any dirt that has risen. The coating always returns.

[Leadforbrains]

Probably lead oxide. I ladle cast using wheel weights and range lead with a little tin added for fill out. I have to deal with this all the time. If it gets to much I just flux the melt and skim. I do this periodically. If you are bottom pouring I don't think it would bother you as much as it does me unless you are running your pot low to the point it clogs up your spout.
Your rejection rate could be do to something else, because it looks like you are being careful enough in your smelting process from what you describe.
I usually have a pretty high rejection rate at first until everything gets to the right temperature.
The mold has to be hot enough, the alloy is hot enough and my casting rate jives with keeping everything right where it needs to be to get good boolits. I still find rejects, but the rate goes way down during this moment of ZEN when I am in my zone.
I am still new at this so somedays it takes me a while to get there if you know what I mean.

[high standard 40]

If you smelted at 600 degrees you should not have any zinc in your alloy. What color is the skim on top of your casting pot? I heard others here say that when melted, zinc appears as clumps of oatmeal on top of your melt unless you flux it in. Can't say for sure on that though. My wheel weight alloy in my casting pot will begin to turn the color blue or purple on top and I have read here that that is normal.

[KYCaster]

#1...Smaller ammounts are not easier to clean. Your side cutters are a good way to tell the difference between Pb and Zn, the difference is very obvious. The frosted/sunken spots are due to the mold being too hot, slow down your casting tempo or cool the mold between casts.

#2...The film that forms on top of the melt is oxides. Left alone it will form a protective layer and reduce further oxydation. I flux only once before I start casting and scrape the sides and bottom of the pot with a wood paint paddle in addition to the boolit lube flux I add on top of the melt. When the flux is completely consumed, you will have bright metal surface with dirt and ash from the flux floating on top which can be scooped off. If there is a metalic film, you haven't fluxed properly. Once the fluxing is done the oxide film will form in just a few minutes...no need to mess with it till the next time you fire up the pot.

Jerry