colored coating on top of melt

[Irascible]

I have a few molds with which I have to cast at 800 degrees. I have noticed, when using rotometals 1 in 25 alloy, that at that temp I get a rust/gold colored top layer which can be skimmed off. Yesterday I added a new block of the same batch of alloy and ended up with a bright blue layer, which could also be skimmed off. I tried fluxing first but ended up skimming. What the ?

[Yodogsandman]

You might be skimming off your tin, leaving close to pure lead behind. Use a wax to reduce the tin oxides back into the alloy before skimming. A golden color could indicate tin but more likely the blue, rust and golden colors on top are showing pure lead.

[OS OK]

If you are using a bottom pour pot and clean ingots to start with...just give the melted pot a butter bean size chunk of wax, stir it well and don't skim.

By the way, when I skim, I put it all in a little pot and re-run that pot whenever I am smelting 40 lb. batches of new ingots. I try to hang onto all my Sn.