Cover Image
close this bookClay Materials - for the Self-Reliant Potter (GTZ, 1990, 98 p.)
View the document(introduction...)
View the documentPreface
View the documentAcknowledgements
View the documentThe Author:
View the documentOrigin of clay
View the documentProspecting and mining clay
View the documentClay washing and clay body preparation
View the documentNature of clay
View the documentClay bodies
View the documentTesting of clay

Testing of clay

The tests which are described in this chapter will not all be appropriate for small pottery industries. Cottage industries will be content with making, at most, shrinkage and moisture content tests. However, industries or ceramics training centres involved with development and research work should find most of the tests useful and relatively simple to undertake.

caution: Testing should only be done for good reasons. Most private cottage and small industries do not have time or resources to carry out regular testing. However, many production losses can be prevented by testing materials when they seem to behave differently than usual. When clay arrives it is inspected, and by making a simple rope test (see p.13) its plasticity can be judged. Am experienced thrower will immediately feel differences in the clay body, and should suggest testing to the manager.

consistency: In carrying out these tests the most important thing to keep in mind is; consistency. That means, the tests should be made in the same manner each time, so that the results can be compared.

The individual tester builds up his own set of standards by which he evaluates his test results.

equipment: For most of the tests a scale, a ruler and a ceramic kiln are sufficient. A small drying oven is useful for moisture tests and thermal crazing tests. A transverse strength tester is needed for testing the strength of the green or fired clay bodies. These two pieces of equipment can be made locally.

object: The object of testing natural clays and clay bodies is :

1) to find out the properties of clay from new deposits and to assess their useful- ness in production. (research)

2) to be sure that new supplies of familiar clays are similar to former supplies of the same clay. (quality control)

3) to test the quality of new clay body mixtures (research)

4) to control the quality of clay bodies used in production (quality control).

research: Research is done in order to find out the qualities of new materials or develop new clay bodies.

quality control: Quality control is to check ongoing production, so that a uniform product can be supplied to the market.

The tests required for the various situations differ. The table below suggests what test to make.

The quality of clay taken from locations only a few feet apart in the clay deposit may differ. Each lorry load of clay delivered may also differ in quality.

Therefore, the clay samples for testing should be collected from at least 4 different locations within the clay deposit or from where the clay has been dumped.


1) Mix the four samples well .
2) Divide the mixture by a cross cut in the pile.
3) Two portions facing each other are removed and the remaining two portions are mixed well.
4) This is again divided by a cross cut and steps 1-4 are repeated another 3 times.

This process is called quartering, and ensures that the remaining portion is "average quality". It is then used for carrying out the clay tests.

2. Moisture Content.

Even if the clay looks and feels dry, it will contain some water. When clay is purchased it may contain any amount of water, and if the clay is bought by weight the water is also paid for. So knowing the water content helps your profits.

Ideally, each batch of clay delivered to the pottery is tested. This test is also done for checking moisture content of plastic bodies, and semi-dry clay bodies for press molding.

procedure: 1) Sample clay by quartering. 2) Weigh out 200 g (W moist) of the moist clay. 3) Place the clay in a clean cup and heat it to 150° C for 2 hrs. 4) Find the weight (W dry) in g of the dry clay.

3. Particle Size.

A quick test to check clay and sand content of new clay supplies is done by screening procedure:

1) About 500 g clay is dried at 150° C for 2 hours.
2) Weigh the clay.
3) The clay is made into a thin slurry
4) The slurry is screened through one or more fine sieves.
5) The residue left on each screen is dried and put on a set of scales.

This figure indicates the amount of sand in the clay. A 200 mesh sieve holds back particles bigger than 0.0076 mm and some fine sand will pass this screen, but for comparing the quality of new batches of clay with former supplies it is accurate enough.

4. Plasticity.

Atterberg Number: The more plastic a clay, the more water it will absorb without becoming fluid. So the range of water content over which a clay is plastic is a measure of its plasticity. This is termed the Atterberg Number.

plastic limit: The plastic limit, Pw, is the minimum amount of water required to make the clay plastic so that it can be formed.

liquid limit: The liquid limit Lw, is the minimum amount of water required to make a clay slip flow under its own weight.

