November 2007 Update

Pottery Rocket Stoves - Now at the ECC

The Eastleigh Community Centre, of Nairobi, Kenya, ( is beginning production of several new products, shown in this update. The picture at the right shows a section of a pottery rocket stove, under construction. Rocket stoves are a highly fuel efficient alternative to the other stoves in the markets.

Reduced fuel consumption and portability of stoves are urgently needed, important in addressing those environmental and health issues that are implicit to cooking with such fuels as wood or charcoal. For example, in the slum of Mathare, opposite the ECC, many residents cook indoors, using less efficient stoves, and as a result there is a serious issue of respiratory illness. Outdoor space in such communities as Mathare is hard to come by and to monitor, but outdoor cooking with such fuels is always preferrable, in a well ventilated area.

The high fuel efficiency is partly because the short, chimney section of the design causes the flame to be sucked in, and the cook pot is heated both from the bottom and the sides. The sizes of the internal spaces of the stoves should also be optimized.

For the section of the stove shown, the first course of brick has been laid, using a mortar of 50% clay and 50% sand. After the sheet metal is tightened to the diameter of the stove, additional mortar may or may not be pushed into the space behind the brick. This section should be tight, with good integrity.

For the rocket stove of the ECC, the amount of fuel needed should also be greatly reduced because the curved brick used, as shown above, is insulating. The light weight of the stove sections also makes these portable. The curved brick is made to be insulating by adding a lot of fine, wood sawdust to the clay composition. This sawdust burns out in the firing of the brick, leaving voids, and this contributes greatly to the insulating properties, as well as the light weight.

For the pottery rocket stove shown below, the diameter of the cook pot is 30 cms. Note that the side of the pot is a uniform 12 cms. (~1/2 inch) from the inner wall of the stove. Thus there is very little loss of heat energy. This is one of several reasons why the amount of fuel required is very low.

The pottery rocket stove is portable in the smaller models, for cook pots of under 40 cms. diameter, approximately speaking. The environmental and health considerations of improved stoves address important issues on the household scale, just as is the case with pottery filter systems.

It is important to note that the preferred method of delivering water is when it is piped in, from a pure source. Similarly the use of wood and charcoal as fuel is not a preferred practice. Rather it would be better to prepare meals with a gas stove or a solar cooker.

But thus far, for the environmental, best practices of cooking, the issues of effectiveness, price and user-acceptability have proven to be difficult to address, so the use of gas or solar have been slow to take root. In the meantime it is urgent to reduce consumption of fuel wood and charcoal to a bare minimum, as is the case with the use of the rocket stove.

A Kiln Design that is Traditional  in Rural Kenya Helps Facilitate the Firing of Stove and Filter Components

The kiln shown below has been in common use in some of the rural potteries of Kenya, and this design is now proving it's usefullness at the pottery of the Eastleigh Community Centre. Kilns of this type typically fire to no more than 500 or 600C, however this temperature is quite adequate in achieving good strength and the over all integrity of a stove. Similarly the kiln is adequate for most of the water and sanitation applications.

The kiln shown above is constructed using common red, construction brick. The bricks are mortared together using a 50/ 50 mix of clay and sand.

Note that in a number of locations, conventional ceramic wisdom has put a lot of emphasis on the need for a higher firing temperature for pottery stoves and other pottery ware, closer to 1000C, earthenware temperatures. This emphasis is largely because such higher temperatures are normally thought of as being important in giving better strength. But this viewpoint appears to be erroneous.

The reason that pottery rocket stoves can be fired at 500C is that the strength needed tends to be compressive strength and not fracture strength. One result of this is that many of the products shown can be fabricated using the simple technologies of the rural potteries.

The picture below shows the inside of the the outdoor kiln, and this shows a one-piece, pottery rocket stove at the bottom of the image. Also in the kiln are the curved bricks of the stove that is shown under construction at the header of this update. The outdoor kiln will also contain pottery grog powder, which has been treated with silver nitrate. Thus the nitrate will be burned off.

For the outdoor kiln there is an added advantage to the use of the 50/ 50 clay to sand mortar, in that this does not make a permanent bond, as is the case with concrete mortar. Because of this the kiln can be dismantled for rebuilding in other sizes, as the needs arise. So there is a flexible approach to kiln design and size.

Shown above is the outdoor kiln, prior to firing.  The ware of the kiln is covered with grass or leaves, then this is covered with mud.  Note the openings at the base of the kiln, for the introduction of the wood fuel.

Introducing a New Design for a Disk System

The pottery disk system shown below is intended to be a low cost version, less expensive than the candle systems promoted by the ECC to the present. This disk system consists of a pottery upper container with a food-grade plastic lower container.

The disk of the system shown is mortared into place using common, portland concrete mortar. For such mortar the pottery-to-pottery bond is a simple and very effective one.

Pottery disk systems like the one shown will be inexpensive, priced at KShs. 450/- (US$7.00).

Important to the user of this disk system is an understanding that prior to filling the upper container with contaminated water, the treated water of the lower container should be decanted. Not to decant this water could cause an overflow, once the water of the upper container passes through the disk, however the transparency of the lower container allows for a simple visual check.

Pottery End Caps for Candle Filters Cut Costs

The pottery candle shown at left is in the upside-down position. The candle system consists of upper and lower containers, both of which are food-grade plastic, and the candle is placed in the bottom of the upper container. To the present, however, the candle end caps of the ECC have been stainless steel, making the systems expensive. Pottery end caps have now been introduced, as a much less expensive alternative.

The candle having pottery end caps is shown at left, in the upside-down position. The target price for the candle system is KShs. 700/- (US$10.50).

Treatment Focuses on the Use of Silver Nitrate

The use of colloidal silver, for treatment of ceramic filters, is an excellant option, but for many prospective situations this material may not be readily available. The use of silver nitrate gives an appropriate and simple approach. The candle of such treatment is then re-fired, to the low temperature of 500C, such as in the outdoor kiln shown above, thus driving the nitrate off.

In many locations silver nitrate is available, wherever reagent chemicals are sold, and university chemistry departments are familiar with the shops where silver nitrate can be purchased. In case the silver nitrate is not available, or is very expensive, it is quite possible to make ones own, by boiling pure silver in nitric acid. In this case it is necessary to follow simple safety procedures, venting the fumes of the nitric acid, and handling the salts of the silver nitrate with rubber gloves.

The candle filter shown above is being saturated in dilute silver nitrate.  Because the filter medium is open there is a 'wick effect,' wherein the solution is quickly sucked into all parts of the medium.  Subsequent firing to 500C, in the outdoor kiln above, causes the nitrate to burn off.

Introducing a Sanitary Pit Latrine Liner

The ECC has also put together a first sanitary pit latrine liner, thrown on the wheel. For this latrine liner the upper surface is to be treated with silver chloride.

The simple two step process of the silver application is, 1. to paint dilute silver nitrate onto the upper surface, and within the cylinder, then allowing the liner to dry, then, 2. to paint salt water (table salt, i.e. sodium chloride) onto the same surface. An ion exchange results, with relatively insoluble silver chloride resulting. So that surface should be free of pathogens for a long period. (See the picture below, with caption.)

The sanitary pit latrine liner, shown above, is in the upside-down position. This has been thrown on the wheel in the stoneware pottery of the ECC. The upper surface is to be treated in a two step process with silver chloride, an inexpensive disinfectant.