Working of a wood fired oven
Radiant heat & refractory bricks
How does a wood fired oven work and cook?
When wood burns in the oven chamber, the heat is absorbed by the refractory bricks and then radiates in all directions and heats the floor.
It's this radiant heat which actually cooks the food and not the fire directly.
Similar to a battery, a refractory brick act as some sort of heat accumulator and its heat storage capacity is determined by its chemical composition.
Firebricks are made from fireclay which contains silica (SiO2) and alumina (Al2O3).
It's the alumina content which determines the overall density and hence the capacity to absorb heat.
For baking purposes, around 20% to 30% Al2O3 is a good proportion.
Here's some good reading about refractory bricks:
http://www.traditionaloven.com/articles/84/firebricks-heavy-dense-fire-clay-bricks
http://www.fornobravo.com/pompeii_oven/brick_primer.html
For baking purposes, around 20% to 30% Al2O3 is a good proportion.
Here's some good reading about refractory bricks:
http://www.traditionaloven.com/articles/84/firebricks-heavy-dense-fire-clay-bricks
http://www.fornobravo.com/pompeii_oven/brick_primer.html
Thermal mass
While the composition of the firebricks determines their capacity to absorb and radiate heat, it's the thermal mass which determines the total oven heat storage capacity and the time during which it will be able to bake.
The more mass, the more absorbed heat, the longer time you can bake. But it also means that it will take much more time and wood to heat and reach the desired temperature.
Of course, in order to keep this heat and release it slowly, you have to prevent it from escaping the oven. For that, you need to insulate it as much as possible, like a house.
Insulation
Thermal conductivity
Insulation is primordial for an efficient wood fired oven. Poor insulation means that it will take an incredible amount of wood to reach the right temperature and the oven will cool down very quickly as the heat escapes. So proper insulation should be part of the plan. There's the sorry case of one of my neighbours who has to heat his oven for a very long time (he told me 3 days!) for his oven to reach the right temperature. His cooking floor rests directly on a huge slab of stone and concrete, without any insulation, and you can feel that it gets really hot underneath. So much heat which is not being used for baking! Bah.
The oven has to be generously insulated from all sides. Underneath the cooking hearth, over the dome, and the door as well to retain heat.
There are various materials that can be used for the insulation. Their ability to conduct heat is measured by their thermal conductivity (measured in w/mK). The lower the value, the less they conduct heat, the better insulators they are.
Here are thermal conductivites of some materials/gases. The lower, the better.
Extracted from: http://en.wikipedia.org/wiki/List_of_thermal_conductivities
Here are some materials commonly used for insulating ovens:
Ceramic fiber and calcium silicate are the best insulators but are more expensive. I couldn't find these here so I went for a perlite + Portland cement mix.
The ratio perlite/cement can vary and will influence the density, compressive strength and thermal conductivity.
The higher the proportion of perlite, the lower the density and better the insulation will be, but at the cost of a lower compressive strength.
Here's a table showing the thermal conductivity and compressive strength of different Perlite/Portland cement mixes.
Extracted and consolidated from http://www.perlite.org/library-perlite-info/construction-perlite/Perlite-Concrete.pdf
Air-entrainment agents are additives added to cement to create a multitude of tiny air bubbles. This is mainly used for concrete to sustain repeated freeze/thaw cycles thanks to the air bubbles absorbing the internal stresses. See info about air entrained concrete.
Since a good insulator (apart from the vaccum), is a material which confines air or gas in tiny pockets, preventing the heat from propagating by conduction, so air entrained concrete should have a reduced thermal conductivity.
I couldn't find commercial air entraining agents here but after further research on the subject, I found that liquid dish soap could actually be used as an air entrainment agent. I just added a few centiliters per liter of water.
Since the dome insulation layer doesn't have to support any load, I wasn't too worried about loss of compressive strength so went ahead and experimented with that method.
I cannot really quantify how the insulation was increased due to this process.
A comparative study could consist in the following:
Since I don't have a foam generator, I did a test using citric acid (H3C6H5O7) and sodium bicarbonate (NaHCO3) mixed with liquid soap. The test consisted in first diluting the citric acid in water and soap in a big plastic bottle whose bottleneck was connected to a piece of hose and then add the sodium bicarbonate. When the two are in contact, they produce carbon dioxyde, water and sodium citrate (Na3C6H5O7).
