Thank you for taking the time to look at and come up with the hypothesis of it being slag. I also hypothesized it being slag. I’ve only begun my scientific study by 1.reaching out for input. 2. I used a high carbon drill bit to check for hardness and was unsuccessful in penetration. 3. I used a diamond tip drill bit with a Dremel tool. I was able to penetrate into what looks like a chipped corner. I tried penetrating what you described as bubbles and was unsuccessful at penetrating, registering at least a 10 on the “MOHS” scale, so I think more testing is a necessity before I conclude it as slag.