Glass Machining Expert Discribes Antique Scribed Edge

Glass expert had he opportunity to examine the edge of antique rolled, ground and polished window glass. This glass was removed from a greenhouse built in 1930. The window glass was sized by glass scribing and breaking it from a larger piece of glass. Above is a photomicrograph of the edge of the window. The glass cutting tool was run along the top of the glass. This created the "crush" damage indicated in the photomicrograph. This damage was driven downward to cause separation of the glass along a line. After sizing it appears as if the edges had been dressed by edge seaming. The crush damage is so severe that the seaming is all but invisible on the top of the glass pane. Also, the high degree of crush damage makes this piece of glass vulnerable to cracking. The glass pane performed well for over 80 years as long as it was held in its frame. After removal, it cracked due to thermal stresses. This photo illustrates that scribing and breaking creates two distinct edged. The "crush" edge and the final edge. The crush edge is where most later cracking occurs.

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Glass Expert Witness Discusses Window Failure

Glass failure analysis expert discusses interesting window failure, that of a piece of antique window glass. The window expert performed a failure analysis that determined the failure initiated at crush damage from the scribing operation, and it started as a bending crack. It then transformed into a "thermal crack". That is the crack front was driven by thermally generated stresses ( either residual stresses or temperature differential generated stresses). Termal cracks tend to wander, and after this crack initiatiated it wandered and almost created a semi-circle (crack growth is indicated by the arrows in the above photographs). This window failure is interesting because this glass was made in the 1930's, and it was ground and polished. Present flat glass (i.e. float glass) is solidified on liquid tin and does not require glass machining (i.e.grinding and polishing). Also, modern float glass processing is better controlled than the previous roll forming processes.

Glass Expert Discusses Termpered Glass Cookware

Modern Pyrex glass cookware is now made from tempered soda lime glass. As a result there is a potential that the tempered glass failure will create dangerous projectiles. In one case it was found that if the temper glass stresses are not symetrical, flying glass pieces will occur. In the present case, tempered glass cookware lids were caused to fail; the resulting failure was completely contained by the stainless rim. The rim performs two functions. It protects the vulnerable edges of the tempered glass lids from glass damage. In addition, it appears that it contains any glass particles that could become projectiles.

GLass Failure Analysis Expert Discusses Wire Glass

Safety glass expert has examined numerous broken wire glass doors and windows. In the past wire glass has been mistaken to be safety glass; however, it is just as dangerous as standard annealed window glass. The wire is bonded between two pieces of semi-soft glass at high temperature. When this "sandwich" cools to room temperature, stresses are created that weaken the glass. The only advantage of wire glass is that the wire will hold the glass in place during a fire, and this will help reduce the spreading rate of a fire. Above left is a photograph of the remainder of a wire glass door through which a person had pushed his hand through when pounding on the door. The glass failure analysis determined that this fracture pattern is similar to an equivalent annealed glass failure. The wire did not prevent this glass failure. Above right is a photograph of a wire strand that pulled out of the glass at the time of the failure. One can see that its bond to the glass was very poor. a wire glass expert would attest to the fact that wire glass is not safety glass.