DESCRIPTION: Ceramic stoves crack due to thermal stress, but they can also crack because of poor structural design. Fired clay is a very brittle material which has a relatively low strength and a low resistance to impact, and thus cannot bear heavy loads. Ceramic stoves must therefore be carefully designed to reduce the likelihood of fracture.
When a material is subjected to a load (for instance putting a pot on a stove) the stress (force per unit area), locally, at the imperfections is very much greater than in the body of the material. Cracks are more likely to originate in right-angled joints, where there is a concentration of stress.
Stoves are not only subjected to mechanical loading but also to thermal loading. Stresses are established when one part of the stove is hotter than another, and the greater the local temperature difference, the higher the stress.
PRECAUTIONS: Avoid putting sharp comers in your stove. For example (fig 1.), in joining tunnels and pieces, make sure joined pieces are gently tapered and rounded, avoiding right angled joints (fig.2). Never place the air entrance and flue gas exit on the same side of the stove. Place them opposite each other. The narrow space left in between is particularly susceptible to stress (fig.2). Always try to build a stove out of cylinders to avoid the high stresses that result from bending in flat shapes i.e., exploit the natural strength of curved shapes (arches or domes) (fig. 3). When the food in the pot is likely to be stirred vigorously during cooking, or if large pots are used, it is essential to reinforce the ceramic, or remove the load from the ceramic in some other way (fig. 4). Place mud around the ceramic and put the pot on the mud (fig. 5). Use a metal frame (fig. 6). Use a double-walled stove (fig. 7). In both (6) and (7) avoid a 'wedging' situation between the pot and the edges of the pot opening, otherwise problems could occur due to differential expansion in the pot and the pot seat. Use an outer metal casing (fig.8). Reject stoves that have black marks after kiln firing on the inside or outside of the firebox. The black mark indicates a spot that has been reduced in the firing. Localized reduction may weaken the clay at this point (fig. 8). Avoid features that give rise to high temperature gradients for example, baffles under the second pot seat.
SOURCE: Stephen Joseph, ITDG Stoves Project, Applied Research Section, Shinfield Road Reading Berkshire, .RG2 9 BE, United Kingdom.