1,1/4" granite clad woodstove/oven

[eldaren]

Hello There, This is my first post.
-My terminology on this topic comes from Swedish, so I will be struggling for words here so please bear with me. I hope this query is in the right topic /area of your forum:

I'm wondering if granite is ok to use for this application:The granite (1,1/4" thick 24"x 24" tiles) and will not be touching the body of the stove. and some type of insulation will be used in the gap between, approx 1" thick. The stove unit I purchased is meant to be clad in 3/4"soapstone, but because of the cost diff. i want to use granite instead. this in the end,will save me around $1,500 U.S., which is roughly the cost of the free-standing stove body itself- cast iron and welded steel plate and tempered glass.
I'm aware of the characteristic differences between soapstone and granite- density, heat retention, etc. and I have built one Tullikivi oven for a customer (easy-precut and numbered, all 1200 kilos of it) I know granite, like all stone, varies greatly in composition: And the granite I have is very light- high guartz and/or mica? content. When i purchased it, it was damp- looked dark grey, but when i brought it inside, it dried out to be relatively light colored. As i said price was a big factor here, if I chose black granite, 1,1/2" thick, mirror polished, it sells for around 600$ U.S. for 1 sq. meter from a local stonecutter. The light granite is not as finely polished, imported from (whoknows, probably China) but sells for a non-whopping $65. U.S. which I can afford. If I use my local stonecutter for soapstone, the price is close to the black granite. He orders from Finland.
This stove is secondary heating for my home, if it was the primary source, I would not have chosen a model with so much glass and I would have opted for the soapstone. The firebox unit receives the highest possible rating for efficient burning, tight and has an external ashbox, and air intake control from under. here is a link to the model- as pictured, it is clad in soapstone.
(broken link removed to http://www.contura.se/modeller/c470.asp?c470=utforande&meny=c470)
I went a little nutts with details here, but my main question is the use of the granite, I don't want to build this beast and have it crack on me. Because the unit is self enclosed, a crack would be superficial -as I said, the firebox is air tight in itself (cast iron and welded steel)
Thanks in advance for your attention and advice. If anyone needs tips on building oversized masonry archways, drop me a line.
Cheers,
Bill.
pastorprime(at)hotmail(dot)com

 

[firefighterjake]

Bumping since I hate it when folks post a question and no one gets a reply.

My guess . . . and I'm certainly no expert . . . is that most folks go with soapstone for heating for its heat retention . . . I would guess that granite would work, but not as well. A possible concern might be the granite spalling and chipping . . . something I have seen in buildings that have caught on fire -- granted there was direct, high heat and flame impingement on the granite vs. not having any insulation barrier between the flame and the granite. That said, I would think that if the stove manufacturer recommends soapstone I would tend to stick with what they recommend and play it safe . . . I would rather pay a higher price up front and know everything was safe and built to specifications vs. trying to be cheap and paying for it in the long run with the stone cracking, not gaining the heat longevity, etc.

 

[eldaren]

Thanks, Jake
I have also seen granite chip and literally explode after being heated on open flame. Most (smart) Native Americans don't allow granite (rounded river rocks) to be heated for a sweatlodge for exactly this reason. Gases/water are easily trapped in porous stone which in turn react to extreme temperature change( freezing/ super-heating)
In my case, as I said the granite will be insulated, and being indoors, not subject to water/humidity and open flame.
hopefully my post will catch the eye of an old-timer here.
Thanks for the bump and the 2 cents, it is much appreciated.

 

[Dune]

I don't see any problem with granite, especialy since it is not in direct contact with the flames.

 

[Slow1]

Looking at the pictures of that stove I wonder what part the stone may play in the design of the stove itself. Clearly the stone is not in the firebox - that seems to be mostly glass, but it does look to surround the top of the stove as well as the first (visible) portion of the flue. You mentioned that this stove is considered to be one of the more efficient models available to you. Looking at the firebox itself I'm a little surprised at this since it looks very much like a glass encased fireplace - I don't see anywhere for a secondary burn to happen and it is hard to imagine it being up there above the glass viewing area as it would seem hard to get it hot enough there to achieve smoke-burning temperatures. So - where does this efficiency come from? Is it perhaps from capturing heat from this first part of the flue exit? If so, then if you insulate the flue from the decorative finish (in your case granite) then you may give up this efficiency entirely. This may be fine for you, but you may want to consider this.

On the other hand, perhaps it is already insulated and the stone is nothing but decorative finish. If this is the case then insulate away and enjoy the beauty you can create.

In any case although I'm not an expert in stove design it seems to me absent a secondary combustion of some sort I can't imagine it is going to be a truly efficient heating appliance.

 

[eldaren]

Unfortunately, there are no good pictures of the unit itself with out the superficial stone or "dressing". The re-burn occurs in a large tapered chamber terminating 12" above the door, it only slightly smaller than the visible part of the firebox, which you see in the picture. The whole thing is designed around efficient re-burn. The illustration to the far right shows (400 series) unfortunately it is in Swedish.
(broken link removed to http://www.contura.se/accessoarer/konvektion.asp)

here is the text shorthand translated-
Red arrows, convection air drawn around the outside of the firebox and through 2 - 2" curved pipes in the re-burn area. this can be forced through with the addition of a fan mounted under the unit once the heat builds up.
blue arrows, primary intake for combustion- focuses pre heated air towards the glass to burn off creosote on the glass
green arrows, clean burning system- a regulating valve on the combuston air intake..

One thing that impressed me was that it has a self contained ashbox under the firebox with a seperate door (do not need to open the firebox to clean out the ashes) and the ashbox is not just a little pan, it is solid, rather heavy and 2" deep. 90% of the units i looked at had a thin ashbox more like 1/2" deep.

The local distributor has one of these in his own home and promised me that the glass really does stay clean with a properly built fire. He has used his for two years and never cleaned the glass.

 

[Slow1]

Ok - so then it appears that it may rely entirely on convection (hot air) to carry the heat away from the reburn area and into the room to be heated. If this is the case then the top of the stove should not get warm at all really and the material you cover it with shouldn't matter (other than to keep it non-flamable just in case of a chimney fire or some other disaster). You could test this theory by having someone (the distributer?) take temperature measurements with an IR gun of an operating stove (or simply feel it) to verify that the surface doesn't warm up.

Nice stove btw - I would like to see one in person.