From my first blog, Mr. George Usinowicz asked: “While at PPG, did you know my uncle Red Rhenholz Mutzberg?

Sorry George, but I didn’t have the pleasure. Of course, keep in mind that over my 40 years with PPG the total number of employees ranged between 30,000 and perhaps 38 0r 39,000 – roughly as many as a fair size city.

From my third blog, Mr. Jeff Nixon asked: “Is there a known value for the amount of stress that will develop with a given thermal gradient? I’ve heard that it can be 50 psi per degree Fahrenheit, but no citation was given. If true, then could not a reasonably conceived thermal gradient in the field cause thermal stress breakage in heat strengthened glass?”

To the first part of your question, 50 psi per degree Fahrenheit is a reasonable value to use when calculating the stress caused by a thermal gradient. The value is a function of the modulus of elasticity and the thermal expansion coefficient of soda lime float glass.

To the second part of your question, it is conceivable that a situation could arise where an extreme thermal gradient, combined with heat strengthened glass at the low end of the specified range could result in breakage. While I personally have never seen such an occurrence, a colleague with impeccable credentials says that he has seen it happen. In fairness, he also believes that there was significant edge damage involved. Also, typical heat strengthened glass almost always runs in the mid to upper range of the specified range.

I would also like to thank the several readers who submitted compliments. It is appreciated.

OK, in my next blog we’ll bet back to business!

As someone who has spent the better part of half a century in the glass business, I am personally delighted to see lots of glass being used. After all, I am a PPG retiree who looks forward to his pension check and lots of glass being used is a good thing! Keep on using it, but use it with good judgment.

During the last several years of my time with PPG, I had several customers ask my support in cautioning against the use of excessively large insulating glass units. They were concerned about the long-term performance of such units. Their concerns had merit and, in fact, many insulating glass fabricators have long had restrictions on the size of units that they would warrant. There are a number of factors that impact the long-term performance of excessively large insulating glass units. Some occur during manufacture and some in the field. As a result of my customer’s concerns, as well as my own experience, I authored a technical document for the PPG web-based technical library addressing the subject. The title is “Large Insulating Glass Units – Design Considerations.” My intent is not to recreate that document here, as it can be accessed by CLICKING HERE. However, I will offer the following for your consideration.

Among issues to consider:

· Glass damage due to difficulty in handling large, heavy insulating glass units;

· Ruptures in seals and spacer welds due to the flexibility of large spacers;

· Uniformity and press-out of the primary seal;

· Excessive deflection under wind-load (remember that just because heat strengthened and tempered glass are strong enough to resist the wind-load, they will deflect the same as annealed glass); and

· Increased risk of thermal stress breakage.

In addition, large units have economic consequences. Metal glazing members must be stronger, i.e. thicker, to eliminate excessive deflection due to the weight of the unit and avoid potential damaging contact to adjacent units. While setting blocks can be moved closer to the ends of the horizontals to reduce deflection, they should never be closer than 6 inches to the ends of the horizontal. And, horizontal sill member deflection can result in water not draining properly, which then leads to premature failure of the insulating glass units.

Like I said, I love to see lots of glass used and it’s not my intent to discourage anyone from using it. However, it is important to think things through and consider the “unintended consequences” before simply specifying excessively large units.

Remember our discussion concerning temperature gradients and the fact that … ah nuts, let me just repeat it here: “A general ‘rule of thumb’ is that a gradient of 40° F is sufficient to result in a probability of glass breakage of 8 lites per thousand. With the popular high performance tinted and coated glass products in use today, temperature gradients of 60° or more are easily possible.”

We all know about the popularity of today’s high performance low-E coatings and high performance tinted glass. And, of course, they are often used in combination to create even better performance. They are great products that save energy, improve occupant comfort and offer pleasing aesthetics. They also absorb and/or reflect a high percentage of the incident solar energy. When they absorb the energy, they get hot; and when they get hot the scene is set for very high thermal stress to be created in the glass and for breakage to occur. In order to minimize thermal stress breakage, the specified glass type must be sufficiently strong.

Allow me to use an analogy based on cars. If you want a sedate family sedan with a gas efficient 4 or 6 cylinder engine, then typical “S” rated tires are sufficient. If you step up to a sport sedan with a super-charged V-6, or performance V-8, then you will need a “U” or “H” rated tire. If you really have a “mid-life crisis” and decide you need a real “barn-burner” – say a Dodge Challenger, or my favorite – the Chevy Camaro with the 426 HP engine – then you’ll be looking at “V” or “W” rated tires. The ratings relate to the ability of the tire to provide stability and durability to be able to safely run at higher speeds.

