| Clay materials for the self-reliant potter |
|Origin of clay|
|Prospecting and mining clay|
|Clay washing and clay body preparation|
|Nature of clay|
|Testing of clay|
1. Prospecting Clay
2. Clay mining
2.1. Before mining
2.2. Stockpilling and weathering
1. Prospecting clay
Pottery clay can be found in most countries. In areas where pottery or brick-making have a long history suitable clay deposits will already be well known. However, introduction of new techniques like glazing or high temperature firing may call for new types of clay. In some countries the land may still be virgin from a potter's point of view and before any production facilities are established a reliable source of good pottery clay must be found.
Local authorities: First of all, information about the geology and the soil of the region should be gathered from local authorities, like industrial development organizations, agricultural institutions, National Geological Institutes or mining corporations. They may have little information, and the authorities may even say that no clay is available in the region. However, that is often not true, and should not keep anybody from trying on his own.
Practical people: It is worth talking to people who make water wells, and builders of dams and roads. They should have first-hand information about the soil of the region. Farmers in the area will know about the upper layers of soil on their fields. Sometimes clay is used for other purposes, like whitewashing houses or medicine. In Tanzania iron smelters use a highly refractory clay for their furnaces, and in Nepal the brass makers use a local fireclay for their casting moulds.
Prospecting: When setting out to explore the countryside one should bear in mind how nature created clay. Recent deposits of plastic clay are most likely found in the plains and valleys, or along rivers. They are often close to the surface. Older secondary deposits may be found in the hills where the land was raised and folded, millions of years after the clay was deposited. Such deposits may be covered with a thick layer of other materials.
River banks: A good potter's clay will often be covered by several meters of overburden. Instead of digging test holes through the overburden at random, a general idea of the deeper soils in the area can be obtained by examining the soils exposed in river banks, escarpments and cut areas where a road or a railway was made through a hill. Quarries, wells and ditches should be examined as well. Termites bring soil up from below, and the material from termite hills indicates the quality of soil located 1-2 meters down.
Field testing: In the field a few simple tests can establish whether the clay is worth examining further. First, take a sample from about 30 cm inside the exposed surface, and mix it with water. If it turns into a plastic sticky mass it is clay. Then knead it well and form a rope the thickness of a pencil. If you can bend this rope around two fingers without seeing big cracks, the clay has plasticity (fig.13-1).
If you bite the plastic clay gently with your front teeth you will get an idea of how finely grained the sand content is.
Then rub a sample of dry clay in the palm of your hand until the fine particles are rubbed away. What remains is the grit content, which may be particles of feldspar, quartz or mica.
Small pieces of limestone will cause trouble in pot-making, so the deposit and its vicinity should be examined for signs of white or grey limestone. Lime powder thoroughly mixed with the clay will lower its melting point and is often used for low-temperature pottery. But a piece of limestone the size of a pinhead will crack the pots or bricks when they are exposed to moisture after firing. Screening the clay through a fine sieve reduces this problem, but if possible, it is better to look for another clay. Lime content can be tested by putting a few drops of dilute hydrochloric acid on a sample. If there is lime, bubbles will form from the reaction.
sample collection: If the clay, after field testing, has proven to be of interest samples should be collected for further and more thorough testing. The quality of clay from different places in the same deposit will differ slightly, so in order to get a repre- sentative sample, clay from 4 different spots within a few meters' distance are dug out. The clay should not be taken from the exposed surface of the deposit but rather from 30-50 cm inside since the clay at the surface may be contaminated with other soils or washed out by rain.
The 4 samples are mixed well at the location and a sample of about 5 kg is then packed and labelled with its location .
Make a sketch of the location as accurately as possible, indicating features of the landscape like big trees or rocks. If a motor vehicle is used, note the exact distance on the odometer from the nearest town to the clay site. A photograph of the clay site will help in finding the right site again later.
Probe digging: In areas where initial survey and testing indicates deposits of suitable clay, holes should be dug in a regular grid in order to ascertain the size of the deposit. Initially, holes should be dug with a grid distance of 50 m. and where the best quality clay is discovered, the distance can be reduced to 15 m. It is worthwhile to ensure that the deposit is large enough to supply the planned production for a long time. The holes can be dug with a spade, but if a hole is more than 2 m deep the sides of the hole should be supported by planks.
A bucket auger (fig.14-2) is a very useful tool for taking samples. One or two people rotate the auger, which drills its way into the soil. The shaft can be extended so that samples can be taken from depth of 5 m or more. The bucket auger can be made at a local machine shop.
Map: A map should be made of the whole grid area, and on the map the probe holes are marked with a number, thickness of overburden and depth of clay layer.
The map is drawn with the help of a few fixed features, like trees and large stones (fig.15-1). The distance between three fixed points is measured as accurately as possible. The map is made in a scale of 1:100 (1 cm on the map represents 100 cm (1 m) in reality) or 1:200 (1cm=2m). The three fixed features are then marked on the map like on fig.15-2. The location of the test holes is measured and marked on the map according to its distance from the fixed points. Each test hole is given a number, which is marked by the hole itself, on the test sample label and on the map.
After testing of all the samples has pinpointed the best area, a more detailed plan (e.g. scale 1:50) of that area should be drawn, showing the depth of the various layers of top soil, clays and possibly other materials.
