I was wondering if it was possible he was running his melt too hot causing excessive oxidation or possibly skimming some of the oxidized Tin off the top before it was incorporated back into the melt.
That's better. A way to stall, slow down the oxidation of the tin is to leave burnt saw dust on the surface of the melt. This insulates the surface of the melt from the oxygen in the atmosphere, greatly reducing the formation of any oxides. Some use kitty litter, which just insulates, but does nothing for fluxing. The carbon in the burnt sawdust is an excellent flux.
This ONLY works IF you're using a bottom pour pot. Ladle casting wouldn't work too well, you have to constantly dip the ladle below the surface of the melt. This disturbs the melt, so you see the formation of the oxides much faster.
1chig, the best thing to do is completely empty the pot, clean it out real good. Get that powdery residue off/out, then only use clean ingots when you re-fill.