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close this book Boiling Point No. 31 - August 1993
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Clay Preparation Techniques

by A Koopmans, HSE-Greenfields, P O Box 167, Chiangmai 50000, Thailand

For making stoves, it will often be necessary to blend different types of materials, different clay types (plastic clay with a sandy clay, etc.), clay with sand, grog, ash, sawdust, etc. The moisture content of the clay mixture may also be adjusted .

Clay preparation consists of mixing materials, removal of large stones (say over 5mm), roots, crushing of large particles, etc.

There are 4 main methods of preparation - the plastic method; the wet method; the dry method and the semi-dry method.

The plastic method

The plastic method is used extensively in the brick industry, as well as in potteries and for stove making. It makes use of the moisture available in the clay as delivered. If the clay is too dry, water is added.

The process is simple and cheap and, in principle, no equipment is needed. The mixed clay can be used directly for forming products by handthrowing, coiling, pressing, extrusion, etc.

The main drawback of the method is that mixing of different types of clay or other materials is time consuming, and often results in unevenly mixed clay. The method is less well suited for clay types which do not slake easily e.g. hard and dry lumps do need a lot of time to soak up the water, dissolve and become plastic.

The wet method

In this method, clay is dissolved in water to form a slurry. A very homogeneous mixture can be prepared but only one forming method is suitable: casting which is expensive.

The dry method

Clay is dried, pounded and crushed into a powder form but can only be used for dry pressing, ea. tiles.

The semi-dry method

This method is basically a combination of the dry and the plastic method. It consists of drying and crushing the clay until a powder is obtained. Additives such as sand, ash, etc. can be added and mixed with the clay powder. Once properly mixed and sieved in order to remove large particles, water is added and mixed into the powder till a plastic mixture is obtained. with a consistency suitable for further processing such as handthrowing, coiling, pressing, extrusion etc. This method has the advantage of being able to produce a homogeneous mixture while, after mixing with water, the resulting clay can be used with many different manufacturing techniques. However, the disadvantage of a dusty environment remains.

Deciding upon a Method of Making Stoves

In practice only two methods are available for stove production: the plastic method and the semidry method.

Many considerations will have to be made to choose the most appropriate clay preparation method for a particular case. One, and maybe the most important, consideration is local traditional pottery practices and skills. Where potters are accustomed to preparing clay using the plastic method it may be difficult to introduce other methods unless there are very clear indications or reasons why another method has more advantages than the method traditionally used.

Factors such as the quantity of clay to be prepared within a certain time frame, whether equipment for clay preparation such as crushers, pugmills etc. is available, whether electric power is available, the financial situation of the potters and credit availability, competition from other stove makers, etc. have to be taken into account also. A balance must be found between the required capacity, the required quality as well as the means available (financial, technical, maintenance etc).

In most cases, however, the type of clay or clays which are going to be used will be a decisive factor. With clay which easily slakes ie. easily soaks up water and becomes soft and does not contain many large stones, both methods can be used equally well. Here local traditional pottery practices will dictate the method to be used.

If hard clay has to be used which does not slake easily, or for clay with a considerable amount of large particles such as stones which have to be removed, the semi-dry method would be preferred.

Only in the case where clay is hard and does not slake easily does there appear to be a clear case to choose the semi-dry clay preparation method. But even here, the plastic method could be used, depending on how much clay has to be prepared as well as how much space is available for it. For small quantities and with sufficient space, soaking pits can be used to let the clay soak up water so that it will become soft enough to be prepared using the plastic method.

Clay preparation in practice

Clay preparation starts with the mining of the clay. For small scale production units with the clay pit nearby, most potters will have only a small stock of clay available at the factory and dig and transport the clay to the factory as and when needed. Larger production units may subcontract the clay supply. In both cases sufficient supplies should be available to meet requirements over any period when the clay pit may not be accessible.

Where the semi-dry method is used, the clay will subsequently have to be dried, depending on the moisture content of the clay as delivered. Normally, if left in an uncovered stock, wind and sun will be sufficient to dry the clay.


Fig 1- Crushing dry clay by hand

Crushing

Crushing the clay can be done in two ways: manual or mechanical with roller mills, pan grinders, disintegrators, hammer mills, jaw crushers, etc. Most methods will work well even when the clay still contains some moisture, with the exception of a hammer-mill which needs dry clay.

