http://www.alumaxbath.com/tech/tgp.htm
http://www.cwct.co.uk/facets/pack08/text06.htm#nis

My uncle worked in the glass industry for many years. He suggested this theory. Nickel Sulfide, present during production, can cause sponteanous failure years in the future. Long shot? You bet, but possible. Found alot of info on google addressing this possibility. cheers
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Dan