Using Cob Ovens : 11 questions answered

Published

We are very fortunate because we had the time and resources to build a cob oven from termite mounds in December 2014. The oven is now seven years old and is in its prime.

Our oven is an unusual type with a base that feeds hot air to the internal area from below the dome, and this is called a rocket stove base.

Because we use the oven for more than just cooking, we thought we would share these wonderful ovens’ many uses and maybe convince others to include them in their homes.

We are also asked many questions when people see the oven in use and are curious about it. Some common questions about cob or mud ovens are answered below.

1. Is cob the same as mud?

Cob is very different to mud although there is often some within a cob mixture.

Mud is typically wet earth, and it varies with geography. You want to avoid wet dirt and look for clay as this material is the best for the build.

Cob is a recipe that contains certain materials like clay, sand, and scoria; sometimes, lime is added.

When building, natural fibrous material are mixed with clay before it is applied and can be grass, straw, hemp, etc. Clay is very stable once mixed like this then dried, and can last decades if protected from the elements.

Cob is a very dense, heavy material with the mass to hold high temps for hours. The clay can be thermally enhanced with scoria, as mentioned above, and “Can scoria be used in a cob mix?” explains the benefits in greater detail.

2. How long does a new oven take to dry?

Without lighting a fire inside the dome, the typical cob oven can take months to fully dry; with a slow fire inside, it can take a day or two.

There are many variables to the drying process; the main ones will depend on the wall thickness of your dome and the available airflow.

A typical mud oven will take longer than a rocket-stove-fed oven because of the airflow dynamics of the rocket-stove variation. Many ovens don’t have a chimney and only vent through the front opening.

Affordable Oven Plans Available

This can trap the moisture (clay is wet during the build) within the oven, and the moisture will have to be driven out through the wall to the outside surface. The fire will remove some moisture from the internal surfaces, but most will be external.

Drying a chamber with a rocket-stove base should be faster, and you can measure it as it happens.

In our build, it took 12 hrs.’ of use before we saw results. Our oven has a rocket stove base, so we could measure the process accurately with a thermometer placed inside the oven while operating.

The first two 4 hrs runs didn’t see the temperature rise above 180C, no matter how hard we pushed the hot air into it.

What was happening was the moisture in the cob was offsetting the incoming heat, and the humidity was limiting the temperature.

On the third run, we saw the temperature start climbing, and after about 4 hrs, we saw temperatures above 240C.

We now have it up to temperature in ten minutes as it is completely dry. The more you use them, the better they get until they are dry, stable, and sound. The wall thickness affects how quickly the cob will dry out and be ready for use. To understand the optimum dome design, “How thick does a cob wall need to be?” is suggested reading.

3. How much timber does a oven burn?

The amount of wood a pizza oven uses will vary with oven size and the type of wood available. An average-sized build will use about 35-40 kgs of dry hardwood for a complete firing.

A rocket-stove base will use 15-20 kgs of the same wood, with a slightly shorter cooking time. A rocket-stove-based build’s airflow dynamics are very different from a closed chamber cob dome. These different dynamics can limit the heat the clay walls absorb and store and depend on how long the fire chamber is lit and active.

small kindling for cob oven
Kindling on top with some very hard ironbark rounds.

4. What kind of wood is used in a wood-fired oven?

The best fuel to use is very dry hardwood. The denser the timber, the hotter the coals, and the longer the fire will burn.

While good quality hardwood can be expensive, it is far more efficient than softwoods. Anything wet should be avoided as it will not generate the heat you need to cook.

Our personal experience with different woods has proven the above to be true.

If the firewood is semi-hardwood, the cooking time can be almost double the time that good dry hardwood will take. It can be smoky, and the fire struggles to get hot.

This means we will use twice as much fuel and more time to achieve a satisfactory result.

hardwood used in a cob oven
Assorted offcuts of mixed species hardwood.

5. Can Cob ovens be repaired?

Due to the simplicity of the recipe and ease of building, cob ovens can easily be repaired.

A typical dome will expand and contract with use over time, and this can cause small cracks in the clay dome.

Generally speaking, the cracks are nothing to be concerned about, and they won’t cause the oven to work any different from one that is dry.

The risk of damage is in the initial drying stage, and it occurs when the owner/builder of the oven places too large a fire in the chamber at the first firing. This can create a hard, dry layer on the internal surface while the rest of the cob wall is still moist.

This can lead to structural damage in a bad case.

water damage of a cob oven
Cracks in the base where the oven sits on fibro-cement sheeting.

6. What can be made in a cob oven?

We have used ours to make many things. For some people, the first thing made is a wood-fired pizza and is usually one of the easier recipes attempted.

Over time the list of things made in your oven will expand. This is a list of the things we have made in our oven.

  • Bread
  • Cake.
  • Biscuits.
  • Pies.
  • Pizza.
  • Dried fruit and chillies.
  • Fast roasts
  • Slow roasts.
  • Dried craftwood for craft items.

The list is not limited to what “can” be made in a typical build as we have only shown what we have used our own for.

We occasionally dehydrate certain items in ours, and as they are all different, there are some risks to using it this way. We address this in an article titled “Can a cob oven dehydrate food?“. If you wish to use it to dry non-edibles, we suggest “what can be dried in a cob oven?

7. Is termite mound the same as clay?

A termite mound is made of dry clay particles held together with excretion to form mud with when building their mounds.

