The whole concept of the cob oven relies on pushing heat into the cob oven wall with fire and holding that heat in the structure for as long as possible. It is logical to maximize the potential of every fire you light in your oven, so wall thickness matters. Thicker walls are better.
The wall thickness of a cob oven should be 100mm or 4 inches at a hard minimum but a cob oven is far more efficient at 6 to 8 inches thick (150-200mm). This can be enhanced further with thermally helpful materials added to the thicker cob mix.
How heat is trapped can vary between ovens and the materials used to build them. As we built our cob oven we spent a lot of time researching what to make it with, and how to go about it.
It is with this experience behind us now, some 6-7 years later we think we can safely pass on our recommendations to the next oven builders.
This article is goes into the reasons why you might want to thicken those walls.
Why oven wall thickness matters.
Cob ovens rely on heat retention to cook your food for you. It makes sense to build into the ovens walls the capacity to accept as much heat as possible without damage, while not making the cost of the oven any more than it needs to be.
We can achieve the heat retention in two ways, and they are not mutually exclusive. You can implement both at the same time to achieve the ultimate in thermal capacity.
The first method is by making the wall of the oven as thick as possible, without compromising the aesthetics of the oven. Many ovens are very appealing and that should still be a major factor in the original design of your oven.
The second method is to incorporate materials or ingredients into the cob mix during construction that will enhance the thermal qualities without compromising the structural integrity of the oven.
The thicker the ovens walls are and the better the cob oven can hold that heat within the walls, the more efficient your oven will be and the more productivity you will obtain from a fire.
It is important to consider the cost of the wood and the return in cooking time you receive from it, particularly when some cob owners are not able to harvest firewood from their own property.
It is not hard to imagine the cost of quality firewood becoming cost prohibitive for some people so it makes sense to take the time and build the absolute best thermal qualities into the oven at the start.
Different cob mixes for heat retention.
Cob is a generalized term for clay mixed with sand, gravel, and some type of straw or hay to help bind it into a stable mass once dried.
There are many different recipes that have been used throughout the ages as this type of construction dates back centuries. The predominant factor with the recipe is the local availability of materials.
The benefit of modern life is the availability of materials not so local and if you are not hung up on the belief that it must be very local only, then the only mix you have available to you is the material you have.
To enhance the thermal qualities of your oven you can only build a thicker wall. This is fine. It will work well and last.
However, if you are looking at the cob oven with an eye for the future and are taking into consideration the probable issues with firewood, it might be a logical step to consider what extra ingredients you can add to the cob mix when you build the oven. To make the most of the opportunity while in a design mind set, here is an article that details the design steps to consider before you build. “How do you design a cob oven?“
This is where scoria can have remarkable benefits when incorporated into the cob mix. To get more information on scoria in a cob oven setting, we have an article here.
Drying a thick cob wall safely.
If the oven walls are thicker and heavier than the standard 100mm or 4 inch mantra that is repeated, then be sure that it will take longer to dry out.
We suggest having a small warm fire run for several days as continuously as possible with extra emphasis on warm. Not Hot.
A well dried oven will last a long long time so it pays to take care at the final hurdle of cob oven construction. If you cannot maintain the fire for a few days in one go then break it up into chunks and run several warm fires over several days in total.
Add an extra day to compensate for the loss of stored heat that is gained by a continuous fire. Either way will be fine, it depends on how soon you plan to use the oven for cooking.
I bet that it is yesterday. Ours was.
Will a thick wall cob oven last longer?
If the cob oven walls have been dried properly there is no reason to believe that your oven will not outlast a standard 4 inch thick oven.
It is safe to say that yes, all things being equal it will last longer.
Will a thick wall cob oven use less wood?
A thick walled cob oven should use less wood over it’s lifetime if measured in the amount of food that could have been cooked using the stored energy that a thicker walled oven will hold.
When your oven has the potential for extended cooking sessions, the oven can then be used as a dehydrator when the temperature has dropped enough to dry food without cooking it. We mention this process in this article here.
How to maintain the wall thickness over time.
This section depends on the location of your cob oven.
If it is located where it is open to the elements then it will be wise to have an annual checkup on the external surface to see if there is deterioration of the cob itself.
This will look like a grainy surface that is rougher than the original finish. This is because rain and weathering have dissolved some of the clay from the cob mix.
This will only happen on the outside surface because once the oven is in full use and can take a hot fire, the inside surface of the oven becomes hard baked and can tolerate some hard treatment.
Building a cob oven is one of the best things we have done within our permaculture system/yard. We built the oven with scoria so know first-hand the thermal qualities this rock can give. This article goes into why scoria is so good.
We have a couple of articles that are relevant to the oven and the first is titled “Building a wood fired termite cob pizza oven” on how we built it and how we use it is described in this article titled “Using a cob oven, 11 questions answered”
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.