Now, back to glass: If clear glass or a lightly tinted glass (the family sedan) is used, then annealed glass is likely to be sufficient to resist thermal stress in typical design conditions; but if high performance tinted glass, or coated glass or a combination of both (the “barn-burner”) is used, then it is very likely that strengthened glass will be required to resist the thermal stress.

I have been and continue to be amazed at how often expensive, high performing glass products are specified and used, but fabricated with annealed glass. Believe me, I know the reasons why it’s done, but this is truly a case of “pay now, or pay a lot more later.” The additional cost of strengthening the glass is but a fraction of the total cost of the wall system and is cheap insurance when compared to the cost of replacing glass after the building is completed and occupied.

We all know the old saying: “The right tool for the right job.” Well, use the right glass for the right application!

A final word: Recall that in my previous blog I pointed out that heat strengthened glass is approximately twice as strong as annealed – plenty strong enough to resist the thermal stress generated in any practical application. And, if it does break, for whatever reason, properly heat strengthened glass is much more likely to remain in the opening than tempered glass. Also, properly heat strengthened glass is not susceptible to spontaneous breakage. It is, in my opinion, a better option than tempered glass for all but safety glazing applications, or where tempered glass is specifically required by code.

Enough said. We’ll go on to something new next time.

Annealed Glass: The last step in the production of float glass, before it is cut into stock sheets or cut-sizes, is a long trip down the annealing lehr, where the hot glass ribbon undergoes slow controlled cooling to ensure that it has minimal residual stress.

Heat Strengthened Glass: Glass that has been cut-to-size is heated to 1200 – 1300° F. and then subjected to controlled cooling (quenching) with air to create compression layers on both surfaces and the edges, with a balancing center tension zone. To comply with ASTM C 1048, heat strengthened glass must have a surface compression of 3,500 psi minimum and 7,500 psi maximum. It is generally accepted that heat strengthened glass is approximately twice as strong as annealed glass.

Tempered Glass: Glass that has been cut-to-size is heated to 1200 – 1300° F. and then subjected to controlled cooling (quenching) with air to create compression layers on both surfaces and the edges, with a balancing center tension zone. To comply with ASTM C 1048, tempered glass must either have a minimum surface compression of 10,000 psi, or a minimum edge compression of 9,700 psi. It is generally accepted that tempered glass is approximately four times as strong as annealed glass.

Note that heat strengthened and tempered glass are produced on the same equipment. Whether the glass is heat strengthened or tempered is dependent on how quickly it is cooled. Tempered glass is cooled more quickly by using higher volumes of air in the quench process. It is this more rapid cooling that generates the higher compression stresses and corresponding additional strength.

Some details about annealed, heat strengthened and tempered glass

Annealed Glass
· Very flat and reflected objects will be relatively undistorted if properly glazed.
· Can be cut into shapes, drilled, notched, edged, etc.
· When subjected to a 3 s uniform wind load, a stress of approximately 3,380 psi will yield a probability of glass breakage of 8 per 1000.
· For clean-cut glass edges, an edge stress of approximately 2,400 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, the break pattern will be such that the glass will typically remain in the opening.
· It is not acceptable for safety glazing applications.

Heat Strengthened Glass
· Will cause reflected objects to have some level of distortion due to roller-wave and bow/warp resulting from the manufacturing process.
· Can not be cut, drilled, notched and edgework should be done before strengthening.
· Under some lighting conditions, the strain pattern induced by the air quench may be visible.
· When subjected to a 3 s uniform wind load, a stress of approximately 6,750 psi will yield a probability of glass breakage of 8 per 1000.
· An edge stress (based on seamed edges) of approximately 5,300 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, the break pattern of properly heat strengthened glass will be similar to that of annealed glass and will typically remain in the opening.
· It is not acceptable for safety glazing applications.

Tempered Glass
· Will cause reflected objects to have some level of distortion due to roller-wave and bow/warp resulting from the manufacturing process.
· Can not be cut, drilled, notched and edgework should be done before strengthening.
· Under some lighting conditions, the strain pattern induced by the air quench may be visible.
· When subjected to a 3 s uniform wind load, a stress of approximately 13,500 psi will yield a probability of glass breakage of 8 per 1000.
· An edge stress (based on seamed edges) of approximately 10,600 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, tempered glass will fragment into numerous roughly cubicle pieces and may evacuate the opening.
· Is acceptable for safety glazing applications, provided that it complies with applicable particle size requirements.

