Excellent points CM, and it bears highlighting. I meant to mention that, but forgot. All metals that oxidize when heated have an ideal reduction temperature, IIRC iron's is about 300* above melt point, I don't know what 20:1's is but I'm sure it's not as high as 775*. I would think the cooler the better.
It's a common misconception that tin floats on lead since the RATE of oxidation of tin is higher at casting temps than that of lead. Due to this rate, the tin exits the alloy as oxide scum before anything else in the alloy does, so tin is depleted more quickly since tin OXIDE floats on the melt's surface. The higher oxidation rate of tin is due to the fact that it's melting point is so much lower than that of lead. As we know, the higher the temp, the higher the oxidation rate with boolit metal. Tin has a melt point nearly 200* lower than lead (I forget exactly at the moment) and is much more reactive than lead of antimony at casting temps. This also explains why antimony is the last component to oxidize out.
Lead and tin are fully miscible, meaning they form a true solution and do not gravity segregate after being melted together. (sorry for the misspellings, I'm in a hurry)
Gear