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  #21  
Old 02-02-2013, 05:01 AM
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Location: South Australia
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Default Re: Alternative Insulation

Just out of interest, we have a product we call "black sand". It's furnace slag that has been granulated by pouring the molten slag into a stream of cold water. You end up with a product that looks like coarse black sand. Particle size around 4-5 mm. A litre of this holds nearly half a litre of water in the voids. True story - when I fill a measuring cylinder to the one litre mark, I can pour nearly half a litre of water in.
Maybe this explains how some people came to recommend sand as an insulator under the bricks?
And maybe the same logic behind the people who recommend broken glass under the bricks?
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  #22  
Old 02-02-2013, 05:16 AM
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Default Re: Alternative Insulation

Sand has to be an insulator! If not, how would you explain that on a very sunny day at the beach--the sand on the surface can be scorching hot, but dig down just a little bit and it is cool?

Or is it such a good conductor that the deep underlying sand sucks all the heat out of everything except for the topmost layer?

If the first case is true--being a good insulator, then it would make sense to have a thick layer of sand beneath the firebrick floor!

Last edited by mikku; 02-02-2013 at 05:18 AM.
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  #23  
Old 02-02-2013, 06:27 AM
Peasant
 
Join Date: Oct 2012
Location: Minnesota
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Default Re: Alternative Insulation

David s makes a good point about foil trapping moisture. If you use foil you would certainly want to wait until the the oven has cured and then use a perforated foil if at all possible. Water vapor will pass thru much smaller holes than liquid water. The biggest advantage of foil is that it doesn't take any space.
Related thought some of you WFO pros may want to confirm; the ability to a WFO to bake a pizza in 90 seconds is that it utilizies all three means of heat transfer. Conductive heat from the deck, radiant heat from the live fire and convective heat caused by domed ceiling.
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  #24  
Old 02-02-2013, 08:15 AM
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Default Re: Alternative Insulation

Sand is not an insulator, the airspace between grains is. Sand on the surface is hot because there is no moisture, once you dig through to where there is moisture evaporative cooling takes over.
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  #25  
Old 02-02-2013, 12:00 PM
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Default Re: Alternative Insulation

The thermal conductivity (k) of dry sand is 0.15 - 0.25 W/mK.
Compared to dry loose vermiculite 0.058 which is about three times better.
But add cement to the vermiculite and it drastically reduces the k value.
A 4:1 vermiculite, cement brew has a k value of 0.16, similar to the sand.

Last edited by david s; 02-02-2013 at 12:10 PM.
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  #26  
Old 02-02-2013, 01:18 PM
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Default Re: Alternative Insulation

Quote:
Originally Posted by mikku View Post
Sand has to be an insulator! If not, how would you explain that on a very sunny day at the beach--the sand on the surface can be scorching hot, but dig down just a little bit and it is cool?
Because its wet further down.
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  #27  
Old 02-02-2013, 01:38 PM
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Default Re: Alternative Insulation

Quote:
Originally Posted by david s View Post
The thermal conductivity (k) of dry sand is 0.15 - 0.25 W/mK.
Compared to dry loose vermiculite 0.058 which is about three times better.
But add cement to the vermiculite and it drastically reduces the k value.
A 4:1 vermiculite, cement brew has a k value of 0.16, similar to the sand.
This is the thing: most of the solids we are discussing, sand, glass (which is, after all, pretty much just fused sand), vermiculite, ceramic fibres, etc., are thermal conductors, to varying degrees.
What makes any of them insulators is the ability to hold air in place so that it insulates, because if it's allowed to move it becomes a cooling medium.

In my not-so-humble opinion, insulating over the dome should be OK.
A sealed, empty enclosure will help a lot, or there's bottles inside the enclosure, there's clay mixed with saw dust, etc.

The BIG question. Have we yet found NeilB a decent alternative to go under his oven?

Last edited by wotavidone; 02-02-2013 at 01:41 PM.
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  #28  
Old 02-02-2013, 03:07 PM
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Default Re: Alternative Insulation

Perlite or vermiculite both have tiny holes in them that the cement does not penetrate so each grain in effect is a mini bubble of air. A marble is solid and dense.
If using glass bottles as an underfloor insulator the usual method is to break the glass and lay the floor bricks directly on the bed of broken glass. Bottles are better than flat glass because they create bigger and more air spaces.
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  #29  
Old 02-02-2013, 05:16 PM
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Default Re: Alternative Insulation

I agree with everything that has been said, at least somewhat.
But I think in a desert, on top of a dune if you dig down just a little the sand would be much cooler....(a location lacking a lot of moisture). So the air is the insulator!

As far as "NeilB" alternative insulation question goes, I think a bit of history research is necessary. Then the question is "are there OR have there been "traditional types of ovens" in your location?" And how were they insulated?

That holds true for anyplace on earth. The native peoples developed techniques from available materials to cook or bake. If it worked for them, as a means of survival--then it surely would work for you. The price would be very inexpensive as well! If you have to modify their design a little to reach pizza making temperatures, then that is another challenge.
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  #30  
Old 02-02-2013, 05:20 PM
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Join Date: Jan 2013
Location: Tochigi Prefecture, Japan
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Default Re: Alternative Insulation

I really like to know more about "K" factors. I searched last night for a long time to get anything--then could not understand the numbers when I saw them.
"R" values are understandable, "U" are inverse, but "K" is another bird???
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