In my next entry, we will get back to our discussion on thermal stress.

Thermal stress in glass (or any material for that matter) is caused by temperature gradients. The center of the glass is heated by the sun and expands, while the edges that are captured in the glazing system remain relatively cold and do not expand as much as the center. The result is stress, typically at or near the edge of the glass, which happens to be the weakest part of a glass lite. The absolute temperatures are not the issue; it is the gradient – the difference between the edge and the center of the glass – that causes the stress that leads to breakage. A general “rule of thumb” is that a gradient of 40° F is sufficient to result in a probability of glass breakage of 8 lites per thousand. With the popular high performance tinted and coated glass products in use today, temperature gradients of 60° or more are easily possible.

While the sun and the solar absorption of the glass are the main drivers creating the thermal stress, there are a number of design factors that exacerbate the situation, including:

•  Outdoor shading caused by the glazing system, overhangs, fins, adjacent buildings, etc.;
• Interior shading devices such as drapes and Venetian blinds;
• Interior structural glazing pockets;
•  Heating registers;
• Type of glazing system, i.e., thin, thick, massive;
• Color of the glazing stops; and
• Minimum outdoor design temperature

Some design conditions, such as solar intensity and design temperature cannot be controlled; others, such as the glazing system and overhangs are controllable. It is also important to anticipate design elements that can and very often will change during the life of the building. The most common change that occurs is in the type of interior shades or, most critical, the addition of interior shades where none previously were installed. When a building has been happily performing without thermal stress breakage suddenly begins to experience such breakage, it is very often caused by a change in or the addition of interior shades.

Procedures are available for performing thermal stress analysis for glass products and predicting the probability of glass breakage. The procedures require that someone knowledgeable about the design of the building and glazing system furnish the appropriate design conditions. Remember the old adage: “garbage in – garbage out.” If the designer says there will be no outdoor shading, and there is; or that there will be no interior shades, and there are, the end result may very well be thermal stress breakage when the thermal stress analysis says it should not occur.

More to follow on this important subject.

Now, as much as it hurts, I’m willing to accept that you may actually not want an unending stream of “Tales from Michael.” So, I encourage you, dear readers, to submit your questions and suggestions through the comments area below. Obviously, I won’t be able to deal with every question or topic, but it sure would work best if I write about what interests you, rather than just what interests me. Most of the time, I’m sure that it will be of interest to both of us.

Let me fire you up and “prime the pump,” so to speak. I have often been puzzled and even dismayed when reviewing section 8800 of the project specification. Often, out-dated specifications and products are referenced, or contradictory statements are included. Sometimes, “wish-list” requirements are included that would be great to have if they were possible; unfortunately, when you ask for the impossible, it only complicates the process at best, and can lead to chaos at worst.

I wonder why I’ve seen these issues in the specification. Perhaps you can answer this mystery for me. I am waiting for your comments!

I retired from PPG Industries in July 2007 after 40 years of service in the glass business. During those 40 wonderful years, I enjoyed assignments in many areas of the business, including Glass Research, Technical Services, Marketing, Sales, Commercial Construction Glazing and Manufacturing. Hmm, seems in retrospect like PPG kept trying to find a place for me where they could maximize my contribution … or, perhaps, minimize the damage I might cause. Either way, after 40 years in the glass business, I have had many experiences, both good and bad. As you might imagine, I have many “war stories” to share.

On a personal note, I am married to a lovely lady named Janice (for 33 years and counting) and have three children – Rachel, Joshua, and Alicia. The family also includes two cats (Dante and Lord LT Valentino Omega or Megs for short), both of whom have me well trained. Those of you who have cat friends know what I mean; the rest of you don’t know what you are missing! Our home is in Buffalo Township, a wonderful rural community located about 25 miles northeast of Pittsburgh, PA. It has been and will continue to be my 3-acre slice of heaven. My hobbies include golf (I am a double digit handicap hacker), reading, computers, and trying to outsmart my cats.

My blogs will emanate from my basement office, using one of the four computers (both Macintosh and Windows machines) at my disposal. Actually, it will be from whatever computer is not being used by other members of the family. Hopefully, many of you will find my blog to be somewhat entertaining, and some may also find it helpful or thought provoking.

Please feel free to submit questions to my email address: mbitterice@glass.com. If I can, I will answer your question; if I can’t answer it myself, perhaps I’ll be able to “point you in the right direction;” and, I will tell you if I don’t have a clue.

Well, that’s it about me. In about two weeks we will discuss the most valuable asset in any business and how you might grow and nourish that asset.

See you soon!