Economy: After having established the quality of the raw clay, the approximate size of the deposit and the thickness of the overburden the final decision of whether or not to start mining the clay remains to be done. Many factors control the economy of mining clay:
1) The distance from the deposit to a suitable road, and the cost of transport to the workshop. It may be necessary to construct a small track from the deposit to the nearest road.
2) The cost of removing the overburden com- pared to the amount of clay underneath.
3) The cost of renting or buying land.
4) The quality of the clay. If the raw clay contains large amounts of sand, it may be necessary to wash the clay at the mine in order to reduce the cost of transport.
The total cost of opening up the mine, and the cost of digging and transporting the clay as listed above should be calculated as cost per kilogram of clay. This cost per kilogram is then compared with the cost of possible alternative sources of clay.
2. Clay Mining
2.1 Before Mining
The clay you intend to mine may have been deposited in an ancient lake as shown in fig.16- 1. It may have taken hundreds of thousands of years to fill the lake with sediments, and during that period variations in climate, course of rivers, etc., caused the layers of sediments to vary. Each layer may contain its own type of clay or sand and the thickness of each layer may vary considerably.
clay layers: Before you start to dig the clay, expect it to be limited to certain layers. Today these layers will often be positioned horizontally in the same manner as they were laid down (fig.16-1). But they may also have been turned upside down by later folding of the landmasses and could be positioned as suggested in fig.16-2 and fig.16-3. The digging of probe holes as mentioned above should indicate how the clay layers are positioned.
overburden: First the top soil or overburden has to be removed and piled away from the clay pit where no future clay digging is planned. Care should be taken to avoid mixing top soil with the clay. If the overburden is several meters thick, it may be worthwhile to hire a bulldozer to clear away enough top soil for several years' clay mining.
In some cases, the overburden is too thick to be removed, and underground mining must be considered. This method is especially tempting if the clay is situated on a riverbank, on a slope or on an escarpment so that horizontal shafts can be dug. That will save the tedious task of removing the overburden, but this advantage may be offset by the extra cost of using lumber supports for keeping the walls of the under- ground shafts from collapsing. Underground mining of clay in vault shaped shafts without supports is often seen but is dangerous.
digging tools: Commercial mining of clay on a large scale in industrialized countries uses heavy machinery (fig. 17-1). In most other places manual methods are more economical.
Manual digging is done by spade or hoe. A shovel is no good for breaking up clay, but it is useful when loading the clay into a wheelbarrow or truck. A wheelbarrow is used for bringing clay from the pit to a vehicle.
supervision of clay winning: While digging, the worker should sort out roots, limestone, rocks and other unwanted material. The digging should always be supervised by an experienced person who can judge the quality of the raw clay. As the working of the clay pit progresses it may reach layers of inferior clay. The supervisor should regularly test the quality of the clay using the simple methods described above (page 13). Production of first class pottery demands raw materials of consistent and uniform quality. A potter adjusts production methods to the clay, and if it suddenly changes its behavior, it may ruin the production. If, for example, the clay becomes more plastic it may cause pots to crack during drying.
Even within the same layer of clay the composition of the raw clay may differ. Therefore, it is prudent to dig clay from several levels or locations at the same time, and to mix the material before loading it for transport. A worker can dig 4 to 8 tons of raw clay per day, depending on conditions in the clay pit. If the worker is paid according to quantity (piece work) the quality of the clay may suffer unless the supervision is thorough. In the long run, it may be less costly to pay by the hour (time work) in order to get better quality clay.
safety: Clay is normally extracted from open pits, which are much safer compared to underground mining. However, safety should also be a concern in open pits. Clay should not be dug from a vertical clay face higher than 2 m because a large portion of the face could break loose and bury the worker. The digging of clay in deep pits should be done in benches as shown in fig.18-2. The overburden is removed away from the clay face being worked on, so that no top soil will get mixed with the clay itself. The benches on the clay face are made in steps about 1 m high and 1/2 - 1 m wide. The material from different levels can be thrown to the bottom of the pit for mixing, in order to even out variations in clay quality.
record: A record should be kept of where in the pit the different batches of clay are extracted, so that sudden changes in the quality of clay can be traced to specific locations in the pit, and these can be avoided in the future. The movement of the digging area is recorded on the original map and is compared with the location of the original test holes. In this way, the supervisor can decide in which direction and to which depth to direct the digging, and he will be able to avoid clay beds of inferior quality that are shown on the test hole map.
2.2. Stockpiling and Weathering
planning: Clay mining is impractical or impossible during rainy seasons, and in some areas of the world the ground freezes hard during winter. Therefore, it is necessary to extract sufficient clay during the dry season or during the summer to cover a whole year's production. That means you will have to plan your clay digging and production 1/2 - 1 year ahead, or you will run out of clay.
weathering: Storing the clay in the open, and exposing it to the action of rain, sun or frost is called weathering. The alternate wetting and drying, or freezing and thawing, improves the plasticity of the clay by breaking it into smaller particles.
Weathering will reduce the content of possible organic matter in the clay, and this may have a bleaching effect on the raw clay . Weathering also washes out soluble salts. The clay should not be piled higher than 0.5 m. Clay may be weathered from a few months up to one year.
clay storing: When clay is received at the pottery it should be piled as shown in fig.19-1. Each truck or cart load is spread out in a thin layer covering the whole storage area. This makes horizontal layers.
The raw clay is collected from the clay bin by making vertical cuts in the pile, so that a little clay from each truck load is part of each batch of clay used in production. This procedure will ensure a more uniform clay quality.