Most fireplace glass doors are manufactured using ¼” tempered glass. Tempered glass is simply glass that has been heated in a furnace and then quickly cooled. This process increases the strength of the glass and the ability of the glass to withstand the high temperatures associated with a fireplace. The average thermal shock rating (the temperature at which the structural integrity of the glass will fail and shatter) is between 400 -600 degrees Fahrenheit. This is usually sufficient for most wood and gas fireplaces.
Tempered glass is indeed the preferred glass for manufacturers for most fireplace doors because they have found that the increased strength of the tempered glass can make up for the weak structural integrity of many aluminum frames. Nearly all fireplace door manufacturers offer a lifetime warranty on the tempered glass, but as with all warranties, you must read the fine print. Most fireplace door warranties will tell you not to burn the fire with the doors closed or the warranty is voided; the reason being that a large fire could potentially exceed the thermal shock limits of the tempered glass and crack or shatter the glass.
To be fair, no manufacturer knows the exact condition of a customers fireplace or the size fire a customer intends to burn (turns out many homeowners enjoy bonfires in their fireplaces). So the question is often asked, “If I shouldn’t burn a fire with the fireplace doors closed, what is the purpose of the draft assembly?” Well, the answer is – so you can burn with the doors closed and still have combustion air feed the fire. This is a contradiction, but again, fireplace door manufacturers can’t be responsible for what someone does in their fireplace.
While I would never recommend forsomeone to burn a fire with their doors closed and the draft assembly open (for obvious legal reasons), if you were going to do so I would definitely recommend that you keep the fire about 6” back from the glass.
There is another alternative that is becoming increasingly popular; ceramic glass. Ceramic glass is a great option for fireplace doors for two main reasons:
1. It has a thermal shock rating of around 1380 Degrees Fahrenheit – far exceeding any fire you would want in your fireplace.
2. The process for creating ceramic glass is simplified allowing fireplace door manufacturers to stock and cut it themselves – what does this mean to you? A fireplace door manufacturer doesn’t have to wait for the glass to come in from the glass company which drastically reduces the build time for your fireplace door.
Are there any downsides to ceramic glass? I would say just one. While the thermal shock rating for ceramic glass vastly surpasses that of tempered glass, it does not have the same structural integrity. This could be an issue for aluminum fireplace doors that are not as sturdy as steel doors and are benefiting from the added integrity that the tempered glass provides. For this reason, I would stick with tempered glass in aluminum doors and ceramic glass in steel doors.
Bottom line: ceramic glass is a great upgrade for two reasons:
• Allows you to burn a hot fire with the fireplace doors closed
• Dramatically reduces the build time from 4-5 weeks to maybe just 1 week depending on the manufacturer.
There is a fireplace door on the Woodland Direct web site that features ceramic glass as a standard option. It is called the Sentry Fireplace Door and it has an incredible build time of just five days. Comes with a draft assembly so you can burn with the doors closed and it feature a flat black finish on a 3/16” steel frame.