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Chemical damage is the most common type of damage that affects sight glasses, even more than physical elements like sealing stress and projectiles.

Considering the term, “chemical,” you may be inclined to think that you have nothing to worry about as long as you don’t expose the sight glass or level glass to any toxic substances. Surprisingly, though, the chemical reaction you need to worry about most is water against glass, which, given the right factors, is highly corrosive.

In this article, we’ll explain how chemical damage commonly occurs in sight glasses through water, and what best practices you can implement to prevent or slow the process of corrosion.


How Do Chemicals Damage Sight Glass?

Before we get into the science behind how and when sight glass is susceptible to chemical attack, we need to establish an important fact: what kind of glass is sight glass typically made of?

These days, the most common material used for sight glass is silicate. It’s made up of mostly silicon and oxygen atoms.

Interestingly, the silicon atoms in silicate glass bond easily with other oxygen atoms, not just the ones present in the glass material. This is where the first threat of chemical attack from water comes in.


Water is full of hydroxyls, which is an ion made up of one hydrogen atom and one oxygen atom. Hydroxyls attack glass. This means that when the silicon atoms in the silicate glass bond to the oxygen atoms in the hydroxyls, the hydroxyls are attracted to the surface of the glass and begin to attack it. This is the process of corrosion.

This type of chemical reaction can also occur between glass and other aqueous liquids (meaning liquids in which water is the solvent), not just water. But water is a typical liquid that observers use sight glasses to observe, which is why we make special mention of it here.


Heat And Pressure

Water, as well as other chemicals, can be damaging in more ways than one. For example, high temperatures speed up the effects of corrosion. 

At normal, low temperatures, water does not pose any danger to glass. But when it is superheated and held to that high temperature with pressure, it becomes aggressive and attacks the glass. 

The damage that water causes in this aggressive state occurs over a relatively short period of time, just a few months.


Mechanical Stress

Mechanical stress acts as a catalyst of chemical damage on glass by aqueous liquids. Specifically, the mechanical stress that affects a sight glass or level glass is sealing stress, which we discuss at length in our article, Best Practices In Preventing Physical Damage To A Sight Glass.

Sealing stress is the stress that the gasket setting places on the edge of the sight glass or level glass. It’s significantly higher than the stress placed on the center of the glass, which means that the edge is more likely to crack first. 

The edge of a sight glass or level glass is also more likely to corrode when exposed to water or other aqueous substances. The corrosion weakens the edge, making a split or break even more likely to occur.

This factor also applies to fused glass, where glass is fused inside of a ring of metal. The metal compresses the glass, thereby increasing the tensile strength of the center of the glass. This compression, however, results in tensile stress on the edge of the glass. If this part of the glass is exposed to an aqueous liquid and corrodes, it becomes even more vulnerable to cracking and may even break off a shard in the process.


3 Best Practices For Preventing Chemical Damage To A Sight Glass

Now that we know the most common sources of chemical damage to a sight glass or level glass, we can address the best practices you should be implementing to prevent them.

We should point out, though, that it may be impossible to completely prevent corrosion. Water or other aqueous substances will get on the glass and there is no way to prevent that. But these three strategies should help slow, if not 100 percent prevent, this type of chemical damage.

1.   Coat The Sight Glass To Protect It From Liquid

One method of protecting your sight glass or level glass from the corrosive effects of aqueous liquids is to coat it. Teflon is a commonly used coating and will prevent the silicon atoms in the glass from bonding with the oxygen atoms in the processing liquid.

2.   Use Shielding To Protect The Sight Glass From Liquid

Shielding is a form of protecting the main piece of sight glass or level glass by placing another piece of glass in front of it. That way, as the “shield” corrodes, the main piece of sight glass remains intact.

3.   Use A Sight Glass Other Than Fused Glass

If possible, avoid using fused glass for your sight glass, level glass, or shielding. As we explained earlier, fused glass in a metal ring puts too much stress on the edges of the glass and makes it more likely to cause a significant break.

We recommend using a tempered sight glass, which is not fused and is mechanically strengthened through the process of tempering.



It’s important to develop maintenance strategies that will ensure the highest level of safety and functionality for your level gauge, whether it’s a reflex glass, transparent glass, or a regular sight glass.

In the same vein, it’s also important to use high quality sight glass. At Zight, our borosilicate sight glass, tempered sight glass, reflex glass, and transparent glass are manufactured according to high international standards for our industry: DIN 7080 and DIN 7081. These norms guarantee quality in chemical resistance, a key feature that will increase the longevity of your glass in response to chemical damage.