6 rookie mistakes of tempering

Glass tempering is a bit like cooking. You can have the latest and greatest equipment in your kitchen, but the food can still turn out bad. To be a great cook takes much more than just having the best equipment – it’s about understanding the processes. You have to know the ingredients and techniques as well as the equipment.

Automation does make things a bit easier. And let me tell you, the latest kitchen ovens do have some nifty features. To be honest, this is exactly what we’re trying to do with our tempering furnaces – to automate some of the processes in order to guarantee more consistent quality. Nowadays, tempering furnaces come equipped with more sensors, allowing the operator to pull more data and measurements. They also include more automation, which in turn minimizes operational mistakes.

In the long run, automation will make operating a furnace a bit easier. But still glass processors need to do many things correctly to reach high-quality tempered glass.

I have toured the world and have seen hundreds of glass processing factories. I have helped processors take the next step in their production.

Below, I have collected a list of common mistakes that happen in the tempering process.

1. Not learning the process basics

When a person starts a new project, he/she has to learn something new. Let me tell you, I’ve seen processors who have skipped the basics and ended up with quality problems before they even started to get the proper results. Not an ideal scenario, I would say.

To me, the tempering line is not a magic black box. To be honest, the behavior of glass is relatively simple to understand. If you don’t heat or cool the glass uniformly, it will bend. If you heat it too much, it will have waves.

These are the basics that should be clear to anyone operating a processing machine. Manufacturers often offer their clients the basic documentation and training packages that also clearly state the fundamental facts. But make sure you select a partner who can also train and instruct you beyond the basic documentation.

Take time to learn the process basics! This might take a while, but you’ll benefit from it later. For example, when you introduce new coatings, it is easy to look into the heating section and see how the glass behaves in the process. If you know the basics of heat transfer, you’ll find it easier to adjust the process yourself.

Don’t get me wrong, I’m happy to come around and also train you!

2. Neglecting the quality of pre-processing

The tempering line is usually among the biggest investments in the glass processing factory. This means that the primary attention is directed largely towards the tempering line. However, if we look at the end product’s quality, as well as the production yield, we can conclude that the pre-processing – including the cutting, grinding, washing and more – is just as important as the tempering.

The pre-processing quality has a direct connection to the tempering line’s yield when it comes to breakage and production quality. Sometimes, I see processing plants that have acquired a top-rank tempering line but have left the pre-processing systems untouched and out of date.

Poor cutting or edge work results in more breakage in the tempering line. Conversely, if the edge work is good, the glass can run at a lower temperature in the heating section, which leads to better optical quality and less breakage.

For example, diamond-polished thick glass can be processed with about 20 degrees lower temperature than normal seamed glass. This already has a major impact on quality and, of course, glass breakage.

3. Maximizing loading efficiency

When someone makes a new tempering line investment, very rarely does the tempering line becomes the bottleneck of the factory. Still, many obsess over the highest possible loading efficiency by putting both small and large glass sheets into the same load – long side leading and short side leading in the same pattern but with a different orientation.

Mixing up the loading patterns with different glass orientation – short versus long edge leading – will result in different optical quality. If these glasses are installed in the same building, you’ll see a different optical reflection and, most likely as a result, end up with a reclamation. This is a common rookie mistake. So make sure to avoid it.

Don’t get me wrong, maximizing loading efficiency is a good thing, and it will increase your energy efficiency as well. The point is not to compromise the quality.

4. Having a heating time that is too long

The percentage of glass breakage has been a tempering line operator’s most important KPI for a long time. For this reason, operators naturally do everything they can to decrease the waste.

One way to prevent breakage is to increase the heating time. This will, unfortunately, have a negative impact on the glass quality. The higher the glass temperature is, the worse the quality.

Note that quite often operators think that poor cutting or grinding quality comes from having a temperature that is too low in the tempering process. In this case, the operator is correcting the wrong problem and, as a result, has poor quality glass with poor edge work.

5. Running thick and thin glass in a similar way

Avoid putting a thick piece of glass into a very hot furnace. Trust me, you don’t want to hear the “Boom!” That is the noise of a thick glass breaking in the furnace. In this case, the breakage is caused by heat shock. As a result, the furnace needs to be cooled, and the broken glass removed. As a kind reminder, this process takes about two shifts to complete.

Also, when running a thick glass of around 10–19 mm, the quenching pressure the operators are applying is often too low. A common rule is that low pressure prevents the glass from breaking in the chiller. Most often, the case is almost the opposite. So don’t use pressure that is too low in the production of thick glass.

6. Removing the ghost the wrong way

Many operators try to fix the white haze issue by operating the line to run the glass “sad” in the furnace. In other words, the corners of the glass touch the rollers but the center does not.

Yes, this does eliminate the white haze, also known as “ghosting.” But running thick glass “sad” will harm the rollers. The ghosting can chip off some glass from the sides or cause markings to the rollers. As a result, the following loads will have problems and the thin glasses will not stay stable.

Here’s my rule of thumb: don’t exaggerate the sadness of the glass.


It seems that as we become more comfortable with our work, the more mistakes we make. And the more you think you know something, the more likely it is that your choices lead you down the wrong path. That’s why even us experts should always keep the basics in mind.

If you have some unanswered questions about tempering, I recommend downloading our free eBook. It will give you more basics – and then some.

The Glass Tempering Handbook Part 1

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About the author

Sami Kelin

Sami is Director of Glaston’s Flat Tempering Business Line. He enjoys the tough challenges at work to keep up with the rapidly changing glass industry and emerging technologies. He believes his lifelong experiences have given him an open-mindedness that is impossible to learn from an armchair. So far, he has traveled to supervise installations and work in more than 40 countries. Sami aims to ensure customer success by offering quality products and technology that provide them with the highest profitability. A milestone in his personal life was when he made it to the soccer world championship quarterfinals in 2004 – swamp soccer, that is.