[Stoves] Maputo Ceramic Stove material

[Crispin Pemberton-Pigott]

Dear Friends

I have received a thermal expansion test on the material used for making the MCS.  It is made up of 3 additives and 3 local ingredients.  It was hand mixed with a spade in Maputo, Mozambique.  The moisture content was checked for each of the 6 ingredients and compensation made so the dry mass was as close as possible to the 'ideal' or better said, the target provided by the lab.

The native clays have a thermal expansion of about 7.8 x 10^-6 when fired to 1150 to 1200 C.  This can be considered 'normal' in the range of 0-700 C.  The bonding is ceramic.

The best mixed material PK11 done at the lab has a thermal expansion of 2.3 x 10^-6.  The test result of a part cut from an MCS stove body is 2.6 x 10^-6 showing that in the field it is possible to get a very low thermal expansion material made in a very humble environment largely from local materials.

The thermal stresses imposed by a fire on a clay container are largely defined by the thermal expansion.  Thus a clay stove body run to any temperature made from the regular clay compared with the PK11 is 7.8/2.6 times more likely to crack than the mixed material.

If one wanted to make a ceramic insulative stove component it would be good to start with a mixed material with a very low rate of thermal expansion so that it did not erode, day by day, as it heated and cooled.

The work shows such things may be possible.  We have done this in a difficult working environment with very little in the way of facilities to assist any process.  It is our hope to reproduce this in Lusaka in the coming months showing that the experience gained will speed the development of a strong, stress-lowered clay body with a far longer life.

In addition to the thermal expansion, PK11 is extremely strong allowing us to make the stove as thin as 10 to 12mm in places.  This reduces drying times, lowers the firing cost and creates space in the kiln to make more at a time.

The cost of the material as mixed is about $0.20 per kg.  The single pot stove will weigh between 4 and 5 kg total.

Regards
Crispin