| Village-Level Brickmaking |
Before beginning to make bricks for the first time, it is essential to know if the clay which you intend to use is the right type for making good quality bricks. Although brickmaking clays are abundant in many countries, not all villages have a supply of good quality brickmaking clay nearby. Before beginning to organize a brick production site, check very carefully that your clay is the correct type.
There are two ways you can check the quality of your clay. The first is to contact a soil technician who can have your soil tested in a laboratory (check with the local public works office for the location of the nearest laboratory). The results of the test are usually quite accurate and can be very informative if they are interpreted well.
The second method is to test the soil yourself, using simple field tests that require no special equipment. These tests are usually quite satisfactory but occasionally the results of these field tests can be misleading if you have no experience in brickmaking or soil testing since some interpretation is required.
The basic information which you must get from any set of tests should answer the following questions:
1. Can the clay be prepared easily and moulded into the shape you want?
2. Can the moulded clay brick dry without cracking or breaking?
3. Can the clay brick be fired to a very high temperature without cracking, breaking or distorting?
4. Will the fired brick be strong and suitable for use in construction?
5. What is the total shrinkage of the clay after firing? What should the size of the mould be?
Collecting Samples for Testing
In order to do any soil testing, whether it is done in a laboratory or in the field, you will have to collect samples of the soil which you wish to test. Before digging, clean an area of about one square metre, removing all leaves twigs, grass, and stones. Start digging through the first 300 to 500 mm of soil which contains roots and dead leaves. This layer is called top soil and is never used for brickmaking. Remove this layer completely. The soil sample is taken from the layer underneath.
If the soil is to be analyzed in a soil laboratory or if it is to be transported to another village for an experienced brick maker to test, you will need to provide them with at least 50 kg of each soil sample. Put each soil sample into a separate strong sack. Do not mix them together. Each sack should be identified with a tag with the following information:
1. Your name,
2. The address where you can be contacted,
3. A description of the location of the clay giving as much detail as possible. For example: clay was dug 20 metres from the banks of the N'komati River in a maize field 2 kilometers west of the village of Nanrapa The sample was taken from a hole one metre deep, 50 metres from a large mango tree. A description like this is very important if you have more than one sample and it may be necessary to draw a map of the area showing all your test sites. (If you have many samples, it is a good idea to identify both the sample and the pit with numbers.)
Be sure to put a copy of the identification card inside the sack with all the information in case the outside tag is lost or destroyed. Keep a detailed record for yourself of where you dug each sample.
Good brickmaking clays are made of a certain combination of clay and sand. If the clay content is too high (as in the clay used for pottery), the brick will crack and break while drying. If the sand content is too high, the brick will be weak and break easily.
Important: Soils can change completely over a small area. If you find a suitable clay, be sure that it is not an isolated pocket and that there is sufficient clay to continue to make bricks for many years.
Simple Field Testing of Clay Samples
Simple, basic field testing can be done on site to test a clay's suitability for brickmaking. The simple tests listed here will give you only a very general idea about the suitability of your soil. They are not as accurate as laboratory tests and should not be used as a substitute. However, they can help you to quickly rule out poor brickmaking clays and to identify clays with brickmaking potential.
Test 1: Moulding
Take a handful of dry clay soil and begin mixing it with a small amount of water until it becomes soft and malleable. Do not add too much water as this will make it too soft and wet. Try making different shapes with the clay.
The objective of this test is simply to see if the soil can be moulded easily and to get an indication of how difficult it may be to prepare the soil for brickmaking.
If it is difficult to make any shape with the soil or if it keeps falling apart, the soil can not be used to make bricks because it probably contains too much sand. If the soil holds its shape and can be moulded easily, continue with the following tests.
Test 2: Forming and Drying Clay "Eggs"
With the moist soil, mould it into a form about the size and shape of a chicken's egg. Make 20 to 30 of these "eggs" and leave them in the sun to dry. After about a day or two, check the "eggs" to see if they have cracked or broken apart.
If they have large cracks, the soil probably has too much clay and is not suitable for making bricks. This particular soil may need to have sand added to it in order to stop the cracking. It will be necessary to experiment with different proportions of sand and soil to determine the correct ratio.
If they did not crack apart or if they have only very small cracks, try crumbling or breaking them in your hands. If they crumble easily, it probably means that the soil has too little clay in it and is unsuitable for making bricks. If they do not break or crumble easily, the soil has a potential for brickmaking.
Test 3: Firing Clay "Eggs"
Take a number of the "eggs" that are very dry and have no cracks and put them into a small fire. Keep the fire small for the first thirty minutes and then make a large hot fire. Try to keep the fire hot for at least two hours or longer if possible. At the end of the firing, the "eggs" should be well covered with hot ashes and left to cool until the next day. When cool, examine the clay "eggs" and try crushing them with your hands. If they crumble easily, the soil is probably not suitable for fired bricks. If they do not crumble easily, the soil has potential for brickmaking.
Place a few of the fired clay "eggs" in a bucket of water and leave them overnight. By the next day, if they have dissolved or feel soft, they were either underfired or the soil is not suitable for brickmaking. (This is a problem with field tests like this; without controls you do not know if the clay "eggs" or bricks were fired correctly or not.) Try crushing a few of the clay "eggs" that were left underwater. If it is difficult, the soil is probably suitable for brickmaking but further testing should be done.
Test 4: Making a Brick
If the previous three tests indicate that the soil might be suitable for brickmaking, continue and make a full size brick. If you do not already have a mould, make an experimental hinge mould with internal dimensions approximately 7% larger than the desired size of the finished fired brick. (See Appendix 2 for detailed information on making a hinge mould.) With the soil which has shown the most potential for brickmaking, make 50 or more bricks using the hinge mould. Follow the directions given in the manual for moulding and drying the bricks.
When dry, build a small kiln with the bricks and fire it over a 24 hour period, trying to follow the same procedures as described for a large scale kiln. After cooling, examine the bricks carefully and note if the bricks are cracked, broken, or if they appear to be weak or strong.
(See the section on Problem Bricks, page 80-81.) If you are satisfied with the results, continue to use the same clay on a slightly larger experimental scale and produce enough bricks for a 1,000 brick kiln. Fire this kiln and once more note the results.