Volume 37, Issue 7, July 2002


Defective Glass
        Not All Glass Is Perfect Glass
by Dez Farnady

The float manufacturers have established quality standards for float glass, but I have never seen them in writing. I have relied on American Society for Testing and Materials (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. Now that is a brave statement considering how much glass comes off just one float line, not to mention all of them.

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 are damn near getting it.

ASTM Guidelines 
ASTM 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—even before my time. Back then glass quality was not what it is today. We have kept ASTM as the guideline in order to protect the business 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. Size of the flaw and its location within the sheet of 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, should you need it. 

In all of the years I have looked at glass, there have been only a few occasions when raw factory stock was rejected. When problems do occur the factories take instant responsibility for it, and 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 result of what we do to it. 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 in these cases—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 lammy or roller marks or heat stain on your tempered are not glass defects but results of careless handling or shoddy shop practices and poor quality controls. 

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. For more physics there are rainbows called "Newton rings" in insulating glass resulting from light refraction on surfaces too flat and too parallel or in a word because the glass is too good. There may be visible haze on coated products when the light hits it just right and you see the coating and the wire in the wire glass is not straight.

So, while the quality of the float is outstanding, all glass is not without flaw. The 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 product. 


DEZMAN 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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