Glass Discoloration and Cracking

Our customer obtained large glass parts from China for an outside application.  Within a few months, the glass turned yellow and sometimes cracked.  A borosilicate glass composition had been specified which was not UV-sensitive.  Freshly fractured surfaces of the specified and the yellowed glass were quantitatively analyzed by XPS.  The supplied glass had only one-ninth the boron concentration of the specified glass.  The atomic concentration of sodium was 3 times higher, that of barium was doubled, and there was additional calcium in the Chinese-supplied glass.  The yellowed glass also had potassium, chlorine, and zinc concentrations not found in the clear glass.  The substituted glass did not have a small concentration of fluorine found in the specified glass and carbon found in it had carbon-oxygen bonding not found in the specified glass.  The specified borosilicate glass was found by TMA to have a lower thermal expansion than the substituted glass, which prevented the cracking found sometimes in the Chinese substituted glass parts.  The cracking was due to a mismatch in the thermal expansion of the Chinese substituted glass with the structural materials that held the glass in place.