

Can Someone Explain What Happened in this Picture
I made an ingot of a tin-bismuth alloy I mixed myself. As the ingot cooled these very tiny almost perfect spheres formed on the surface exposed to the air. I tried to removed one with tweezers but I couldn't. I've never seen this happen before as I've done castings numerous times including working with this alloy. The only thing I can think of is this is a gas bubble encased in metal that slowly crept above the surface. Pictures attached. Photo taken with my Pixel 10, tweezers for scale in the second image.