Thin skirts need high lead heat and a little bit of tin in the mix for better flowing.
If the heat is too high, the tin will burn off more quickly, however I run high heat when casting pure lead balls (or bullets for low vel. BP Sharps loads) and add 1/2" of tin per 20 pounds of really soft X-Ray room sheet lead from walls.
This helps with reducing the amount of dross produced by oxidation.
It also produces high quality, perfectly formed balls and bullets.