WHich Oven ?

[brickie in oz]

Re: WHich Oven ?

so if wool is 600c then I have chosen well .

Plus you dont get that burnt hair smelling pizza.

[deejayoh]

Re: WHich Oven ?

I think you all mean 600F. 600C is 1112F, which is hotter than the inside of my oven gets AFAIK

[deejayoh]

Re: WHich Oven ?

I'd be interested to see data that shows this. My guess is that there would be little difference, provided both were insulated the same.

Unlikely. Firebrick was developed for energy efficiency. The metal content lets it capture heat more quickly and hold it longer. Using the same amount of wood and time, an adobe oven won't get as hot as a firebrick oven. It might work as well if you release those constraints, but then what is the point? I can communicate with as many people using a quill pen to write letters and then mail it using and horses and boats as I can typing this message on the internet - does that make those two equal?

[Tscarborough]

Re: WHich Oven ?

I am pretty sure he is talking about mineral wool, not sheep wool.

[david s]

Re: WHich Oven ?

Unlikely. Firebrick was developed for energy efficiency. The metal content lets it capture heat more quickly and hold it longer.

I presume you are referring to aluminium when you say metal content. Although aluminium is highly conductive it exists in a different form in a clay brick. It is aluminium oxide(alumina) This substance has quite different properties and a much lower thermal conductivity. It is the same if the clay is either unfired or fired. Check link.
I am way too busy building ovens to have to get sidetracked making test samples of fired and unfired clay and then testing their relative thermal mass and insulative properties, but in the absence of any information provided by anyone to support the assertion that fired clay is way superior, I might just have to, it is an interesting question.
I think folk who go to the trouble of using free raw materials that have not required massive use of energy (fossil fuels) in an effort to keep their creations sustainable and traditional, are to be applauded rather than vilified.

Aluminum Oxide | Al2O3 Material Properties

[Tscarborough]

Re: WHich Oven ?

The advantage of light duty firebrick, is, in fact, it's LOW thermal conductivity. Compared to medium or high duty, much less soapstone, it is very low, thus not burning the bottom of your pizza. The biscotti produced by Italian oven makers is also a low thermal conductivity material, but it is also much softer than low duty firebrick.




Woodfiredoven, can an adobe oven reach and hold temps over 900 degrees Fahrenheit? I am asking, as I have no idea.

[david s]

Re: WHich Oven ?

The advantage of light duty firebrick, is, in fact, it's LOW thermal conductivity. Compared to medium or high duty, much less soapstone, it is very low, thus not burning the bottom of your pizza. The biscotti produced by Italian oven makers is also a low thermal conductivity material, but it is also much softer than low duty firebrick.

I'm pretty sure that the two reasons the Italians use low temp firings for their fired clay ovens is firstly because of cost, it takes less than half the fuel for a biscuit firing than a stoneware one. Secondly the low temp actually gives you better thermal shock resistance, like the low fired Sth American pottery you can put on a stove.

[deejayoh]

Re: WHich Oven ?

I presume you are referring to aluminium when you say metal content. Although aluminium is highly conductive it exists in a different form in a clay brick. It is aluminium oxide(alumina) This substance has quite different properties and a much lower thermal conductivity. It is the same if the clay is either unfired or fired. Check link.
I am way too busy building ovens to have to get sidetracked making test samples of fired and unfired clay and then testing their relative thermal mass and insulative properties, but in the absence of any information provided by anyone to support the assertion that fired clay is way superior, I might just have to, it is an interesting question.
I think folk who go to the trouble of using free raw materials that have not required massive use of energy (fossil fuels) in an effort to keep their creations sustainable and traditional, are to be applauded rather than vilified.

Aluminum Oxide | Al2O3 Material Properties

No, I meant Aluminum Oxide, which is exactly what is in firebrick. Which is also what you linked - and according to your link gives things "good thermal properties" - so I am not sure what point your are trying to make.

test samples of fired and unfired clay and then testing their relative thermal mass and insulative properties

I don't think statement describes the two things we are talking about comparing, unless your definition of adobe is different than mine (or perhaps its an Australian thing):

Adobe 1.earthen brick: brick made from earth and straw and dried by the sun.

And no one is vilifying anyone. A builder came and asked for advice on an oven type, and the claim was made that adobe is as good as firebrick. It's fair to discuss that. It's for sure cheaper, and I don't doubt it will get just as hot. As far as sustainability goes, it is not going to be as efficient in terms of energy consumption so pick your poison. Trees or alumina

[david s]

Re: WHich Oven ?

Does anyone have any published evidence to show either the thermal conductivity or insulating value difference between unfired clay and brick?
Since bricks are made of clay, I would expect that the difference would be very minor. Apart from the 10% or so of shrinkage which increases the density of brick and the addition of straw to the mud which reduces the density of adobe, I can't think of another reason why they would differ.
The main disadvantage of adobe over brick is that it is not as durable and tends to abrade more easily.

I found some interesting data although it's pretty inconclusive as the readings for clay are given in a range (presumably because of clays variability).Still may have to do some tests to see if a given unfired clay sample produces a different result to a fired one as a cob oven is essentially unfired clay.

Thermal conductivity W/(m.C)
Aluminium 205
Aluminium oxide 30
Clay 0.15-1.8
Firebrick 1.4
Sand 0.15-1.8

Thermal Conductivity of some common Materials and Gases

[Tscarborough]

Re: WHich Oven ?

Clay is not adobe either. The rating for Firebrick is high, it is not for low duty firebrick.

[david s]

Re: WHich Oven ?

Clay is not adobe, but adobe is unrefined clay with the addition of some straw as reinforcing and perhaps some added sand, so it's still essentially clay.

[Tscarborough]

Re: WHich Oven ?

The numbers for adobe do not match those for clay as in a clay body used to make brick, in any respect, thermally.

[deejayoh]

Re: WHich Oven ?

The clay in adobe is like the clay in a common house brick. It's not the same as the clay in fire brick. Adobe < clay < fireclay. You can keep researching but I'm done with this one.

[david s]

Re: WHich Oven ?

The clay in adobe is like the clay in a common house brick. It's not the same as the clay in fire brick. Adobe < clay < fireclay. You can keep researching but I'm done with this one.

Yes, I'm about done on the topic too, but it's been a good discussion, thanks. The thing that interests me though is whether there would be any measurable difference in unfired or fired clay for any given sample. It is also possible that low duty bricks with low thermal conductivity are actually less thermally conductive than a cob mixture.

[david s]

Re: WHich Oven ?

The numbers for adobe do not match those for clay as in a clay body used to make brick, in any respect, thermally.

Thanks TS, can you post those numbers and save me from doing my own tests?
Not interested in the max service temp as that is way beyond what we use anyway, just the thermal conductivity at the temp range we use.
Thanks,
Dave