Better Bonfire Kiln
Project Snapshot:
Category:Food and Ag
Sub Category:Devices
Location:Kenya
Latitude: N/A
Longitude: N/A
Members Involved:Practical Action (formerly Intermediate Technology Development Group or ITDG) (lead); Keyo Womens’ Group at Kisumu, Kenya; Overseas Development Agency-Joint Funding Scheme
Time Period: 1996
Capital Costs:unknown
Key Words:construction guide, kiln, mud, bricks, reduced fuel consumption,
Websites:
Practical Action – Practical Answers website page:
Summary:
In 1989, Practical Action (then called ITDG), began a project teaching groups in Kenya how to build an improved clay stove called the Upesi stove. The Better Bonfire Kiln grew out of this project.
One of the most important steps when producing clay products is firing. This is when the handmade clay items must be heated to very high temperatures and held there for several hours. Traditionally, firing was done in a bonfire, but this tended to use a lot of fuel and gave mixed results. Practical Action decided to design an efficient kiln that would reduce fuel consumption, provide a consistent fired clay product, and would be easy to build from local materials. This design became the Better Bonfire Kiln. Practical Action also created a complete technical manual, describing how to build this kiln with step-by-step instructions and easy to follow pictures.
When firing clay products in a bonfire, the dry clay bodiesare placed in a pit and surrounded with wood and dry grasses. Then the bonfire is lit and as it burns, a group of people keep adding wood and grass to the fire. After a sufficient firing time, the bonfire is allowed to go out and cool and the clay productsare retrieved. The problem with this type of firing is that it is very inconsistent. It is hard to make sure the bonfire is burning evenly and heating all the clay bodies the same amount. Some would get too hot, while others would not be heated enough. Experiments with the Upesi stove showed that it had to be heated to between 600°C and 700°C and held at this temperature for two to three hours.
It is also hard to control the rate of heating in a bonfire. Either the fire is burning or it is out. This means that the stoves were heated very quickly once the fire was started and cooled quickly after the fire was put out. This rapid heating and cooling caused many of the clay products to crack. Under fired, over fired, and cracked pieces all had to be discarded.
The bonfire also wastes a lot of fire wood and dried grass. Since the bonfire is built around and on top of the clay pieces being fired, all of the heat from the flames above the pieces was wasted.
The Better Bonfire Kiln design solves all of these problems by building a permanentkiln structure. This structure is made up of bricks and mud and has several small spaces at ground levelwhere wood can be placed to build a fire under the kiln. All of the heat from this fire would be directed into a single central chamber. The clay products to be fired would be placed in this chamber, and then the chamber would be sealed with a mud dome. This arrangement would allow potters to add wood slowly to make sure the kiln did not get too hot too fast, and since the camber was sealed it would cool slowly as well.
Practical Action’s technical guide gives a list of materials needed to build kilns of two different sizes. The big kiln can fire up to 120 stoves at a time, where the small kiln can accommodate 30. The guide also lists common problems that can arise during firing, such as underfiring or cracking and gives descriptions of the possible causes and solutions to these problems.
Documents:
Technical manual for kiln construction – better_bonfire_kiln.pdf