Tempered fireplace glass explodes

[Ron]

For all you doubters just read this post.

[Rick Brandt]

It has always amazed me how few people know that glass doors are supposed to be closed when you are burning a fire. I always try to appeal to the pure logic of it. If they are not supposed to be closed while the fire is burning then what is their purpose? The fireplace already has a flu to close when you are not burning a fire so logically there is no purpose served by glass doors at all unless they are closed while the fire is burning.

I mean do these people think they are there to keep the cat out?

[TomCase]

I just had some FP doors shatter recently, after getting my money back, I plan to replce the shattered glass with something called "NeoCeram". Some kind of transparent ceramic stuff that good for 3x the heat( saw a video on the net one time where they poor ice water on a sheet while a propane torch is firing up at the bottom) A bit pricey at $39 sq ft.

[mm]

No one wants a toasted cat.

Well, no one who still likes cats wants a toasted cat.

Remove NOPSAM to email me. Please let me know if you have posted also.

[mm]

What is DAGS?

Remove NOPSAM to email me. Please let me know if you have posted also.

[Wes Stewart]

Do a Google search.

[barry]

Screened opening makes sense if you're trying to heat the county, at the expense of cooling your house, or if you want ventilation in spring or fall; else you gotta be nuts. A fire at 550F is a smoker; over 1000F you're cooking.

Forget the screen. Go for a properly-designed woodstove or insert and hook it in, and get some useful heat out of it without having to wear a flack-jacket and full-visor.

J

[Chris Jarshant]

Yes, this is what I understood when I first got the house - open the door, start the fire, wait for it to get going a little, then shut them. Else it sucks in air from inside the house, causing the house to suck in ouside air, cooling off every single room except maybe the one the fire is in. My fireplace is specifically designed to work this way. It has a closeable vent to the outside so that it can draw outside air into the fireplace instead of heated, indoor air.

In any case, I guess it's just my bad luck that the thing broke. I do believe fires burn hotter when the doors are closed and the overall efficiency is better.

One more question. If I could heat up tempered glass slowly to a ridiculous temperature, what would eventually happen? Would the glass liquify, or shatter first?

cj

[dnoyeB]

It was probably damaged to begin with. Just not visible to the naked eye.

My mother had glas doors on hers and we loaded that sucker with tons of wood and it heated the whole living room and kitchen.

Shouldnt be anything around the fireplace that is flammable with high heat. Oddly enough people tend to load their furnace rooms with all sorts of cardboard boxes though...

[g.a.miller]

Let me know what you find out. Mine did the same thing! Regards in fullest Anthony [e-mail]

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[Michael Daly]

It would anneal. Well, actually, it would reduce the stresses in the tempered glass and would end up annealed if you then cooled it slowly. If you just kept heating it, it would melt.

It will only shatter if it has a good reason. There has to be a reason for a crack to form and/or propagate. If it already has a crack (or scratch) as a stress riser _and_ the heating causes sufficient differential expansion that the stress riser goes into tension, the crack will grow rapidly and the glass will shatter. If the tempering process has created sufficiently high internal stresses that heating one side causes the expansion (or even annealing) on that side to result in serious tension stresses, it might crack as a result of the high stresses without a precursor stress riser.

Personally, I take claims of tempered glass shattering "with no reason" with the same enthusiasm as claims of people spontaneously combusting.

Mike

[Chris Jarshant]

Ok, I do remember a few weeks ago, I had piled 5 or 6 pieces of wood inside the fire place, and as the fire burned, the log stack became unstable and the top-most one fell forward and smacked into the inside surface of the glass. I guess that must have started something that two weeks later resulted in the shattering. The glass is attached to the frames rather loosely (to allow for expansion I guess) and one theory may be that the glass was forcefully scraped across the metal pressure tabs holding it in place when the log hit, starting a tiny scratch.

One other thing, the glass door is actually 4 separate pieces of glass with 1/4" or so of space between each piece. Maybe when the fire gets real hot, the differential heating caused by the sucking in of room air through that crack causes local low temperatures near the spaces and thermally stresses the glass too much?

cj

[Charles Spitzer]

sorry, it wouldn't anneal.

it would become untempered once you get it over bending temperature, which for ordinary and normally found clear float glass, is around 1030F. on cooling, it would no longer be tempered but regular window glass.

annealing is the process of reducing temperatures slowly enough so that stress is evened out across the entire sheet of glass. if the temperature on one part of the glass is different enough than another, when cooled, it would set up uneven stresses, leading to eventual cracking and shattering.

in order to temper glass, it is heated up in special tempering ovens to around 1200F (which takes about 5 minutes), then cooled to around 700F in another 5 minutes. the tempering oven i'm familiar with has a 60 hp blower to remove that amount of heat quickly enough. the equipment to do so starts around $2million. it's not possible to temper glass without use of this type of equipment.

there are other glasses, like pyrex or borosilicate, which can be tempered the same way, but just at different temperatures. pyrex usually isn't tempered at all.

regards, charlie

[Ron]

Didn't I write that I'm sure there is a "scientific reason" for it.

Hardly worth getting into in the grand scheme of things.

[Michael Daly]

Ok.

[Norminn]

clipped

I would have to re-take physics to answer that one, and I didn't do that well the first time :o)

[Charles Spitzer]

well, yes, but tempered glass takes stress of heat differentials much better than untempered. there is a limit to this stress though, and any small chip or scratch makes that limit a WHOLE lot less.

regards, charlie