The water range, Lw - Pw is called the Atterberg number. The water contents of Lw and Pw are expressed as a percentage of the weight of the wet clay.

procedure: 1) A portion of clay, about 300 g, is stirred with water to form a liquid slip of creamy consistency. The slip is poured into a clean plaster mould. After a minute or so a knife is dipped into the slip at intervals, noting whether the incision disappears again. When the incision just remains, a sample of the clay around the incision is removed and its weight, Wwet is found.

3) The remaining clay in the plaster mould is turned around in the mold until it can be formed in the palm of a hand. Knead the clay between the palms of your hand until it starts to crumble. At that point the plastic limit of the clay is reached and the sample is weighed to find W plastic. Dry the clay in the drying oven at 150 C for two hours and find its weight, Wdry.

The higher the number, the more plastic the clay. Examples of Atterberg numbers:

Kaolin clay 10 - 15 Ball clay 10 - 50

5. Loss on Ignition.

Pore water is lost around 100° C. Chemically bonded water is released at 350° - 600° C from inside the crystal structure of the clay minerals. Loss of chemically bonded water corresponds directly to the amount of clay minerals present in the clay. Chemical water is not released from sand or feldspar.


Kaolinite: Al2O3 . 2SiO2 . 2H2O Al2O3 + 2SiO2 + 2H2O loss in form of water 14%

Montmorillonite: Al2O3 . 4SiO2 . H2O Al2O3 + 4SiO2 + H2O (clay) loss in form of water 5%.

A sandy clay with kaolinite clay crystals and showing a loss of 7% would, therefore, contain only 50% clay minerals the rest being non plastic materials like sand, mica or feldspar.

Limestone: CaCO3 CaO + CO2 loss in form of CO2 44%.

If the limestone test sample shows less than 44% loss on ignition we can estimate the amount of sand or other impurities. Lime decomposes at 825° - 900° C.

The loss on ignition test is mainly used to check raw materials supplies and their quality from batch to batch. If the loss is lower than the standard, it shows that the material of that batch contains more sand or other impurities than normal.

procedure: 1) Dry a sample, about 200g, in the oven at 150° C for 2 hrs in a clean bowl. 2) Find its weight, Wdry. 3) Fire the test sample to 1000° C or higher in an unglazed bowl. 4) After cooling but when it is still warm from the kiln, find the weight, Wloss. 5) Compare with results of former tests. Look in Appendix (p. ) for examples of Loss on Ignition.

6. Shrinkage.

Drying shrinkage depends on the fineness and plasticity of the raw clay. Firing shrinkage indicates the degree of vitrification of the clay or clay body at the temperature at which it has been fired (the higher the temperature, the more shrinkage).


1) Mix the clay with water to plastic consistency and knead it well.

2) Form 10 test bars of each clay to be tested. The test bars are molded in a standard mould.

3) Mark the test bars with two incised lines exactly 100 mm apart.

4) Turn the test bars several times while they dry to prevent warping.

5) After the test bars are completely dry (open air) measure the distance between the two cuts in mm.

Drying shrinkage in % = 100 - dry length (mm)

For the most accuracy, take the average of 10 test bars.

6) Firing shrinkage is found by firing 5 of the test bars to the intended firing temperature.

7) Measure the distance between the lines on the test bars in mm.

Firing shrinkage in % = dry length - fired length(mm)

Total shrinkage in per cent = 100 - fired length (mm)

The firing shrinkage is found as an average of 5 test bars.

Here it is recommended to make many test bars in order to ensure more reliable results.

7. Softening Point.

Clay becomes soft at high temperatures, because various impurities like feldspar, lime and iron oxide start to form a liquid mass between the clay particles. This will cause the ceramic ware to warp during firing and if the clay is used for refractories (like saggars), these may bend under load.

This test is used to compare the refractoriness of different clays.


