these are dust like particles of Antimony.They are easy to deal with just follow these steps.
Melt and clean your lead then add tin to the amount you want. Next spread the amount of antimony over the mix. Let sit for 5 min then put a layer of flux over the top. Let sit for 30 min then stir slowly for about ten min. The animony will dilute into the mix what is left is excess animony since the lead mix will ony hold so much And what ever trash you did not get out in the first cleaning. Contact Bill Ferguson at the antimonyman.com for prices