|Clay Materials - for the Self-Reliant Potter (GTZ, 1990, 98 p.)|
1. Washing at the Pit
After extracting the clay from its pit, it is normally brought directly to the pottery for further treatment. However, some raw clays contain so much sand that it is more economical to dispose of the sand at the clay pit, thus avoiding the cost of transporting this unwanted material. The true clay content found in primary clay deposits may be only 15% or less. The whiteness or the refractoriness of such clays may still make it profitable to mine them.
commercial kaolin mine: In some commercial kaolin mines, clay is extracted by washing the clay face in an open kaolin pit with a high-pressure water-jet. The fine clay is carried away with the water leaving behind most of the coarse materials. In older mines, the resulting clay slurry is run through troughs, where the non-clay materials settle, after which the clay slurry is pumped into settling tanks. After siphoning off the clear water, the clay slip is de-watered in filterpresses and dried in ovens. In more modern works, the trough type settling tanks have been replaced by centrifuges.
Smaller clay works cannot afford such machinery. Instead the raw clay is mixed with water, and stirred either manually in ponds or in a washmill.
washing ponds: Next to the clay pit and close to a source of water two or more shallow ponds are dug, measuring about 4 m by 2 m and 0.5 m deep. The sides of the ponds can be lined with bricks or simple wickerwork plastered with clay.
The pond is half filled with water and the raw clay is added until the pond is nearly full. The raw clay is then stirred with a shovel, until all the clay is separated from the sand. With dry sandy clays this may take less than 20 minutes. Finer clays need longer stirring and for very fine ones it may be necessary to let the clay soak for a day. However, clay that fine is in no need of having its sand removed.
When all the clay is suspended in water and the material at the bottom of the pond is only sand (without a clayey feeling), the clay slurry can be transferred to a settling pond. If possible the settling pond should be located at a slightly lower level so that the clay slurry can run in by itself. Otherwise, the clay slurry is transferred by bucket, which is filled in a small pit connected to the stirring pond with a pipe. In this way the bucket will not disturb the settled sand in the stirring pond.
When a very pure clay is desired, the clay slip is led through several settling pits before filling the final settling pond. During the slow flow through the intermediate pits, the fine sand particles will settle and only the much finer clay particles will flow on.
After the clay has settled in the settling pond, the clear water on top can be transferred by pump or by bucket, back to the stirring pond, which first has been emptied of its sand. The bucket or pump intake should not be dipped into the settling pond since that would stir the settled clay. Instead the surplus water is led to a small pit next to the settling pond, and water is taken from here. For each stir- ring pond several settling ponds are needed. The clay is then left in the settling pond until it is stiff enough to be removed for further drying.
washmill: For large quantities of clay an animal powered washmill is useful (fig.21-2). The circular tank is half filled with water and raw clay, preferably dry and without large lumps, is added while stirring, until the tank is almost full. After stirring for 1-2 hours (depending on the properties of the clay), the clay will be suspended in the water, while stones and coarse sand settle at the bottom. The clay slurry is then run into settling ponds.
The washmill can also be operated continuously; first it is filled as described above and when the clay and sand have been separated, more raw clay is added gradually. The added raw clay will sink to the bottom, where it is worked by the harrows, and an equal amount of clay slurry will run off at the upper outlet. Fresh water is added from a pit with an inlet at the bottom of the tank. After operating the washmill continuously for some days, sand and stones accumulated at the bottom of the tank has to be cleaned out.
sand separator: If the fine sand content of the clay is not desired, the slurry is run (on its way to the settling tanks) either through a fine mesh sieve, a grooved tray (fig.22-2), a series of settling tanks (fig.22-3) or a sand separator (fig.22-4 & 22-5).
washmill construction: The washmill is built of bricks laid with cement mortar. The centre section supports the beam that turns the harrows. The beam turns in simple bearings, that can be made of hardwood which is kept well greased. A sluice system with adjustable outlet levels can be fitted with a grate to catch roots and other organic material. The harrows should be made of iron or of wood reinforced with iron. Fig.22-1 shows how an extra set of harrows hinged onto the main harrows improves the disintegrating action. The tank should not be more than 1 m deep, and increased capacity is achieved by widening the diameter of the circular tank. A tank with a diameter of 4.5 m can wash at least 6 ton of raw clay at a time.
The washmill can be powered by one or two persons or by a draught animal. A motor could also power the washmill, but since a rather high gear ratio is needed, the washmill becomes much more costly. Alternatively, the raw clay can be washed in a high speed blunger as described on page 27.