1) Two test bars, formed as described in the shrinkage test (could be two out of the five used for shrinkage test), are suspended on two points placed 100 mm apart. This leaves the test bar between the two marks unsupported.

2) During firing, the test bar will bend and the degree of its bend is a crude measure for the clay's tendency to sag under load.

The degree of its bend (see fig.70-2) is measured in mm.

8. Porosity.

water absorption: A piece of clay fired to about 600° C will be very porous. As the temperature increases, the feldspar, lime and other impurities will begin to melt together with the silica in the clay, forming a glass. This will gradually fill the pores of the clay, making it less porous. This process is called vitrification, and the more a clay vitrifies the less porous it becomes.

Porosity can be tested by measuring the fired clay's ability to absorb water procedure:

1) Heat a sample of a fired body without glaze to 150° C for 1 hour, or take it directly from a warm kiln after firing.

After drying, find its weight, W dry.

2) Immerse the sample completely in water, and leave it for 24 hrs.

3) After 24 hrs. take the test piece out of the water, and dry its surfaces with a piece of cloth.

4) Find the weight, Wsoak, of the soaked test piece.

Examples of water absorption:

Red bricks: 20 - 40 % Earthenware: 5 - 20 % Stoneware: 1 - 5 % Wall tiles: 15 - 20 % Floor tiles: 3 - 5 %

A body with water absorption of 1% or less, and with low lime content, is termed acid proof. However, floor tiles for dairies and most other uses, where acid proof materials are required, can safely have a water absorption up to 3% .

Bodies for kiln furniture should have water absorption of 18% or more, otherwise they are likely to break due to thermal shock.

9. Transverse Strength

A transverse strength tester is used for testing the strength of:

1) Dry green test bars of clay bodies or raw clays. 2) Fired bodies. green strength: The strength of green bodies is also an indirect measure of the plasticity of the body. The more fine clay particles a clay contains, the greater its strength. Clay from new deposits can be compared with known clays. By comparing the strength of new clay supplies with old ones from the same source,we can control the quality of our supplies.

fired strength: By testing the fired clay body, we can check its strength. The strength of fired bodies (either glazed or unglazed) depends to a high degree on the firing temperature. Around 900°C feldspar, lime and other impurities in the clay start to melt, forming a glassy mixture which "glues" the clay particles together. This process is called vitrification.

The graphs in fig. 72-1 show the relationship between fired strength, firing temperature, and vitrification. As the temperature rises, more and more glass will form until all spaces between the clay particles are filled with it (total vitrification). At that point the clay becomes similar to glass, and it is brittle with little strength.

transverse strength tester: Test bars similar to the ones used for shrink- age tests are used for testing of transverse strength. The test bar is supported at two points (adjustable) and a load is applied to a point half-way between the supports (fig. 73-1). The load is a bucket which is gradually filled with water (or dry sand). When the test bar breaks, the filling of water is stopped and the weight of the water is found.

Testing of fired bodies should be done close to the pivot (short L2) whereas testing green bodies can be done farther from the pivot (long L2). The test bar is placed evenly on the supports and the load W2 is placed exactly half-way between the supports. An empty bucket is placed on the hook, and is counter- balanced with the counter weight. The load is made to just touch the test bar by adjusting the balance with a screw-weight.

After reading L1 and L2 on the ruler and measuring width (b) and thickness (t) of the test bar, the transverse strength can be calculated (all measures in cm and kg).

The same test bar can be tested 3 times by first testing it with (a) at maximum and then test the two resulting pieces again. Normally at least 3 test bars should be tested and the final result is recorded as an average of all 9 results.

The transverse strength is also called modulus of rupture. Examples of green transverse strength: kaolin 7 - 15 kg/cm2 ball clay 20 - 90 kg/cm2

moisture: The moisture content of the test bar greatly influences the transverse strength. Therefore, the test bars should be dried to the same degree of moisture. With changing weather, that may be difficult to ensure. A safe way of establishing consistent results is, as a standard, to dry all test bars at 100° C for 2 hours.