Combustion Chamber Options

Option #1: Baldosa Tile

In Guatemala there is a Baldosa floor tile that makes an economical long lasting combustion chamber. It is easily worked with a hand saw. Figure 1 shows a three dimension view and Figure 2 shows the dimensions. The tiles are about 29.3 x 29.3 x 5.6 cm with holes as shown in Figure 1. These holes should be vertical, except the top and bottom, so that the heat will rise to the container. Note that the bottom does not reach the end of the sides, which can be filled in with mortar and left over tile or other pieces.

Three deminsional view of Chamber Dimensions of Chamber
Figure 1 Figure 2

Option #2: Insulative Ceramics

Source: Aprovecho document. See for more information.

These recipes are intended to assist stove promoters in making insulative ceramics for use in improved wood burning cook stoves. Each of these materials incorporates clay, which acts as a binder. The clay forms a matrix around a filler, which provides insulation. The filler can be a lightweight fireproof material (such as pumice, perlite. or vermiculite), or an organic material (charcoal or sawdust). The organic material burns away, leaving insulative air spaces in the clay matrix. In all cases, the clay and filler are mixed with a predetermined amount of water and pressed into forms (molds) to create bricks. The damp bricks are allowed to dry, which may take several weeks, and then fired at temperatures commonly obtained in pottery or brick kilns in Central America.

Our test samples were made using low-fired "raku" clay obtained from a local potter's supply store. In other countries, the best source of clay would be the kind used by local potters or brick makers. Almost everywhere, people have discovered clay mixes and firing techniques, which create sturdy ceramics. Insulative ceramics need to be lightweight (low density) to provide insulation and low thermal mass. At the same time, they need to be physically durable to resist breakage and abrasion due to wood being forced into the back of the stove These two requirements are in opposition; adding more filler to the mix will make the brick lighter and more insulative, but will also make it weaker. Adding clay will usually increase strength but makes the brick heavier. We feel that a good compromise is achieved in a brick having a density between 0.8 gm/cc and 0.4 gm/cc. The recipes in Table 4 indicate the proportions. by weight, of various materials. We recommend these recipes as a starting point for making insulative ceramics. Variations in locally available clays and fillers will probably require adjusting these proportions to obtain the most desirable results. Insulative ceramics used in stoves undergo repeated heating and cooling (thermal cycling), which may eventually produce tiny cracks that cause the material to crumble or break. All of these recipes seem to hold up well to thermal cycling. The only true test, however, is to install them in a stove and use them for a long period of time under actual cooking conditions.

  Filler Clay (damp) Water Fired at Density
Type Wt. (Grams) Wt. (Grams) Wt. (Grams) (Degrees C) gr/cc
Sawdust 490 900 1300 1050 0.426
Charcoal 500 900 800 1050 0.671
Vermiculite 300 900 740 1050 0.732
Perlite Mix 807 900 1833 1050 0.612
Pumice Mix 1013 480 750 950 0.770

In this formulation, fine sawdust was obtained by running coarse sawdust (from a construction site) through a #8 (2.36-mm) screen. Clay was added to the water and mixed by hand to form thick mud. Sawdust was then added, and the resulting material was pressed into rectangular molds. Excellent insulative ceramics can be made using sawdust or other fine organic materials such as ground coconut husks or horse manure. The problem with this method is obtaining large volumes of suitable material for a commercial operation. Crop residues can be very difficult to break down into particles small enough to use in brick baking. This method would be a good approach in locations where there are sawmills or woodworking shops that produce large amounts of waste sawdust.

In this formulation, raw charcoal (not briquettes) was reduced to a fine powder using a hammer and grinder. The resulting powder was passed through a #8 screen. Clay was hand mixed into water and the charcoal was added last. A runny slurry resulted, which was poured into molds and allowed to dry. It was necessary to wait several days before the material dried enough that the mold could be removed. Dried bricks were fired at 1050°C. Charcoal can be found virtually everywhere, and can be used when and where other filler materials are not available. Charcoal is much easier to reduce in size than other organic materials. Most of the charcoal will burn out of the matrix of the brick. Any charcoal that remains is both lightweight and insulative. Charcoal/clay bricks tend to shrink more than other materials during both drying and firing. The final product seems to be lightweight and fairly durable, although full tests have not yet been run on this material.

In this formulation, commercial vermiculite (a soil additive), which can pass easily through a #8 (2.36 mm)
screen, is mixed directly with water and clay and pressed into molds. Material is dried and fired at l050°C.
Vermiculite is a lightweight, cheap fireproof material produced from natural mineral deposits in many parts of the world. It can be made into strong, lightweight insulative ceramics with very little effort. The flat, plate-like structure of vermiculite particles makes them both strong and very resistant to heat.
Vermiculite appears to be one of the best possible choices for making insulative ceramics.

Perlite Mix/Clay:
For best results, perlite must be made into a graded mix before it can be combined with clay to form a brick. To prepare this mix, first separate the raw perlite into three component sizes: 3/8' to #4 (9.5 mm to 4.75 mm), #4 to #8 (4.75 mm to 2.36 mm), and #8 (2.36 mm and finer). Recombine (by volume) two parts of the largest size, one part of the midsize, and seven parts of the smallest size to form the perlite mix. This mix can now be combined with clay and water and formed into a brick, which is dried and fired. Perlite is the mineral obsidian, which has been heated up until it expands and becomes light. It is used as a soil additive and insulating material. Perlite mineral deposits occur in many countries of the world, but the expanded product is only available in countries that have commercial "expanding" planes. Where it is available, it is both inexpensive and plentiful. Perlite/clay bricks are some of the lightest usable ceramic materials we have produced so far.

Pumice Mix/Clay:
Pumice, like perlite, produces the best results when it is made into a graded mix. Care should be taken to obtain the lightest possible pumice for the mix. Naturally occurring volcanic sand, which is often found with pumice, may be quite heavy and unsuitable for use in insulative ceramics. It may be necessary to crush down larger pieces of pumice to obtain the necessary small sizes. The mix is prepared by separating
pumice into three sizes: 0.5 inch to #4 (12.5 rnm to 4.75 mm), #4 CO #8 (4.75 mm to 2.36 rnm), and #8 (2.36 mm) and smaller. In this case, the components are recombined (by volume) in the proportion of two parts of the largest size, one part of the midsize, and four parts of the smallest size. Clay is added to water and mixed to form thin mud. The pumice mix is then added and the material is pressed into molds. Considerable tamping or pressing may be necessary to work out the air and form a solid brick. The mold can be removed immediately and the brick allowed to dry for several days before firing. Pumice is widely available in many parts of the world and is cheap and abundant. Close attention to quality control is required, and this could be a problem in many locations. It is very easy to turn a lightweight insulative brick into a heavy non-insulating one through inattention to detail. Pumice (and perlite as well) is sensitive to high heat (above 1100°C). Over-firing will cause the pumice particles to shrink and turn red, resulting in an inferior product. Despite these concerns, pumice provides a great opportunity to supply large numbers of very inexpensive insulative ceramics in many areas of the world.

There are many viable recipes to make lightweight refractory ceramic combustion chambers. Using insulation around the fire helps to boil water more quickly, makes the stove easier to light, and saves firewood. It is necessary to create very high temperatures in a combustion chamber in order to clean up dangerous emissions. Unfortunately these high temperatures quickly degrade metals including stainless steel. Refractory insulative ceramics provide a material that is both long lasting and does not lower combustion temperatures as do materials with a higher thermal mass.