This produced a thick foam coming out of the hose which I mixed with the perlite and cement. However, since I added a lot of perlite, I didn't feel that the cement was more aerated than before so I didn't pursue the experimentation. For my tests I used about 50g of sodium bicarbonate and 32g of citric acid - which corresponds to the balanced chemical reaction between these two components.
Here are thermal conductivites of some materials/gases. The lower, the better.
| Material | Thermal Conductivity (W.m−1·K−1) |
| Carbon dioxide | 0.0146 |
| Water vapor | 0.0471 |
| Air | 0.024 |
| Nitrogen | 0.0234 |
| Oxygen | 0.0238 |
| Perlite | 0.031 |
| Extruded polystyrene | 0.029 - 0.39 |
| Rockwool | 0.04 |
| Dry wood | 0.04 |
| Water | 0.563 |
| Concrete | 0.8 |
| Iron | 71.8 |
| Aluminium | 204.3 |
| Copper | 385 |
| Graphene | 4000-5000 |
| Helium II | >100000 |
Here are some materials commonly used for insulating ovens:
- ceramic fiber board/blanket
- calcium silicate boards
- vermiculite/perlite + Portland cement mix
- light expanded clay aggregate
- rockwool (as a secondary dome insulating layer).
- ...
The ratio perlite/cement can vary and will influence the density, compressive strength and thermal conductivity.
The higher the proportion of perlite, the lower the density and better the insulation will be, but at the cost of a lower compressive strength.
Here's a table showing the thermal conductivity and compressive strength of different Perlite/Portland cement mixes.
| Cement (part volume) | Perlite (part volume) | Water (part volume) | Dry density (kg/m3) | Wet Density (kg/m3) | Compressive Strength (kg/cm2) | Thermal Conductivity (W/mK) |
| 1 | 8 | 2.6 | 352 | 593 | 6.3 - 8.75 | 0.0779 |
| 1 | 6 | 2.1 | 432 | 673 | 8.75 - 714 | 0.0923 |
| 1 | 5 | 1.8 | 481 | 737 | 16.1 - 21 | 0.102 |
| 1 | 4 | 1.6 | 577 | 801 | 24.5 - 35 | 0.12 |
Air-entrained concrete
Looking for ways to further reduce the thermal conductivity, I read about air entrained concrete and tried to apply it to the dome insulation cement-perlite mix.Air-entrainment agents are additives added to cement to create a multitude of tiny air bubbles. This is mainly used for concrete to sustain repeated freeze/thaw cycles thanks to the air bubbles absorbing the internal stresses. See info about air entrained concrete.
Since a good insulator (apart from the vaccum), is a material which confines air or gas in tiny pockets, preventing the heat from propagating by conduction, so air entrained concrete should have a reduced thermal conductivity.
I couldn't find commercial air entraining agents here but after further research on the subject, I found that liquid dish soap could actually be used as an air entrainment agent. I just added a few centiliters per liter of water.
Since the dome insulation layer doesn't have to support any load, I wasn't too worried about loss of compressive strength so went ahead and experimented with that method.
I cannot really quantify how the insulation was increased due to this process.
A comparative study could consist in the following:
- make two batches of concrete - a air entrained one and a normal one
- pour them in identical mold (eg: square tiles)
- let cure in exactly the same conditions for a few weeks.
- heat one side with a localised source of heat, like a butane torch, and measure the temperature on the other side at regular intervals. The tiles should be wide enough or masked with an insulator to eliminate heat transfer by radiation.
Foamed cement
Since the building was a laboratory for experimentation, I also tried to make foamed cement in one of the perlcrete batches. Foamed cement is made by generating foam using a foam generator and injecting it in the cement. The result is tiny air bubbles which act as an insulator.Since I don't have a foam generator, I did a test using citric acid (H3C6H5O7) and sodium bicarbonate (NaHCO3) mixed with liquid soap. The test consisted in first diluting the citric acid in water and soap in a big plastic bottle whose bottleneck was connected to a piece of hose and then add the sodium bicarbonate. When the two are in contact, they produce carbon dioxyde, water and sodium citrate (Na3C6H5O7).
This produced a thick foam coming out of the hose which I mixed with the perlite and cement. However, since I added a lot of perlite, I didn't feel that the cement was more aerated than before so I didn't pursue the experimentation. For my tests I used about 50g of sodium bicarbonate and 32g of citric acid - which corresponds to the balanced chemical reaction between these two components.