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close this book Boiling Point No. 31 - August 1993
View the document Clays for Stoves
View the document Effects of the Clay/Non Clay (C/NC) Ratio on Stove Behaviour
View the document Clay/Non Clay Test Procedure
Open this folder and view contents Clay Testing - 5 Country Reports
View the document Clay Preparation Techniques
View the document The Sudanese "Muddy" Stove
View the document Cement Stoves from India
View the document The Chencottai Chulah
View the document Zambian Double Wall Clay Stove
View the document GTZ News
View the document Kenya Downdraught Kiln for Stove Liners
View the document The KCJ - from Artisan to Factory
View the document Working with Village Women in NWFP, Pakistan
View the document Smoked Maasai
View the document Kachel Ovens
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The Sudanese "Muddy" Stove

by Sosan Elhassan, Energy Research Institute, Improved Stoves Centre, Khartoum, Sudan

The Muddy stove was found by the Special Energy Project (SEP) to have good thermal efficiency but poor durability when transported and tended to burn rather quickly when fuelled with charcoal made from cotton stalks. The project concluded that if the "muddy" material could be improved the stove could be marketed and help towards achieving the SEP's overall aim of replacing wood charcoal by cotton stalk charcoal as well as lowering energy consumption.

The clay used to make the muddy stove is known as black cotton soil. It is highly plastic and swells when water is added. The potters mix donkey dung with it in the ratio 1:1 and leave it to set for 24 hours before using it to make water cooling pots (Zeers) or stoves. The zeers are fired around 700C but the muddy stove is used unfired.

The donkey dung is believed to act as a fibrous binder which accounts for the clay's green strength. It also helps to increase porosity, and reduce shrinkage and cracking as well as providing heat when it burns out during firing. The potters were taught their skills by their forefathers and do not use sand or any other additives.

Using very simple test methods the clay from the Gezira region was identified as Montmorillonite with the properties given in Table 1.

Table 1- Some Properties of Gezira Clay

Property

Value

Shrinkage

13 1%

Swelling

38%

Sand Content

only some small stones

Clay Content

very high and fine

Firing at 800 to 900C

spelling & disintegration

Firing above 1000C

vitrification

When a muddy stove brought from Katfia Village was fired at 800 to 900C, the clay proved unsuitable for making stoves.

During firing, the stoves experienced shape deformation (partial fusion), cracking and spelling. Moreover the strength of the stove decreased considerably causing breakage on handling. Close investigation of the broken stove revealed the presence of round white granules which presumably cause the body spelling and weakness. This proves the presence of lime which after firing absorbs atmospheric moisture and consequently expands in volume.

Because the clay was the only type in the Gezira region and importing clay from another area was too costly, it was considered necessary to try to improve the existing clay with additives.

Experimental Procedure

Small stones (up to 3mm in size) and white granules were removed by soaking the clay for 2 days in water, producing a thin slip. The slip was then filtered through a 24-30 mesh screen in mosquito netting. The sand or additives were then added and the mixture was left for a week in order to bring down the moisture content to a workable level. The slip and later the body were thoroughly mixed daily in order to avoid sand precipitation and uneven drying of the body.

After very good wedging the clay body was hand moulded using a two part mould. When the two parts of the stove approached the leather hard condition they were assembled to form the complete stove. At least five stoves were produced from each test body mixture.

Preliminary experiments proved that punching the stove grate holes leads to uneven drying of the bottom part of the stove which develops cracks). Therefore the stove grate is partially removed leaving an open area which will accommodate a wire mesh grate. Most cracks occurred at the bottom (base) and at the joint between the two parts.

In order to avoid cracking during drying the stoves were left in shade until completely dry. Sometimes covering with hessian fabric was necessary as cracks started developing at early stages of drying. After complete drying the stoves were fired at the temperature range of 800-900C.

Thermal shock tests were not considered at this level as we were mainly concerned with finding an appropriate mixture that would not crack either during drying or during firing.

Test Results

The main parameter used to judge the suitability of a body mixture, at this level, is cracking percentage during both drying and firing. The following Table 2 presents the results of several tests using clays containing different proportions of clay, sawdust, sand and dung.

Table 2 - Clay Mixtures & Stove Body Test Results (values in %)

No. of Mixture and Composition (ratio)

Material

A

B

C

D

E

F

G

K Clay

3

4

3

3

1

3

2

Sawdust

2

0

2

0

0

0

0

Sand

0

1

0

2

1

4

1

Donkey

0

0

0

0

0

0

1

Dung

             

% crack in drying

100

100

80

60

40

50

0

% crack in firing & after

-

50

100

100

40

80

60

Fragility

-

MF

-

VF

VF

VF

MF

F - Fragile

MF - Moderately Fragile

VF - Very Fragile

Conclusions

The muddy stoves as made by the women from Katfia crack and virtually disintegrate when fired. This explains why the women do not fire them. The Zeers stoves, although made from the same material, have a lower strength after firing than in the green state, but cracking or spelling are not evident. The firing temperature for Zeers may be as low as 500 to 600, ie. some 200 300C lower than the trial stoves were fired at.

It is likely that firing stoves to the lower temperatures will help reduce some cracking and spelling but we do not feel the ceramic will be sufficiently strong to withstand cooking. The inclusion of sand was beneficial but was not effective enough. It appears that clays containing montmorillonite are unsuitable for making good quality pottery stoves. If such stoves are to be strong enough to withstand normal handling and cooking practices which cause thermal fatigue and shock during use, then the clay must be kaolinite and able to withstand temperatures up to 1200C.