You see it all the time in action movies: a skylight shatters sending thousands of deadly shards of glass showering on the hero who miraculously isn’t hurt. The reality, though, is that if that had happened to the skylight, it wouldn't be so dramatic or deadly. Most skylights today are made from tempered glass, the same glass on most of our fireplace doors! Tempered glass is made with your safety in mind. When broken it does shatter, but in smaller, harmless pieces. Tempered glass is also much more durable than ordinary, annealed glass. Ordinary glass breaks at about 6,000 psi (pounds per square inch) while tempered glass generally breaks at 24,000 psi. Tempered glass is made using a heat process (or in some cases a chemical process) that creates tension and compression in the glass. This gives the glass the durability it needs to stand up to the heat of your fireplace!
Ceramic glass is a little different. Formed in a two step process, ceramic glass (called PyroCeram) encourages crystalline growth in the second phase of production. In this process the growth of crystals is controlled, allowing uniform growth within the glass. Ordinarily crystal growth isn’t a good thing, but with ceramic glass you get the advantages of glass with the durability of ceramics. This may not seem like such a big deal, but consider that your wood stove's glass window is made out of ceramic glass due to its durability and it's ability to withstand extreme heat. Originally ceramic glass wasn’t even designed for cooktops or fireplace doors and stoves, but telescopes!
Ceramic glass is most commonly used with wood, coal and pellet stoves because ceramic glass can withstand continuous temperatures exceeding 1,000 F. Wood burning stoves are used with the door closed, so it’s important that the glass used in the window be able to handle the intense heat put off by the stove. Ceramic glass is also used with fireplace inserts and some masonry fireplace doors when manufacturers think that the heat may exceed what tempered glass can handle.
Tempered glass is far more common when it comes to fireplace doors! Since you shouldn’t burn your fireplace with the fireplace doors closed, tempered glass isn’t exposed to the extreme heat that could potentially damage or shatter it. It tends to be less expensive than ceramic glass, making it easier to replace damaged panels on your fireplace door.
There are many benefits to both ceramic and tempered glass. For example, with ceramic glass you get a glass that’ll handle extreme temperatures, more so than any other glass. Tempered glass is more economical and pretty durable as well. Most ceramic glass has a lower CTE, meaning it won’t expand as much. While this can be a nice benefit, it doesn’t really apply to most fireplace doors and only matters with fire products that get very hot. Ultimately it all depends on what you have and what you want, but to avoid violating warranty and potentially damaging your fireplace door, remember to only put the type of glass on your door that it’s designed for. If your zero-clearance fireplace door has tempered glass (they all do) please don’t order ceramic glass. The same thing applies to your fire product that uses ceramic glass!
Let us know how we can help make your fire products safe today! Our U.S. based customer service is trained in fireplaces and fire products, so we can help direct you to the product you need to make your fireplace safe for years to come.