Lead, tin, and antimony (plus others), are impurities in zinc that cause intergranular corrosion that will eventually cause the castings to break apart.jscarmozza wrote: ↑Tue Mar 30, 2021 7:52 amI've been making some pot metal castings and began to run low on metal, so I added a zinc ingot and some thin aluminum trimmings to the pot, melted it and continued to pour with good results. When I ran low again I did the same thing, but threw in a piece of 50/50 lead tin solder this time...that changed things! The mold filled beautifully, the casting finish went from bright shiny silver to frosted whitish silver, BUT, the casting was weak and broke into many pieces when removed from the mold. You could snap the casting easily just like a piece of chalk. Obviously it was the solder that made the difference, do you think I could dilute the problem by adding more zinc and aluminum to the pot?
Mantua model trains with the die cast zinc boilers were known to break apart over the years by just sitting. This is the reason "pot metal" got a bad name. Some carburetors also suffered from this.
Toss everything that you have added the lead and tin to, including the pot you melted them in, and start over.