No, cob ovens don’t require fire bricks at all. They are useful, but absolutely not a necessary component. There are methods of design that do the same thing as the fire brick at very little cost.
When it comes to building a cob oven, there are many variables that need sorting out and the cooking surface is one of them.
Are you going to use the oven primarily for pizza cooking or are there plans to expand into bread baking, dehydrating, and complete roast meals? All of these are able to be cooked in a cob oven without the need of fire bricks.
They are great, but as you will see, there are alternatives that are quite suitable for the average cob oven builder. The information below might help you come to a design that meets your needs.
How does a cob oven operate?
A typical cob oven operates by acting as a designed heat sink with the ability to cook food.
There are many design considerations to consider when undertaking a cob oven build. Most of the design criteria revolve around thermal capacity and heat retention.
Fire bricks have been used for centuries and have proved to be one of the best materials that can be used for purposes like a cob oven. They are mostly a uniform size, and hold heat similar to the cob. they are simple to place, and will last for years.
Fire bricks are not the only materials that a cob oven can benefit from. The cob base below the fire bricks can have added materials that can create a better thermal mix than simple cob, and we go into this in this article titled “can scoria be used in a cob mix?“. Before you start your build we recommend reading this article.
How do fire bricks work in a cob oven?
Fire bricks work by storing heat and releasing it slowly, allowing food to be cooked from that heat.
There should be certainty in using proper fire bricks and not just general clay bricks as they are prone to cracking and eventually crumble.
While general construction bricks can be used, the effort over time to maintain the ovens base will deter you eventually. They will not last as long as dedicated fire bricks usually but the odd batch will out perform. It is impossible to tell with long term use if they are any good for the oven.
The fires that are often lit in a cob oven can be extremely hot as almost all radiant heat is reflected back at the fire constantly. This radiant heat escapes a fire that is out in the open.
Are there alternatives to using fire bricks?
The alternatives to using fire bricks for a cob oven base include a basic cob floor, and also using pizza stones that rest on the cob base.
While fire bricks are good to have and use, they are not critical in an oven build. The biggest benefit of fire bricks is the ability to cook directly off the oven floor, after it has been cleaned of ash and coals, of course.
The alternatives require something else to cook the food on, like pizza stones, metal trays, and trebuchets. These alternatives allow the cob oven to be made completely from cob, and this will suit many of us as cost can be kept to the very minimum.
A completely different approach to a cob oven is one that we ended up building. It is heated from below via a rocket stove, and we cook with the traditional pots and cooking pans. There is more cleaning up after cooking but that cost is nothing.
You can find details of our build here in this article titled “building a wood fired termite cob pizza oven” that you can look at for ideas if it suits.
What are the benefits of using fire bricks in a cob oven?
The benefits of using fire bricks as the cob oven base are several. Fire bricks can be cut during cob oven construction and so can cover the entire oven floor if desired. This will maximize the cooking surface and this helps with cooking for large groups of friends.
Fire bricks can be very attractive when used in conjunction with similar bricks well laid on the outside, instead of using cob as a wall material.
The expense rises with this process and we will leave this alone for now as it is a detour away from the topic at hand which is cob ovens and fire bricks.
Are fire bricks expensive compared to a cob floor?
Fire bricks can cost around $2 USD or in Australia upwards of $6 per brick. A cob floor can be costless if the clay used is dug from your property.
Cob ovens can be basically build from materials located within your own location, at almost zero cost when no fire bricks are purchased. This fits into a self-sufficient life, and that is why we built one.
Are fire bricks glued in to the cob oven base?
Glue should not be used at all because of the heat involved when the oven is in use. There is very high potential for harmful substances to be released while you are cooking food over the very source of the substances.
Fire bricks should be placed tightly together on a level base within the oven. The gaps should be minimal. The surface below the bricks can be leveled with fine sifted sand that has been leveled off with a straight edge or screed.
Please don’t use any method of glue or cement to try and place them. It is not needed, and can be dangerous.
The design of your oven will depend on the end costs, and while cost can be a deciding factor for many, aesthetics play a part in every ovens end shape and style. Fire bricks are not required in a cob oven, but do suit many designs. It comes down to personal preferences and needs.
We have a sizeable repository of information on cob oven construction here on site and the way to find it all in one place is to use the search tool at the top of the page and enter “cob oven”. You might find something that helps you. To start with design ideas, this post is recommended. “How do you design a cob oven?“
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.