I recognize those inclusions, but mine didn’t always disappear with sizing. I always wondered if those were from dropping spruce cuts into the pot while metal oxides were floating. Do you use a bottom pour & are you dropping in spruce cuts or culls?
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Is the level of antimony here higher than you are used to using?
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At the end of my casting session, I clean my alloy and empty the pot into the ingot mold… because of these inclusions. Then I flux again at the beginning of the next casting session, so ingots are cleaned several times. This helped my purchased 92-6-2 alloy.
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I remember when I received the 8lb ingots and melted them down into 1lb ingots - a whole lot of trash was stuck to the inside walls of the pot. This is the cheapest source of lead with 2% tin I can find (regardless of antimony content), so might be a “you get what you pay for” kinda thing.