My alloy control method is to go on bullet weight. My ingots are all labelled with the average weight of the last bullets I cast from them. The leftovers from each casting session become labelled ingots. Before casting, I mix as necessary to get the required weight. For my #U321297HP mould, which is the subject of this post, my objective is a bullet as close as possible to 170 grains.
In the past I have noticed that my bullets tend to be just a whisker heavier than indicated on the ingots, which I have put down to some minor loss of tin and/or antimony from the alloy with each melt. This does not bother me, and I cover for it by calculating for a slightly lighter bullet.
My latest project has been to brew up about 10 lbs of alloy equating roughly to Lyman #2. Yesterday I set out to use it up. Bullet was #U321297HP. This time I actually measured the bullet weight difference between sequential casts.
Setup is a 10 lb + pot, electric stove top, Lyman dipper, and SC Lyman mould.
I did the cast in two sessions. After the first session, I culled some bullets on appearance. Then I weighed and sorted the survivors into 0.1 gn intervals, culling the lights. Keepers (175) were 169.7 to 170.2 gns, average weight 169.97 gns.
After returning the culls to the pot, remelting and fluxing, I ran a second cast and applied the same culling criteria. Keepers (123) were 169.9 gns to 170.3 gns, average 170.14 gns.
Conveniently, a 1% difference in bullet weight for a 170 gn bullet is achieved by about a 1% variation in SN/Sb content. For the 170 gn bullet therefore, the difference of 0.17 gns represents about 0.17% combined Sn & Sb out of a total of 10% or so. This is a pretty small deal in relative terms.
You are only as good as your library.
