Why does fireplace glass break? Why does wood stove glass break?

We will start with the oldest kind of wood stove glass, Mica Glass.  Sometimes called Isinglass, Mica is a very, very thin product.  Thin-like paper.  It kind of looks like real thin plexiglass.  Mica glass is made from naturally occurring minerals, mined in India, Africa, and China, and pressed into Sheets.  This mineral product is very heat resistant – it can handle over 1800 degrees f.    

Mica is used in many products where heat can be an issue.  As far as wood stove use goes, it began to be widely used in the late 1800s to early 1900s as a sight glass to view the fire in pot-belly stoves.  Because of the expense, and type of use of the stoves at the time, most of the sight glass pieces were very small.   The Mica glass is thin enough to bend to the curvature of the potbelly stove also.

As with most things though.  Mica’s long-lasting exposure to heat will cause the sheets to break down.  Becoming more and more brittle over time.   After some time, you can push your finger right through the glass.  Mica is not used much in the newer wood stoves or fireplaces. Most all Mica used now is for replacement in the older stoves.

woodstove mica glass

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Next up is Tempered Glass.  Now Tempered Glass is a little different.   Made by taking regular Annealed glass (Like your window glass) and rapidly heating it, then rapidly cooling it “Tempers” or hardens the glass.  This locks the molecules in place, hardening the glass, increasing its thermal break rate up to about 500 degrees.  But once that temperature is reached, lookout. The glass will explode.  Sending pieces flying. 

Because the glass is so tense after being tempered, when it breaks it releases that tension, popping, and breaking into a thousand pieces.  So there you have it.  The biggest cause for tempered glass breakage is due to overexposure to heat. 

This is why tempered glass should never, ever be used in wood stoves.  Woodstove generates well over 1000 degrees.  The intense heat will cause the glass to break, sending glass flying and the fire exposed. 

tempered glass sheet

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Last but not least is Ceramic glass.  Ceramic glass is actually not glass at all, it is a transparent Ceramic product.  First made by Corning, it is used in many applications for food prepping products.  Ceramic glass is sometimes referred to as Pyro-Ceramic, Neo-Ceramic, or Robax glass.  These are just different brand names for the same type of product.

Used most widely now for Woodstoves and Fireplaces, Ceramic glass can withstand up to about 1400 Degrees.  A chemical called Borrite keeps the glass from expanding and contracting when heating up.  Allowing it to handle these high temperatures. This chemical also gives the glass its Amber tint. 

Ceramic glass will also break down with time and exposure to harsh chemicals.  Coal-burning will cause the glass to pit and fail rapidly.  Cleaning with oven cleaner will damage the surface of the glass and cause pitting and glass failure over time.  But the most coming cause of Ceramic glass breakage is installing the glass too tightly. 

Ceramic glass is made to withstand high temperatures but what people don’t think about when installing is the expansion of the metal woodstove itself.  People tend to install the glass snugly to get a tight seal.  But in doing so they are pinning the glass in place.  The stove then heats, the metal expands, the glass does not expand, and pops.  The glass is broke.

So here at woodstove-fireplaceglass.com, we recommend installing the glass in the following manner. 

Use the factory-recommended gasket and fasteners to secure the glass in your stove. Some glass originally had gaskets and others do not.  We recommend using the gasket material.  

Every stove is a little different, but the glass replacement concept is similar. When removing your old glass, pay attention to how the glass is secured into the stove. Taking pictures of your old glass in stages as you remove it helps with re-installation.

Be careful when tightening down the bolts or screws when re-installing. Fasteners should just hold the glass in place. We call this "Finger Tight" = Just enough to hold the glass in place.  “snug” but not “tight”. You must allow for expansion as the glass is heated. Do not over-tighten the fasteners.  You want to allow the glass to “float” or move around. Try not to allow any metal on the door system to touch the glass, it will almost always result in the glass breaking when the stove is heated.

Ceramic Glass Sheet

A basic rule with stove glass:

#1 - The Frame the glass is mounted in should be clean, flat, and free of debris.

#2 - Gasket material provides an airtight seal where the glass and stove meet, this prevents the glass from making direct contact with any metal on the stove.

#3 -Some stoves are equipped with an “air wash System” to help keep the glass clean. Sometimes a section of the glass gasket at the top or the bottom of the glass will be removed to accomplish this.  If you see a section of gasket “missing” this is most likely why.

#4 - When you think you have the glass installed correctly, back each screw/bolt off about a 1/4 of a turn, just to be sure.