A Detailed Description of my Venn Diagram of Geekery
I have seen some of that on my castings also. Not sure what it is. What are you using to heat up the metal? I didn't realize you had gotten that far.
I use a portable electric oven burner. Apparently the problem may have been from not using enough talc and/or clamping too hard. I've made some adjustments and it is working great now.I've cast about six figures. I'll try to get a pic up soon.