Removing slag/bubbles from Brass?

My brother and I were trying to cast some bulk brass yesterday for knife making and we are having a difficult time removing slag. We remove it, melt the brass until its liquid and pour it into a sand mold. We keep getting very large, deep, air bubbles! We've tried sand, steel, aluminum molds and we can not get rid of the voids no matter what. Any tricks?

See:

https://www.iforgeiron.com/topic/35189-so-i-want-to-smelt-some-brass/

https://ourpastimes.com/how-to-melt-brass-12291112.html

I'll keep looking for you ....