NEWBIE question:

I made a mix today of the following:
93.5% Lead
4.3% Antimony
2.1% Tin

I used WW for most of this. I was careful to sort the WW and then to monitor the temp to makesure that I was not going to melt Zn if any got into the mix.
I added one pound of Linotype that I melted down from strips.
I used a 60 / 40 solder to increase the tin percentage.

So I think my mix was clean, but I noticed the sprues were brittle. They would break into small pieces when I knocked them off. The bullets looked OK and I even took one and hammered it to make sure it was not brittle. It held together even after multiple hits.

Why is this? Too much tin? Antimony?

Should I be worried?

Any advice would be greatly appreciated.