Zight Glass

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The most important quality of glass, aside from the type, is the way it is strengthened.

There are two glass strengthening processes: tempering and annealing. At Zight, we always temper our glass during the manufacturing process, but some companies prefer to anneal it instead.

In this article, we will explain the difference between a tempered glass disc and an annealed glass disc, as well as why we always use the former for our industrial glass products.


What Is Annealed Glass?

The process of annealing involves heating up a glass disc to a high temperature, usually around 850 – 900 degrees F. This temperature range is the point of stress relief for the glass. The glass is heated all the way through until the entire piece is the same temperature, at which point it is allowed to cool down. 

Once the annealing treatment process is complete, the glass can be cut or drilled without fear of activating an internal stress and causing the disc to shatter.

The problem with annealed glass is that if and when it does break, it breaks off in long, jagged pieces. These shards are extremely dangerous, even on a small scale as in a sight glass. They can easily cut skin open or impale sensitive body parts like an eyeball.

Annealed glass is also not the most suitable medium for industrial applications because it’s simply not that strong. It can’t withstand high amounts of pressure or heat, making it an inferior option for sight glass.

On the upside, however, annealing is a cheap process, which makes the glass cheap, as well.


What Is Tempered Glass?

When glass is tempered, it is heated up to an extremely high temperature – over 1,100 degrees F. After the heating process, the glass is cooled down in a matter of seconds using high pressure cooling air nozzles.

The result is that the tempered glass disc is now up to 4 times stronger than it was before tempering, making it more durable than annealed glass. It’s also safer; when tempered glass breaks, it breaks into tiny pieces with dulled edges that are unlikely to cause an injury.

Because there are more steps in the tempering process, tempered glass is more expensive than annealed glass. But the tradeoff is worth it. Tempering is more effective for industrial glass, like the kind used for sight glasses, than annealing.



Now you know why at Zight, we manufacture tempered borosilicate sight glass instead of annealed borosilicate sight glass. The tempering process yields a much stronger product than the other, which is what you want in a sight glass to observe corrosive liquids at a high temperature and pressure.