What is the cause of vitrified investmest stuck to castings?

Hey All I’ve been having very persistent vitrified investment plaster remaining on some castings after devestment. I say vitrified as it has a glass like quality and it is quite stubborn in terms of removal. Has anyone determined a cause and/or a solution for this problem?

I don’t cast, but just wondering, could it be flux? That being the case, soak it in water for a while and see if it goes away…Rob

It doesn’t come off with a thorough pickling so a water soak is unlikely to work. Thanks though. Ken

What kind of investment is it? Regular plaster-based investment isn’t supposed to vitrify, but I suppose it’s possible if it’s severely overheated. Are you casting platinum or stainless? These things require special investment, or ceramic shell - which does vitrify and is a big pain to get off. Molten salt is supposed to work for getting it off, but I’ve never tried it.

It’s regular Satin Cast investment. I’m just casting silver and bronze.