Volume 38, Issue 8, August 2003


Defective Glass
Who Do We Blame When the Glass Isn't Perfect?
by Dez Farnady

Float glass manufacturers have established quality standards for float glass, but I have never seen them in writing. I have relied on ASTM flat glass specifications and defect tolerance tables for some 25 years. My take on the float business has always been that if the float manufacturers used the ASTM tolerances for allowable defects they could not sell any glass. That is a brave statement considering how much glass comes off any one float line, not to mention all of them.

Virtual Perfection
The point is that float glass quality is so good and the manufacturing standards are so high that we can’t even see the minuscule flaws, if there are any. We have come to expect virtual perfection from today’s flat glass products, and in my opinion we very nearly get it.

ASTM is the American Society for Testing and Materials. It is an independent testing agency that publishes quality standards for the industry. ASTM Designation: C 1036 – 91 for Standard Specification for Flat Glass is current and has been approved for use by agencies of the Department of Defense to replace Federal Specification DD-G-451 (d), the former bible of the glass industry. Still, C 1036 is probably only a leftover, in some form or part, from the days of crystal and polished plate. Back then glass quality was not what it is today. We have kept ASTM as the guideline in order to protect our businesses and ourselves from having to guarantee perfection. It is a great tool and a generous standard, and that is why no one wants to mess with it. Seeds, reems, scratches, stones, gaseous inclusions of all sizes and shapes are allowed and are acceptable within the ASTM guidelines. The size of flaws and their locations within the glass, the thickness of the glass and the viewing distance are the determining factors in identifying acceptable glass. (Table 3 Glass Quality q3, “Glazing Select Quality,” is on page 5 of the guidelines, should you need it.)

In all of the years I have looked at glass there have been very few occasions when raw factory stock was rejected. When problems occur factories take instant responsibility, for it is usually a very obvious one that got missed in the final quality-control check.

Inherent Characteristics 
Most of what we consider glass defects are the results of what we have done with the glass. There is a litany that can be recited and, surprise, surprise, it all results from glass handling. Shipping rubs and handling scratches are not glass defects. It is not the glass that is bad, but rather it is what has been done to it. Dirt inside an insulating unit is not a glass defect, and don’t tell me to look at the ASTM spec. Crap is crap, so replace it. Fingerprints inside the unit or foreign objects in the lami or roller marks or heat stain on your tempered are not defects, but results of careless handling, shoddy shop practices and poor quality controls. 

Of course, there are some other things that just come with the product. I like to call them “inherent characteristics.” You may not like them, but they are not defects. The most famous is the tempered roller ripple. This is a surface distortion that may not even be measurable, yet it shows up when the light hits it just right. Then there is the tempered strain pattern that shows up under polarized light. There are multiple reflections in insulating glass. There are rainbows and distortions in multi-layered laminates. If you want to know why, just check out your high school physics book. 

So, while the quality of the float is outstanding, all glass is not without flaw. Our job is to understand and be able to distinguish between the limitations of the product as opposed to damage resulting from mishandling. Understanding the limitations may even help to prevent the accusation of having provided defective products. 


Dez Farnady Dez Farnady serves as general manager of Royalite Manufacturing Inc., a skylight manufacturer in San Carlos, Calif. His column appears monthly.


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