2. Clay body preparation
Two different methods are used for preparing the clay for production: dry and slop. Each method has many variations, and the right choice of techniques and machinery depends very much on the nature of the clay and on the type of ware to be produced. Therefore, before deciding on any method, the clay should first be tried out thoroughly - from prepara- tion of clay to forming, glazing and firing.
2.1 Dry Method
After weathering, the clay is dried completely. If the clay can be used as it is, without having sand or stones removed, the dried clay is wetted directly and left until it has a plastic consistency. Then it is kneaded manually or in a pugmill. Fig.23-1 shows such a system with dry clay stored in the back, a slaking pit in front, plastic clay covered under wet cloth ready to be pugged.
In must cases, however, sand and other impurities have to be removed. The dried clay is then first pulverized either manually by a lever hammer, or with the help of a hammer mill or pin mill.
lever hammer: The lever hammer is operated by two persons. One steps on the short lever, thereby lifting the long lever onto which a heavy iron or wooden hammer is fixed. This falls on the clay, which is fed to the hammer by the second person, who may also screen the powdered clay.
In many countries, the same traditional machine is used for rice hulling.
hammer mill: A hammer mill or plate mill of the type used for grinding grain may be cheaper to operate compared to the lever hammer. It can be powered either by an electric motor or small diesel engine. In the hammer mill, a number of small exchangeable hammers are mounted on a rotating disk. The hammers disintegrate the material by impact until the particles are small enough to pass through a curved screen. The screens can be changed, so that coarse material like grog can be produced as well.
The rotation of the hammers produces suction at the centre and pressure at the perimeter. Therefore, the inlet of material should be at the centre the point of suction. The pressure helps to blow the ground material through the screen. The outlet should lead into a big cotton bag that retains the clay but lets the air through. Some hammer mills have the inlet at the perimeter, and that produces a lot of dust, which is a health hazard to the operator. The dust nuisance can be reduced by fixing an outlet with a long tube on top of the mill casing to release air pressure.
pin mill: This mill has a rotor fitted with beaters which rotate between stationary pins. The material is fed through a hopper to the centre of the mill. The rotating beaters hit the material and fling it outward where it hits stationary pins. When it is fine enough, it passes the sieve surrounding the rotor.
The pin mill is suitable for slightly moist clay materials, which otherwise tend to clog the hammer mill screen.
screening: After the clay has been pulverized, it can be mixed with other dry materials like sand, limestone, feldspar, kaolin, talc or grog (see p. 47). The mixture should be screened again, or put through the hammer mill an extra time in order to ensure proper mixing of the different materials.
For larger potteries a vibrating screen as shown in fig.24-2 will be useful. The screen can be replaced so it can be used for different particle sizes.
wetting: Water is then added to the clay, either by pouring water into a pit in the centre of the clay and leaving it to be absorbed, or by sprinkling an even amount of water onto about 5 cm thick layers of clay, which are then covered by subsequent layers. After soaking, the clay is kneaded either by foot or hand. The clay should then be left in a moist place for at least a week. This could be a clay cellar, or a clay pit covered with wet cloth and plastic sheets.
Prolonged storage will improve the plasticity of the clay and give it a stiffer consistency.
kneading: After storage, the clay should be kneaded again, either manually or in a pugmill. A vertical pugmill can be constructed locally. An animal powered pugmill of a type often used in brickworks is shown at fig.24-4. The barrel is made from metal or wood and a vertical shaft is fitted with blades set at a slight angle, which forces the clay downward.
conclusion: The advantage of the dry method is that little equipment is needed. Its drawbacks are that the workers are exposed to unhealthy clay dust, and that the clay only develops its full potential plasticity after prolonged storage. In some cases, troublesome impurities can only be removed by the slop method.
2.2 Slop Method.
The clay is made into a slip, other materials are added, and after screening the slip is dewatered until the clay has the right consistency. There are numerous variations of the slop method and a few examples are described below.
2.3 Industrial Slip House.
In industrial production of white ware, clay bodies are made up of several different clays, with additions of feldspar, quartz and other materials. Feldspar and rock quartz are first crushed to the size of gravel ( 2-4 mm ) before further grinding in a ball mill. For that, one of the following machines is used.
jaw crusher: This machine is used for the initial crushing of rocks (fig.25-1). Large lumps of rock are reduced to 15-25 mm pebbles. This initial reduction can also be done manually with a hammer. A jaw crusher is often used for producing grog.
roller crusher: Crushing rollers are used for breaking down lumpy clay or shale. The rollers may be smooth or with grooves, and they rotate in opposite directions to each other. The space between the rollers can be adjusted.
