We had a discussion on heat treating cast bullets over on facebook, and I threw out my speculation on what the role of arsenic is. I thought I would share it here and see what you had to say.
Carbon in steel forms iron carbides, which are MUCH harder than steel. There are several different phases of iron carbide, and the properties of the steel depend on which iron carbide phase is present, and how much of it is there. The purpose of heat treating steel is to convert one iron carbide phase into another. Antimony in lead alloys is roughly analogous to the iron carbides in steel, in that it is much harder than the surrounding matrix and that the properties of the alloy are dependent on how much is present, and what form it is in. That's pretty much where the analogy stops. Iron carbides are insoluble in steel, and always form a separate phase. Antimony is slightly soluble in lead, and like many things, the degree of solubility changes with temperature. At room temperature, antimony is soluble in lead to the tune of less than 1%. At molten lead alloy temps, antimony is much more soluble (e.g. 20-30%, depending on the alloy and the temperature). As many experienced casters can tell you, getting pure antimony to dissolve in lead alloys is a major pain because it dissolves so slowly. Why would antimony rich phases in a cast bullet be any different? I believe that the role that arsenic plays in the heat treatment process is that it forms an intermetallic compound with antimony, and the arsenic-antimony adduct is much more easily soluble in the lead matrix (much like the tin-antimony intermetallic adduct, e.g. Lyman #2 alloy). IF the arsenic-antimony adduct formation is roughly a thermoneutral reaction, then it will also be reversible, meaning that the arsenic atom could "pop off" after the adduct had migrated into the lead rich phase. If this model is accurate, then arsenic would be essentially serving as a shuttle to facilitate transport of the antimony into the lead during the heat treatment process.