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Using pyrex in a RMH?  RSS feed

 
                        
Posts: 278
Location: Iowa, border of regions 5 and 6
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I thought this video:



was really interesting, because the guy got a pyrex dish from the dollar store, heated it up over a fire to make sure it could take the heat, and used that to form the front part of the burn tube of his RMH!  Now he not only has a butt-warmer, but one that will give that nice cheery flame effect like a regular fireplace!

Do you think the guy's a raving loony for trying something like this?
 
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This is pretty cool.  I wonder how much heat loss comes through the pyrex though.
 
pollinator
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Muzhik wrote:
I thought this video:

http://www.youtube.com/watch?v=zMYlu8FQEL4

was really interesting, because the guy got a pyrex dish from the dollar store, heated it up over a fire to make sure it could take the heat, and used that to form the front part of the burn tube of his RMH!  Now he not only has a butt-warmer, but one that will give that nice cheery flame effect like a regular fireplace!

Do you think the guy's a raving loony for trying something like this?



He seems to have picked the best place to put it. You can see the fire well, the air flow is away from the glass so it will not get sooty and this has got to be the coolest place you can watch the flames from so Pyrex may not be needed. It should not effect the operation of the heater either.
 
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Not sure this is a good idea.  Can't find the reference but there was a significant discussion on using pyrex in a wood stove forum and it was found to thermal fracture quickly compared to the correct glass.  If it's a bit away from the flames and air flow prevents overheating it could last.
 
pollinator
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I might want something to catch flying shards, if thermal shock should ever occur. Maybe a wire cage (potentially decorative)?
 
                        
Posts: 278
Location: Iowa, border of regions 5 and 6
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mekennedy1313 wrote:
Not sure this is a good idea.  Can't find the reference but there was a significant discussion on using pyrex in a wood stove forum and it was found to thermal fracture quickly compared to the correct glass. 



What is the "correct glass"?
 
                          
Posts: 211
Location: Northern California
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The current formulation of Pyrex is not very thermal shock resistant at all; if this worked it was due to other factors (like location, as Len mentioned).
 
Joel Hollingsworth
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Muzhik wrote:What is the "correct glass"?



Transparent zero-CTE glass ceramic might be the best. Fused quarts would probably also perform fairly well.

The window from an old oven might be worth looking into, if you have one lying around and don't want to use it for a solar cooker or some other project.
 
Max Kennedy
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In my experience the correct glass is usually called Pyroceram III.  It is treated to withstand thermal stress up to 1650 Fahrenheit. Robax or Neoceram are other brand names.  Though similar to Pyrex they have been treated to withstand higher temperatures.
 
Joel Hollingsworth
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mekennedy1313 wrote:
In my experience the correct glass is usually called Pyroceram III.  It is treated to withstand thermal stress up to 1650 Fahrenheit. Robax or Neoceram are other brand names.  Though similar to Pyrex they have been treated to withstand higher temperatures.



It's similar in that it's meant to be transparent and to withstand thermal shock.

The fundamental strategy is entirely different, and so are the composition, structure, etc.

The products you have listed all have a crystalline component, like milk glass or Corningware. As with Cornigware, the crystals that form within the glass have a negative CTE (they contract as they warm up, and expand as they cool down). The volume fraction of glass vs. crystals is adjusted until the overall CTE is zero (the glass expands exactly as much as the crystals contract). Usually these crystals are based on lithium compounds.

To make such a composite transparent, each individual crystal must be much smaller than the wavelength of light. The manufacturers almost achieve this: only the bluer wavelengths of visible light are short enough to be scattered by these very tiny crystals. Greens and reds pass through undisturbed, which is why we see an amber color.

Pyrex, on the other hand, is entirely made of glass. Boron is used to replace sodium and calcium, which tend to dissolve out of the glass just slightly, producing tiny flaws on the surface of normal (soda-lime) glass. The boron also leads to a slightly lower CTE, greater strength and higher melting point (within the special definition of "melting point" one needs for glassy materials) all due to stronger bonds.
 
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Maybe keep your eyes open for an old stove going to the dump that has glass doors and recycle the doors?

Kathleen
 
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the front of the feed tube is indeed the best place for it up till you get a severe temp difference between the coals in the feed and the air flowing over the feed edge.
Try it and let us know how it works over time. remember the air wash in a wood stove is to cool the glass so it wont have a great temp difference between top and bottom so wood stove glass IMO wont work very well for this application.  as usual i might be wrong so giving it a try would be worth it.
 
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I think if the glass cracks, but stays in place, it will still be good. 

I also think that if you have a bit of a gap of air between the fire and the glass, that it won't be as hot.

 
Ernie Wisner
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I dont know for all types of glass, just those we tested. all the glass we tested broke and would not hold together well enough to be used. if folks are trying all kinds of glass i would like to know about those that work.
 
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