An excerpt from the linked article, note the first sentence.
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Premium shells in North America will have up to 7% antimony to make the pellet hard. Premium Italian shot will have a maximum of only 3% yet are as hard if not harder than North American pellets. They use an old trick called tempering. We used to do this with our cast lead bullets for long distance metallic silhouette shooting. One drops the still soft lead cast bullet into a 10 gallon pail of water. This sudden lowering of temperature will change the molecular structure of the lead alloy. Hardness of this lead alloy will go up tenfold! With less antimony, the pellets have higher mass than 7% antimony pellets. Higher mass retains more energy at distance. Higher mass gives you better breaks at distance! I know now why so many Italians use 8.5s and break long distant target very well: higher mass of the 8.5 compared to NA 8s with many more pellets than 7.5, ensure a better kill. RC Cartridge uses tempered lead shot even for the Supernik. The Supernik is a nickel plated shot. The nickel plating does not make the pellet harder. It is there to increase the smoothness of the pellets thus reducing friction among each other and allowing an easier transition in the barrel.
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So the topic title itself is incorrect.
Antimony, having a lower specific gravity than Lead, when added to Lead in any significant amount lowers the specific gravity of the alloy but I don't think the higher density/mass of a 3% alloy over a 7% alloy will live up to the hype further down in the paragraph. I am also not aware of any research done on heat treating Lead or its alloys that shows significant increases of hardness by heat treating. The topic has been raised many, many times over the years in relation to cast bullet hardness but nothing ever came of it that I am aware of. If anyone can show me a scientific article that shows differently I will be glad to have my opinion remodeled.
Don T.