pan grinder: One or two heavy wheels made of granite or steel rotate on a pan of similar material and crush the pebble size material to sand (fig. 26- 3). In some cases, the pan is perforated. The pan grinder can also be used for crushing grog and for preparing clay bodies, especially granulated bodies for dust pressing of tiles.
hammer mill: The hammer mill is widely used because it is so versatile. It is mainly used for softer materials like clay, limestone, talc and gypsum, but can also be used in an emergency for grinding feldspar and quartz, provided these have first been shattered by calcining above 600 C. The hard materials will quickly wear out the hammers and the sieve of the mill. ball mill: the final grinding takes place in a ball mill, which can grind materials up to coarse sand size. A ball mill is a hollow mild steel cylinder, lined with special bricks made of hard rock (like granite), or porcelain, or a thick rubber sheet. The ball mill is filled about 50% with pebbles of flint or porcelain, 25% material for grinding and 20% water (measured by volume). The ball mill is rotated slowly, so that the pebbles constantly roll down the inner slope of the cylinder, and the material is ground by the rubbing action between the pebbles. (See Appendix p.84).
In large factories, each raw material is ground separately and then mixed in a blunger according to its slop weight. In smaller factories kaolin, feldspar and quartz are measured by dry weight and milled together. In that way the total ball milling time can be reduced by adding kaolin after the harder materials (quartz and feldspar) have been milled for some time.
A coarse screen fitted to the ball mill holds back the pebbles while the clay slip is poured out and led to a blunger through wooden troughs.
blunger: In the blunger (mixing ark) plastic sedimentary clay (often ball clay) is added to the milled materials. The blunger shown at fig. stirs the clay slip by rotating two sets of blades at 17 rpm (rotations per minute), and blunging time for ball clay is more than 10 hours. A high speed blunger with a single shaft propeller (fig.27-2) cuts blunging time to 1-2 hours and is now replacing the slower type.
screening and de-magnetizing: After blunging, the clay slip is screened through a fine mesh screen (80-150 mesh per sq.inch). The screen is vibrated to prevent it from getting clogged. By fitting a coarser sieve mesh above the fine mesh screen the clogging problem can be reduced. 2- and 3- deck screens are commonly used.
In production of white ware, the clay slip is passed through
magnets that catch iron compounds, which would otherwise produce brown specks in
the fired product. The magnets are either permanent magnets or electromagnets.
Old loudspeaker magnets suspended in the blunger are
filterpress: The clay slip contains about 50% water when being screened, and half of that must be removed before the clay has the right consistency for forming. In modern industries, dewatering is done in a filterpress. This consists of a series of frames which form chambers when fitted together. Each chamber is lined with a filter cloth, and the slip enters the chambers through holes in the frames. The filter cloth is hung over both sides of the frame and the two halves of the cloth are sewn together around the inlet hole. The frames are fitted together in one long row, and sealed by tightening a heavy screw.
The clay slip is then pumped into the filterpress under pressure (7-10 kg/cm2) and the water is forced out through the filter cloth. The water drains away through grooves in the frames.
Filtering time varies according to the particle size of the clay
and the pumping pressure. Filtering of a coarse grained kaolin clay may take
only two hours, whereas a highly plastic clay may take 8 hours. After the water
has stopped dripping from the frames, the tightening screw is opened and stiff
clay cakes are removed from between the frames and transferred to the clay
compressed air pump: A durable and simple filterpress pumping system can be made with the help of an air- compressor and a tank capable of handling pressures up to 10 kg/cm2. At the bottom of the tank a pipe connects to the filterpress inlet. After filling the tank with clay slip, compressed air is pumped into the tank. An adjustable pressure valve maintains the desired pumping pressure. When the clay in the filterpress is dewatered, the compressed air is shot off, the pressure in the tank is released and another batch of clay slip is loaded.
This system has several advantages over a conventional piston pump system; 1) The clay slip is forced through the filterpress under a constant pressure, thus reducing filtering time and lamination problems. 2) Maintenance cost is very low, since no moving parts come in contact with the abrasive clay slip. 3) The pump system is much cheaper and it can be made from locally available parts. 4) It uses very little energy.
The pressure tank is the most expensive part, but this cost can be reduced by using a smaller tank, which is then charged twice during one filterpress operation.
pugging: The clay is then left to mature in the storage area for at least two weeks in order to improve its plasticity. However, the clay still requires hand or machine kneading to remove entrapped air and make the clay uniform.
kneading table: A kneading table works the clay with horizon- tal and vertical rollers (fig.29-1). The upper horizontal rollers press the clay down, and afterwards the vertical rollers press the clay up. This alternate movement up and down squeezes entrapped air out of the clay and gives it a uniform consistency. The clay is laid in a circular wad on top of the kneading table, and each operation takes about 50 minutes.
