The cob oven is a wonderful technology that suits good times. There are two types to consider, and they offer different cooking methods to the user.
This article details both styles’ good and bad points, and the information below should help you decide which cob oven to build.
How does a cob oven work?
A cob oven operates through a two-stage bi-directional process of thermal heat transfer.
The first stage is where a fire is lit within a chamber that pushes heat into the oven wall and is banked within the cob structure for use during the second phase.
The fire is contained within the oven chamber, where thermal and radiant heat soaks into the cob wall and base over time.
The effectiveness of the oven is dictated by several things.
- Fire intensity
- Oven airflow
- Cob oven wall thickness
- Cob mixture thermal capacity
Oven chamber fire intensity
The cob oven runs better with a high-intensity fire, primarily driven by the quality of the firewood. Hotter fires come from dry hardwood. Because the oven is made of cob (basically earth), there is little to no chance that the oven will burn. This means that you can light that fire and really get it burning.
Cob oven chamber air flow
Another factor is the airflow within the oven chamber itself. Some cob ovens have a chimney incorporated into the design, while many do not.
Both ovens (with and without chimneys) operate similarly in heat transfer. Still, they differ in how smoke can be directed as the fire burns and the associated airflow direction that comes with the changed oven exhaust pattern.
Cob oven wall thickness
Another significant factor is the thickness of the cob wall, which is flexible to a degree.
The single factor that makes the most difference to the way your oven cooks your food is the oven wall thickness. If the wall is too thin, the capacity to retain heat will diminish, worsening with thinner and thinner walls.
This doesn’t mean that the oven will not function, but it means the oven will cook food for a shorter time. The oven walls will heat up faster but will cool down more quickly.
In essence, the thermal heat bank has less embedded potential held in the cob and, therefore, less to release into the oven.
Cob oven wall thermal capacity
Thermal capacity is the measurement that defines the allowable cooking time in your oven. Different cob mixtures or recipes will have more or less capacity depending on what ingredients are used in the cob mix.
A cob wall that is entirely solid (has no air pockets as insulation) will allow heat to soak into it well. It will also release that heat slowly once the fire has been removed.
A cob mix can hold thermally beneficial additives like porous granular pebbles such as scoria. This is introduced into the cob mixture when the oven is being built. It can supercharge the oven walls’ ability to hold heat internally within the wall structure for longer. This makes the cob oven more thermally efficient. We have an article dedicated to this subject titled “can scoria be used in cob mix?“. We recommend reading it.
The second phase of using the oven begins once the fire has been removed from the chamber and is when food can be introduced into the oven.
The food is cooked by reversing the heat flows from out of the oven wall and back into the oven chamber, so in effect, the food is cooked by thermal radiation escaping from the wall.
This is where the oven wall thickness is vital to get right when the oven is built. The better the cob oven wall can retain the heat, the more efficient the oven is. This means more cooking can take place. The importance of wall thickness is discussed in this article titled “How thick does a cob oven wall need to be?“
Obviously, the longer cooking times that can be achieved from a fire, the less wood that should be required to run the oven. If the walls retain and then release the heat more efficiently, the cheaper the oven will be to run over time, assuming you buy in firewood.
How does a rocket stove oven work?
A rocket stove cob oven is very similar to the heat retention process of the oven chamber.
The difference between the rocket-fed oven vs. the standard oven described above is how heat is introduced into the chamber. The typical method of this oven is to have the oven chamber on a raised platform of some description with the heat source beneath this platform.
The fire pit is placed at ground level at the edge of the support platform, and the connected burn chamber runs horizontally towards the center of the oven structure, still at ground level.
The burn chamber then turns upward and rises into the base of the oven chamber itself. This section, where it turns from the horizontal plane to a vertical plane, is a chimney where the smoke burns off once the fire reaches the required intensity.
The resulting noise when the fire reaches its full potential is where the name “rocket stove” is derived from.
What are the benefits of a cob oven.
The main benefit of the closed chamber cob oven is the extended cooking times that this style of oven potentially offers. Anecdotal evidence suggests food can still be cooked the day after the main cooking event. Each oven is different, so use that information wisely.
Another benefit of the cob oven is the simplicity of the design. Building a cob oven is a straightforward affair if the few basic design requirements are sorted out before the build takes place.
If you need help with the design process, we have an article titled “How do you design a cob oven?” which might help with the build design process.
What are the benefits of a rocket stove cob oven?
There are a couple of benefits that this type of oven offers. The rocket stove cob oven’s most significant advantage is its cooking methods versatility.
Because heat is introduced via the chimney below the oven floor, the air within the oven becomes hot very quickly. This slowly transfers into the oven walls like the standard cob oven.
Because the air is already hot and the fire is external, there is an opportunity to commence cooking once the oven temperature has been reached. This can be as short as ten minutes from lighting the fire.
While the food is cooking, the heat is also transferred into the oven walls, allowing the oven to act just the same as a standard cob oven once the fire has been put out.
For this to be effective for food cooking, the rocket stove heat source must be run for an extended period. Once the oven walls are up to a suitable temperature, the opportunity arrives to use this type of oven as a dehydrator. We go into this in detail in a post titled “Can a Cob Oven dehydrate food?” and it also covers some downsides.
Another significant benefit of the rocket stove Cob Oven is the ability to cook food safely using just a tiny amount of wood. This is beneficial if the oven only needs to be run at 180C for just a few hours.
The cost of firewood can be far lower than the standard Cob Oven if short cooking times are required. It balances out when the rocket stove is run for extended periods.
What are the disadvantages of a Cob Oven?
The primary disadvantage of a standard Cob Oven is the amount of firewood required to operate it as a food-cooking unit.
This is associated with extended waiting periods as the oven needs time to transfer heat from the fire into the walls and base. This oven is made for entertainment events and isn’t very time efficient.
What are the disadvantages of a Rocket Stove Cob Oven?
A couple of disadvantages come with the rocket stove Cob Oven. The first and main disadvantage is the intricate/complex design and the potential for poor heat flow at the fire pit end (aka draw). It is easy to get wrong.
The second disadvantage is the need to tend the fire constantly. Because the fire pit is connected to the chimney with a horizontal burn chamber joining them together, the wood is consumed quickly when the fire reaches optimum intensity.
You need to tend the fire often because the fire pit is small. It is a heat engine you need to feed frequently. There is another downside that needs mentioning.
Because the heat is coming from an active fire below the oven, some light ash will enter the chamber when the fire is disturbed. Wood that is a softwood also can create some smoke at times.
What kind of cob oven is best?
The best cob oven is one that meets your needs as designed. It does not matter which style you choose because they are both effective.
The traditional cob oven with the internal fire can add an ambiance that the rocket stove style cannot match.
The rocket stove cob oven can be more fuel efficient and allow simple outdoor cooking quickly at any time without having to burn 30-40Kgs of wood to begin cooking.
The meals that can come from a cob oven are often very memorable. Shared with friends and family, they become a focal point for good conversation and fellowship.
You could do far worse than beginning the process of designing your very own cob oven.
If you are looking for more information on cob oven design, construction, and use, we have many articles here on site. You can access them all in one place by using the search icon at the page top and entering “cob oven.”
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.