Lead and antimony or arsenic do not require tin to alloy (see attached chilled shot MSDS page for an example of a tin-free alloy of these metals). Tin makes it easier to alloy them both because of its lower melting point and because it helps the lead wet things, which is why it improves mold fill. There is actually a whole string of elements you can alloy with lead to harden it, and they cover quite a range of solubility. Thallium is the most soluble, while copper (used in hard babbit) and barium are the least soluble. In order of decreasing solubility, tin, antimony, and arsenic span the middle solubility range, with tin being the easiest to dissolve and arsenic the least soluble among the three.
Here's what some will find surprising:
all these alloys and their combinations quench harden in water. What is different is how long they can hold their hardness, post quench, with the most easily soluble elements holding it for the shortest time. Lead/tin quickly moves from the quench to a large crystal form, so it doesn't hold the added hardness for a useful length of time. Adding antimony prevents the coarse grain formation for a longer period, but you still need to use the bullets within weeks or months after they reach peak hardness to take advantage of the effect. Adding a little arsenic makes it take a very long time to lose hardness, and it seems to prevent a complete loss of heat treating hardness from occurring. That is important to the shelf life of chilled shot, obviously, but to that of other heat treated bullets as well.
Randy Garrett (Garrett Cartridges) used to put expiration dates on his cast bullet ammo, but stopped after he'd found his water-hardened lead/antimony/arsenic bullets were still holding BHN in the twenties after a decade. A similar result is obtained by Marshall Stanton at
Beartooth Bullets who finds that his bullets heat treated to BHN 22 drop only to BHN 21 after their hardness peak, which they then hold indefinitely.
This paper at the LASC is where I first got some of this information. It covers it in a good bit more detail.
Bustoff,
Quenched bullets typically need about two weeks for the crystallization process to reach full hardness, so your 24/hr - 8 month measuring looks like it skipped over the peak. Once the peak is passed, the hardness goes downhill. How fast depends on the alloy, as I described above. I've heard it recommended that you size quenched bullets within hours after casting so the surface hardness corruption caused by sizing is done before the final hardening really occurs, allowing that hardening to extend to the sized surface. I have not tested how truly that works out? But, obviously, if you size the bullets in something like the Lee sizers using mineral spirits as the lube, you can let the mineral spirits dry off, then oven heat-treat afterward and before filling the lube grooves.
If you have more tin than antimony, peak hardness is reported to go downhill faster than is the case when the reverse is true. The hardness two weeks after the quench is pretty easy to get to BHN 28 or more with controlled temperatures and quenching, but it's subsequent hardness loss goes slowly over time and appears to have a 1/e^t + k shape, dropping more quickly early on, and at a slower and slower rate over time; the constant, k, being the final hardness that is indefinite.