Black metallic spots on fused glass backs

what is causing ugly black metallic spots on the back side of fused glass.  Been teaching this for 10 years and never saw it before.  Tried everything.  Only thing different is using Wissmach glass (ran out of Spectrum).   Maybe something new added to our glue?  Alene's quick dry.  Anyone else having a problem?  It has happened about 5-6 times.  I could post a photo, but don't see a place to do it.  mcadele1@hotmail.com

black spots

I too have black spots appearing after fusing.   I'm wondering if it's Wissmach glass as well.  Just switched to Wissmach never seen this using Spectrum.  Anyone out there have any idea what's up?

Black spots

A new case of black spots.  It is actually between the layers of glass and this time came to the surface through a bubble.  Tried to cover it with another piece of opaque glass.  It rebubbled and I drilled it out the black stuff with my Dremel., filled the hole with white frit and covered with more opaque glass.  Still not pretty and of course the back is still black.  No answers to this black stuff, yet.

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