Ok guys,
The light finally came on & I believe I’ve figured it out. After a little more research and another small experiment, it became clear that unseen voids in bullets have a direct affect on measurements of specific gravity (SG). Archimedes’ principal, which is the bases for SG measurements, states that “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object”. Therefore, since a bullet will displace the same volume of water regardless of the size of internal voids, SG measurements are inversely proportional to the size of the void. I.e. when using the air versus water weight technique to measure SG, the SG of a bullet with a large void will be lower than the SG of an identical bullet with a small void. Hence, the large-void bullet measurement will imply a smaller lead/tin ratio. E.g. using a 530gr bullet cast with 20:1 alloy, if another bullet from the same batch weighs 529gr due to a 1.0gr void, the SG of the 529gr bullet will incorrectly suggest the alloy ratio is 18.6:1. Therefore, when measuring SG to determine the lead/tin ratio of an alloy, it’s wise to use bullets that fall within the upper end of the weight spread, indicating minimum voids.
So, given the above, I reviewed my data from the original experiments that incorrectly suggested the lead/tin ratio of the alloy was increasing as bullets were being cast. I found that when starting the casting sessions all initial measurements of the SG were based on bullets that were slightly lighter weight than those cast later in the session. So it’s a good bet the initial bullets had slightly larger voids, which diminished as the mould temperature increased. Although I was using a hot plate to keep the mould hot between sessions, apparently it was not hot enough.
As to why the weight spread diminished from session to session, I have no clue. It may be the result of subconsciously improving my casting and/or measuring techniques.
BTW, I now know the original experiments and my assumption were incorrect. And I’m convinced that lead/tin alloys are a homogeneous solution. The tin will not stratify in the alloy solution and neither will the lead/tin ratio change due to fluxing or removing the dross. Therefore I plan to remove the original article from my website.
Wayne
Last edited by texasmac; 03/24/17.
