Some may have read my earlier articles on constructing simple efficient wood fired ovens and water heaters. To date the ovens I have described are what are commonly known as “Black” ovens , now the mental image that the description “black oven” conjures up may not be particularly appealing but the reality is I assure you far from it. Basically a black oven is one where the food being cooked is exposed to the combustion gasses of the wood so technically a wood fired pizza oven is considered a black oven. Now most i’m sure will agree that a good wood fired pizza is a beautiful thing. Having almost exclusively used a black oven these last three years for all my families roasting and baking i can attest to how tasty food from one of these ovens can be.
There are a number of broad differences between white and black ovens and i’m going to generalize here so professional pyromaniacs and oven builders please remain calm. Generally black ovens are stored heat ovens ,that is you burn wood in them and the mass of the oven stores the heat of the fire. You then clean the ash out and put your food in, be it pizza, roast or bread (again i’m glossing over a vast body of information here) and it cooks, all the while slowly losing heat. Now we’ve all seen wood fired pizza ovens where there is a fire going in the chamber. This is merely an adaption to the need in a commercial situation to cook for extended times or for ovens without adequate mass to store enough heat in the first place. Where my black ovens differ is that there is no attempt to store heat. You cook while the fire is burning with little or no need for preheating.
Funnily enough though one of the most commonly asked questions is “doesn’t the smoke taint the food “ well to be honest i like the flavor it imparts to the food but by and large if the oven is well designed and tended then there is very little “added” flavor.
As i mentioned earlier we bake in ours, cakes, scones, biscuits lots of things with delicate flavors that wouldn’t benefit from a smoky flavor. You’ll notice i added the caveat “if well designed and tended”. If its not well designed and smokes then it will taint the food ,ditto for wet or green wood or of particular horror if the wood is treated or painted. This cannot be overstated Use Only Untreated Wood. And of course some people simple don’t know how to start or tend a fire (i would hazard to say most) so if the fire is poorly tended again it will smoke.
Now if your with me on this you’ve probably already guessed there is such a thing as a white oven and yes a white oven simply keeps the food and the combustion gasses separated. Now compared to the black ovens i’ve designed using old electric or gas ovens the white oven is a slightly more complex beast to construct. Firstly we have to have a gas tight inner chamber where the food cooks, this is then surrounded by a chamber where combustion gasses flow and heat the inner chamber through conduction, convection and radiation. In the interests of efficiency we also can have an outer insulating layer to trap and make better use of our heat.
So to recap we have black ovens where food is in contact with combustion gasses and these fall into two main categories and one sub category, those that cook with stored heat(mass ovens) those that cook with direct heat(Insulated ovens) and the third category being those that combine a bit of both (mass ovens with a ongoing fire).
Guess what ? White ovens fall into two main categories with a third sub category. Those that cook with stored heat, those that cook with direct heat, and those that do a little of both. Now when i talk about mass ovens and insulated ovens i don’t mean that mass ovens have no insulation, in fact the best most efficient ones do. However the insulation goes outside the mass and keeps the captured heat from escaping to the environment thereby staying hotter longer. By contrast an insulated oven has comparatively little mass and uses the heat of combustion as its produced.
White ovens that use mass to cook use ….well a lot of mass ! This can make them quite an undertaking to construct and because of the long preheat time removes a lot of the spontaneity that is the spice of life. As previously argued http://permaculturenews.org/2012/11/23/so-where-does-the-appropriate-in-appropriate-technology-come-from/ they are also very inefficient when used in the wrong context. So setting this type aside for a future date i want to concentrate on low mass insulated ovens.
But first a quick look at our sub category of mixed white ovens (some mass and ongoing heat).
Mixed white ovens have been around for some time and seem to often incorporate 200l barrels (44 gallon in old speak and 55 gallon if your from the US). I think that this is purely because they lend themselves so well to the concept and are cheap and easy to get.
Above we see a more recent design by Max and Eva Edleson of www.firespeaking.com adapted from similar ovens they saw in South America which use some mass and ongoing heat.
Interestingly enough the ovens don’t appear to use any insulation as such and so must lose a fair amount of heat to the environment. However because of the slow heat transfer rate of the Adobe bricks around the barrel and the large fire chamber, more than enough is soaking into the barrel. Also of interest is that the fire is a traditional sort in that logs are piled in, a fire lit and off you go. As a person who has spent a fair amount of time over the last few years mucking about with rocket stoves the design cries out for a big dose of efficiency in the shape of a rocket combustion chamber, less mass and better insulation.
This Photo is of one of the oldest barrel ovens i know it dates from the 1940’s and is at Moreton Telegraph station in Far North Queensland Australia. Note the similarities to the much later design above. These were know as Ant bed ovens as they utilized the material from termite mounds broken up and mixed with water to form the outer shell. You can clearly see a layer of ant bed (think cob) packed in between the inner and outer drum to create a space for the hot combustion gasses to flow around. It was probably this oven that really sparked my initial interest in wood combustion back in the mid 90’s.
This leads us by a long and tortured path to the brief for my latest design which is a white oven with as little mass as possible using a rocket stove as the heat source. Combining these two features should i hope produce an oven that is very quick to heat up and is very frugal in its use of wood. Its always a mistake to pre-empt a design but i’m guessing this oven should hit 300 deg c (great for Pizza) on very little wood. Stay tuned.
This article is reproduced here with Tim’s permission from his original August 27, 2013 post for NZ’s Koanga Institute there titled “A Whiter shade of pale.”