The kneading table is mainly for bodies with low plasticity, like porcelain bodies, but is little used today.
pugmill: The pugmill has replaced the kneading table in modern ceramics plants. It is a more costly machine, but it produces a better quality clay, especially if the clay is de-aired during pugging. The pug mill consists of a large cylinder with an axle running through its centre.
Sets of iron blades are attached to the axle and these both cut the clay and move it forward. Clay is fed from one end and pressed out through a mouth piece at the other end in a continuous column, that is cut up in convenient pieces ready for forming.
The flow of clay through the cylinder is not even. The clay at the centre moves at a different speed compared to the clay next to the cylinder wall, and this produces different densities in the clay body. That causes stress in the finished ware, which may warp during drying or crack during firing (these special cracks are called lamination cracks). Hand kneading after pugging is often required.
A de-airing pug mill takes the clay through a chamber where a strong vacuum is maintained by a vacuum pump. In this vacuum all entrapped air is sucked out of the clay, which greatly improves its plasticity. At the same time, de-airing reduces the problems of lamination stresses in the finished products.
2.4 Simple Slop Method
The complicated slop method used in larger industries can be modified for use in small scale industries. Generally, it is cheaper to set up clay preparation equipment for the dry method described above. But if the raw clay needs to be cleaned of small impurities causing either pinholes, or colored specks in white clay bodies, the slop method should be tried.
slaking: Raw clay can be slaked in water in concrete pits, or in oil drums cut in half, until it forms a slip. Slaking takes less time if the clay is dried first. The clay slip is then screened through 60 or 100 mesh, other screened materials like talc or feldspar are added, the slip is thoroughly stirred, and in order to ensure a uniform mixing the slip is screened again.
blunging: Where electric power is available, a high speed blunger is used. This is cheap and simple to make. A vibrating screen is practical for larger production.
settling tank: After screening the clay, the slip is poured into a tank where it remains until the clay has settled. The clear water on top is then siphoned off, and can be reused (unless it contains soluble salts).
In the thin slip, coarser particles will sink, and before trans- ferring the clay to dewatering trays it should be stirred again.
dewatering tray: In place of the filterpress system, dewatering trays as shown in fig.31-1 can be used for stiffening the slip. The tray is made from porous bricks or tiles set in either a clay mortar or cement mortar. The tray can be lifted to allow air circulation for increased evaporation.
The slip is carried from the settling tank to the dewatering tray, where the clay is further dewatered. The soft clay paste is stirred now and then to ensure even consistency, and care has to be taken to prevent fast drying on the surface. The tray should be protected against direct sun, since the sun would cause a crust to form. Cotton cloth placed on top of the slip increases evaporation and prevents a crust from forming.
In the rainy season, the trays can be heated by circulating hot air underneath them. A small chimney can be constructed at one end of the tray, and a simple firebox at the other (fig.31-2).
In areas with long periods of dry weather, shallow pits in the ground can be used for dewatering. The pits may be lined with cloth or bricks, or left without lining.
Plaster of Paris bowls can also be used for dewatering, but plaster is more expensive than bricks.
manual filterpress: Clay paste from the settling tank can be filled in cotton or polyethylene bags and placed in a manual press as shown in fig.31-3. This is simple to make, and it works surprisingly well.
vacuum filterpress: H. Davis has described a filterpress which uses vacuum instead of pressure for extracting water from the clay slip .
Filter frames of lighter construction than normal are filled with clay slip and placed in an airtight metal box, in which a vacuum is created by a vacuum pump. Vacuum pumps used for milking plants or refrigerators can be used.
storage: After the dewatering process is over, the plastic clay should be stored for at least a week. The clay should be slightly softer than desired for forming, because during storage the water will slowly penetrate clusters of smaller clay particles and surround each with a water film. This will improve plasticity and make the clay stiffer.
The clay should be stored in a cool and moist place. One or more clay cellars can be con- structed underground as shown in fig.32-1 . The clay can also be stored in the workshop, covered with wet bags and plastic sheets.
kneading or pugging: When clay is taken from storage, it must be kneaded or pugged in order to reduce problems of particle orientation (see p.43) and to make the clay less stiff. The kneading can be done by hand or foot. Industries with a large output can make use of a simple vertical pugmill. Due to laminations occurring during pugging, the clay may require kneading by hand before being formed.