A termite mound is very similar to clay; so crushed termite nest is an excellent ingredient to add to your materials list.

The termites have already screened the larger raw materials and the sand from the ground while building their towers, so you can add these into your cob mix as needed.

We added some scoria to give the cob some structure and body. It helps with heat retention as it is volcanic in origin and is similar to pumice but harder and heavier.

8. Will termite cob burn?

Cob will not burn, because it is created out of earth and that is impossible to burn.

The cob mixture that you make should have some binding materials like straw or similar, which will burn off if they stick out of the cob when it is dry.

The rest of the cob will not burn, but the door can burn if it is made from wood, which most are. This is our door after seven years of use, and it looks like we will be building another soon.

cob oven door
The timber is untreated pine.

9. Will these ovens last a long time?

Cob ovens will last decades if maintained appropriately. Cob is similar to adobe, and there are examples of this that are centuries old.

These structures are primarily clay dwellings in design and are not subjected to the intense temperatures that our ovens are; however if the build is made well and looked after, there is no reason that it won’t last for your lifetime.

We expect our oven to do this. It is seven years old and the clay cob improves with use. We think it is a terrific example of sustainable natural cooking.

A bit of cosmetic repair work to the outer clay layer will have the cob back to its best when we get to it. In the meantime, the oven works away.

10. Will rain damage the cob mix?

Water can damage an untreated or unsealed clay mix, but how much precipitation will determine the amount of damage suffered. It is rare for a cob oven to be damaged beyond repair, meaning most repairs are simple and cosmetic affairs.

What is typical of water damage is that the outer clay layer dissolves, and the clay is washed out of the cob mixture. The effect is a rough sand layer where it was once smoother. Many owners eventually build shelters over the cob to save this extra work.

water damage of a cob oven wall.
Light washing away of the clay component leaving a sandy texture. 7 years old.

The internals of the oven should be ok, as the high temperatures make the cob into a version of clay terracotta.

This is how our oven has acted, but mainly in the fire chamber of the rocket stove section. The dried clay cob glows red when in we are cooking. It looks like the cob is a rudimentary form of pottery.

fire chamber of a rocket stove cob oven.
This fire chamber of the rocket stove base glows red when the fire is well lit and roaring.

To make repairs, mix a thick slurry of clay and sand without any straw, and smear this over the clay surface. It is that easy. Once the clay mix is dry, it will be stable if kept dry.

Do cob ovens need to be covered?

Cob ovens without a hard roof overhead should be covered with a tarp when not in use. Weathering can hasten deterioration of the cob surface over time. Applying a lime plaster coating can give a little more protection but cannot replace a hard roof or good tarp.

11. Do these ovens need a chimney?

The conventional wood-fired oven is built outdoors and without a chimney. However, if you want one indoors we suggest building a chimney because of the smoke. Outdoor oven owners prefer not to have a chimney; the reason for this is for maximum heat retention.

A fire requires a few things to exist. Combustible material and a source of oxygen. Airflow is necessary to maintain the flames.

The classic wood-fired style will have the fire contained within the oven’s chamber with the door removed for airflow.

The maximum heat can then be stored in the walls for later cooking. A chimney will remove a portion of the generated heat through airflow from the fire up through the chimney, which is a waste of heat energy.

Our oven is different. It has a rocket stove clay base that feeds the hot air into the cooking chamber via a clay chimney stack. The height of this chimney is important, and is discussed in the linked article several paragraphs below.

The velocity of the air can be very high and that is what gives the name of a rocket stove. They literally roar when operating at maximum draw. This is why height is important. Build it too short and the rocket-effect is not available.

The air then exits the oven chamber via a few cracks in the door and a baffled vent at the back of the oven chamber.

This vent allows the hot air to escape only from the chamber’s base and not from the top levels of the oven, so the chamber retains the hot air for as long as possible.

We cannot achieve the extreme temperatures that a standard wood-fired design can achieve but this is a compromise we happily made so that the fuel requirement can be less.

We suggest “How do you design a cob oven?” to understand both styles and why they operate differently.

We still reach internal temperatures of 240C with comfort, which is plenty for most styles of cooking. The big payoff for our type of design is the lower temperatures should keep the cob in good condition for far longer, but we have to tend the fire more regularly. It’s an easy task.

Uses for a mud oven.

Our uses are many. We have cooked many meals that are listed above plus the non-food uses that we will mention briefly.

We make and sell handcrafted wooden items at local markets, and sometimes the timber we need at the time is still damp.

We used the chamber as a kiln dryer for the wood, which proved effective.

The temperature variations that the rocket stove style can give when trying to run it slowly are challenging, and as we have only used it once this way, we have not developed a method yet to make better use of the technique.

The takeaway for you here is that the ovens are not just for food but can be a drying cabinet for other materials. We commonly use it for drying herbs, Thai chilies, and our favorite smoky mango muesli. We have even dried wet sand in it.

Conclusions.

In all its styles and versions, a cob oven is a valuable tool in the home. They are cost-effective to build, use readily available natural materials like clay, and are fun as well. Family and friends will enjoy the food and the companionship that these tools seem to generate willingly.

We have many articles on design, construction, materials, and use, here on site. To get them all in one place, go to the top of the page and use the search icon. Looking to make your own build? If you have the space and the urge, we strongly recommend taking that step. You will not regret it.

Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.