Introduction

The main purpose of this manual is to give guidance on the selection and, when necessary, the modification of natural or raw clays from which durable pottery stoves and stove liners can be fabricated. It is intended for use by project technicians who can then advise potters and producers of the best possible mixes. The tests are simple, but we recommend that they are carried out in a laboratory. Basic mathematics is the only skill required to do the calculations and a meticulous approach to 'note-taking' helps to build up a long-term record of clay and its characteristics.

Stoves are ideally suited for manufacture by experienced rural potters or in small factories producing, for example, bricks and tiles. Here, the fabrication practices are likely to be suitable for stoves but the clay body mixes are certain to require some modification. Stoves are subjected to more severe service conditions than bricks and tiles, or even cooking pots.

To ensure good sales, stoves should not fail in use from thermally-induced stresses and they should be sufficiently strong to withstand the mechanical loads and knocks to which they are subjected. To make them durable is likely to require more than the use of a suitable clay body mixture. The design and the shape are also important, as is the care taken in their manufacture. We include in this manual some comments on design and fabrication but as this is a very inadequately researched area we can only offer the most basic suggestions.

A research project was carried out at Sheffield University by A. Gaspe under the supervision of P.F. Messer to investigate why pottery stoves or liners made at some locations failed in service through thermally-induced stresses, while those made at other locations did not. The project was funded by the Overseas Development Administration of the UK Government and administered by ITDG in Rugby, UK. The results are presented in the following chapters.

Finally, for those with some scientific background, we provide some information on the factors determining the strength of ceramic materials and the factors affecting the development of thermally-induced stresses.

Factors affecting strength and thermally-induced stresses

The strength, σf, of a ceramic is the tensile stress (force/unit area) at which the material breaks. It depends on the Young's modulus of elasticity, E, of the ceramic, its effective surface energy for fracture initiation, γi, and the size of the fracture-initiating flaw, c, in the following way:

σf = constant x ([Eγi/c)0.5

The constant depends on the shape, position and orientation of the fracture-initiating flaw.

The product Eγi determines the toughness of the ceramic. The fracture toughness, Kic, is given by:

Kic = (2Eγi)0.5

Flaws are regions from which the material is missing and through which mechanical load or force cannot be transmitted. Flaws concentrate the stress around their peripheries. A spherical pore is a flaw but, because of its rounded shape, the maximum stress is only twice the average value as shown in Figure 1. The maximum stress does not change with the size of the spherical pore. Rounded pores, even large ones, are unlikely to be fracture-initiating flaws on their own. To be severe, a flaw needs to be either a sharp crack or a pore linked to a sharp crack. This is because the stress next to the crack tip is magnified by a factor very much greater than two.

Inclusions such as quartz grains are often partially or wholly detached from the matrix by cracks or by associated pores or fissures (a fissure is an elongated pore which concentrates stress by more than a factor of two). Therefore, inclusions often act as quasi-pores (see Figure 3), which can be linked with sharp cracks to become fracture-initiating flaws.

When the mechanical load on a material is increased, the stress within the material is also increased. Adjacent to a flaw, the stress will reach the very high value required to pull the atoms apart, whilst the average stress is at a modest level. When the bonds between the atoms are ruptured, the flaw grows in size at the crack from the flaw tip and propagates across the material causing the material to split into at least two parts.

We know from the examination of fracture surfaces that large pores – both of rounded and elongated shape – and inclusions cause fracture. We know theoretically that these flaws must have had a sharp crack-like feature associated with them to make them severe flaws. The equation for strength tells us that the strength of a ceramic decreases as the flaw size increases.

The terms E and γi in the equation both depend on the effective porosity in the ceramic; that is, on the volume function of pores and quasi-pores. Both E and γi decrease as the effective porosity increases.

For E, this occurs because pores and quasi-pores cannot support and transmit a force. Consequently, the material around the pore or quasi-pore is more highly stressed, as shown in Figure 1. A material is therefore stretched more for a given mechanical load when it is porous than when it is fully dense. A porous material therefore has a lower value of E.

Energy is required to fracture any material. Part of the energy is required to form the new surface. Energy is expended in other ways, such as heating the material. The total energy requirement to form each unit area of fracture surface is γi. During fracture a propagating crack is attracted towards pores and quasi-pores lying close to its path, i.e. it takes the path of least resistance. If it intersects the pores and quasi-pores, less surface has to be created when the material is fractured. Consequently the presence of pores and quasi-pores lowers the value of γi.

For a flaw of fixed size, type, position and orientation, the strength will decrease as the effective porosity is increased (E, γi and Kic decrease).

When an object is heated non-uniformly, the temperature varies with position throughout the object. As materials expand when they are heated, the object expands but by different amounts throughout its volume because of the temperature variation. The part which is at the highest temperature wants to expand the most. Its expansion will be constrained by a neighbouring part which is at a lower temperature and wants, therefore, to expand less. The hotter part is constrained to be smaller than it wants to be, whilst the cooler part is forced to be larger. This is how thermal stresses arise. The hotter part is in compression, whilst the cooler part is in...