Crushing the clay manually with the help of heavy wooden pestles and hammers can be done, but it is a tedious, time consuming and dusty method. Two people working could possibly prepare from about 500 to 1,000kg of clay per day, depending on the hardness of the clay.

Another method is a foot operated crusher with the wooden or metal (or stone) pestle attached to a lever beam. By stepping on one side of the beam and by letting it fall by its own weight the clay is crushed in the mortar. The system is simple and cheap and can easily be locally made. The capacity is low and two people are needed to operate it: one stepping on the lever beam, the other adding and removing crushed clay in the mortar.

When using mechanical means, the equipment available or which could be locally made, as well as the availability of capital, will be a limiting factor. A cheap and simple solution but which needs sufficient space is a heavy concrete roller on a concrete floor. The concrete roller is attached to a beam, fixed to a central pole. A draft animal is used to rotate the roller by pulling it in a circular fashion. One person turns the clay and keeps it in the path of the roller. However, for very hard clay this system does not work well. A more mechanized system is a pan grinder. Two heavy cast iron rollers, turning at a speed of about 200 rpm move in a circular way inside a tub-like enclosure. Part of the bottom of the tub is perforated and part is not and scrapers push the clay in front of the rollers.

The crushed clay in all cases will have to be sieved to remove the larger particles. The upper limit of what can be allowed in a clay mixture to be used for stove production will have to be found out experimentally. In Kenya an upper limit of 5mm is taken as it has been shown that these particles will dissolve in the subsequent process of mixing with water and soaking or "souring". Any material sieved out can again be fed into the crushing equipment for further crushing and pulverising.

Each step in the process of producing and handling dry pulverised clay is a dusty undertaking. Workers should be supplied with face masks. Breathing in the clay dust can cause silicosis, an irreversible lung disease. Daily exposure to large amounts of dust can lead to death within a few years and bronchitis in a matter of months.

Pre-mixing

After having been crushed and/or pulverised, the next step is mixing and/or pugging, where the clay and any additive such as sand etc are proportioned, mixed, soaked etc.

Proportioning of the dry powdered clay or the wet clay and sand should be done using the same bucket or pan or whatever is used to measure quantities by volume. The best way to prepare clay mixtures is to spread them out flat in layers on a clean floor or in ? soaking pit (preferably made from concrete or bricks). Normally a soaking pit is about 1.5 by 1.5 metre or 1.5 meter diameter with a depth of about 40-50cm.

In case two parts clay and one part grog or sand have to be mixed, first a layer of clay is spread out, followed by a layer of grog or sand and a layer of clay. Water in the required quantity is sprinkled over it and the next layers of sand and clay can be added. This heap should then be left for some time (preferably a day and a night, the longer the better) so that the water can soak in and any larger particles can disintegrate due to the moisture. This soaking or "souring" ensures that the clay is better workable as the plasticity improves. After the soaking period, the material is moved by digging the heap vertically with the spadefuls stacked on top of each other.


Fig 2 - Mixing clay, sand, sawdust and water by foot

Hand Mixing

After this first pre-mixing stage, the clay has to be further mixed until a homogeneous clay mixture is obtained. This again can be done either manually or mechanically.

Manual mixing basically involves using the feet to knead and mix the clay till the proper consistency has been reached and stones have been removed.

Depending on the softness of the clay mixture, production ranges from about 150 to 500 kg per man day. For larger quantities, a pugmill can be used.


Fig 3 - An example of a horizontal pugmill or extruder

Pugmills

There are many varieties of pugmills but they can be sub-divided into horizontal types and vertical types. Horizontal types are also called extruders as the clay is moved forward by means of a screw inside a round barrel. The capacity of these horizontal pugmills varies from about100kg per hour to a few thousand kg of clay per hour for brick making. In Thailand locally made equipment is available for as little as 1,000 US$ and has a capacity of about 200kg per hour, depending on the softness of the clay mixture. Imported equipment normally would cost about 3-4 times as much.

Vertical pugmills are usually locally made and can be driven by animal power. They consist of a vertical barrel with a heavy duty axial shaft which is fitted with angled blades which mix the clay and push it downwards. In the lower part is an opening through which the clay is pushed out.

Storing

After the clay has been mixed and kneaded properly to the right consistency, it should be stored properly before being used. Whenever possible, the clay should be covered with plastic sheeting or some other material which keeps the moisture inside. Storing improves workability and may prevent problems in drying and firing.