The cob oven operates in extreme conditions with intense fire in the oven chamber, while the external surface should only be warm to the touch.
This temperature difference is no easy design feat, but the properties of the cob mix show how some natural building methods can outperform many modern materials.
The cob oven relies on the fibrous strength of straw to bind the mix together, showing how simple this building method is. But in a cob mix, are all straws the same?
Any grass stalk that has tensional strength in the stem section is able to be used in a cob oven mix recipe. Coconut husks can also be used.
Why does cob need straw in it?
Cob needs straw or similar in the mixture to reinforce the structure once the cob has dried and hardened.
Straw acts much like the way reo-bar functions inside a concrete slab. The difference besides the strength factor is the easy availability of stalky grass fibers and the zero cost of the material.
Because a typical cob oven is not subjected to demanding physical loads that concrete often is, cob does not require such a stiff reinforcement method.
There will be thermal expansion of the oven when it is being used to consider. As the oven heats up, the oven will expand a little, and as it cools back down, the oven will then contract.
This movement would likely create excessive cracking in the oven structure if hard steel were used. It would expand with the cob but not at an equal rate. Grass fibers will have some flexibility always available.
Will cob last without straw or hay in it?
A cob mix without straw or a grass fiber is not an actual cob mix and will not last. Cob is the name of a blended natural product that requires clay, sand, and straw/hay/grass fiber.
Besides the wording, a mixture without the grass stalk component really becomes some version of mud. and can easily decay.
In the context of a cob oven, the oven chamber will be able to be formed by a clay mix without the fibers, but the structural integrity of the cob oven will be far less.
When a crack appears, there will be nothing to hold the two sides together, so there is every chance the damage will become worse. This will inevitably lead to the oven failing.
An excellent example is this termite mound that has been abandoned on at least one side of it. It is a large mound about 6 feet high and the same across.
When a termite mound is active, the termites are constantly repairing the external shell when either rain or nest attack from animals occurs. Cracks rarely have a chance to grow.
While the nests rarely crack apart, this is more because of the mound’s physical mass rather than the superior clay mix. Termite nests are not a solid mass of mud but a complex labyrinth of chambers and tunnels.
This helps regulate the internal temperature if the nest. Once the mount is totally abandoned it falls into decay and it will eventually collapse.
If there was some internal reinforcing incorporated into the structure, the collapse would take far longer, and it would be more of a dissolving away.
How long does the straw need to be?
The straw fiber should be at least 12inches (300mm) long to be effective. This allows the cob clay component to attach to the fiber with enough grip to reinforce the matrix as a whole.
When you build an oven, the straw fibers are not laid in only one direction. They should cross over each other everywhere to create a multi-directional mat. Short straw lengths will make for a less robust matrix. However, this will be superior to an oven shell with no fibers.
Does the straw in cob need to be dry?
Straw that is used in a cob oven mix should be dry. The reason for this is wet or still green grass fibers are likely to rot inside the cob wall.
The risk of using green fibers is the potential failure or shortened life of your oven. Having rotted grass fibers within the wall of your oven can lead to cracks, as described above.
We should state that the fibers that should be used are the stalks with some leafy material. Not mostly leaf with just a little stalky material.
When you mix your cob blend, there will be some water added to it. This will soften the clay and make the mix plastic in nature.
This should not affect the straw stems at all. Once the oven shell has dried enough for the first fires to be lit, any moisture introduced will be driven out of the cob wall.
On the other hand, if the green straw is used, the moisture in the stems will be more difficult to remove because it is cellular and not transient, similar to added water.
There are nuances to consider in everything here so to be sure, just use dry straw stems.
Will the straw inside a cob oven wall burn?
For a fuel source to burn, oxygen needs to be present. Because the straw is encapsulated in clay and sand in a solid matrix with little air available for ignition, it will not burn.
There are centuries of historical cob construction to lean on regarding the fire resistance of cob, so we don’t need to reinvent the wheel here.
When the cob mix is placed on the shell of the oven during construction, remove all the possible air pockets as you go and the oven will be fine. Keep the wall thickness over 4 inches (100mm) to ensure that there is strength, fire resistance, and good thermal capacity.
This article goes into wall thickness in more detail. It’s titled “How thick does a cob oven wall need to be?“. We recommend reading it. For information on thermal capacity, we also have an article titled “Can scoria be used in a cob mix?”
When building a cob oven, it is easy to overlook a simple thing like straw. The attributes that such a simple item can offer when mixed with other appropriate materials should not be ignored.
We have many articles here on site if you are interested in more information on cob oven design, construction, and use. You can find them all at one location using the search icon at the top of the page and entering “cob oven.” or simply via the menu.
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.