| The Self-Reliant Potter: Refractories and Kilns |
1.6.1 High temperature testing
1.6.2 Refractory materials and bodies
1.6.3 Refractory items
Bagwalls, flue linings or saggars that give in during firing may ruin both the kiln and the ware. To avoid these problems tests can be done to ensure that the materials will withstand the severe conditions to which they will be exposed during many cycles of firings. Therefore before trusting a refractory raw material or a refractory product, say a firebrick, some simple tests should be carried out. Simple tests of clay supplies can also tell us whether we get the clay we expected and which may have been paid for dearly.
The first thing we want to know is: can the clay withstand high temperatures? For this, a kiln which can withstand temperatures of 1300-1400 °C would be ideal for testing. Few potters will have access to such a kiln, but a sample of the clay could be sent through local authorities to the national geological department or ceramic institute which will normally be interested in gaining information about suitable clay sources.
Quicker results could be obtained by firing the test piece in the flue, in front of the firebox, or on top of the bagwall of a potter's kiln. The temperature may not be 1300Ã¹ 1400 °C but it is most likely the highest temperature the material will have to withstand in practice.
In case no high temperature kiln is available, a small test kiln could be constructed. In extreme cases, where there are no proper refractories available for the construction of a test kiln, (this was once experienced by the author in Africa) the test kiln, built of the untried refractories and fired to as high a temperature as possible, becomes the test itself. A small test kiln is also useful for firing glaze and body tests and the one shown in fig. 146 is not expensive to construct. By changing the firebox arrangement it can be fired with firewood, oil or coal.
In most cases it will already have been established whether or not the type of clay in question is suitable for high temperatures and the individual potter or local pottery development centre will only need to check the quality of clay supply and refractory body mixtures. The following tests should be carried out with new batches of clay.
The clay to be tested should be collected from at least four different places at the clay deposit or from where the clay has been dumped. The four samples of about equal size are mixed well on a swept concrete floor. The sampled clay is then divided into four equal portions. Two portions opposite each other are set aside and the other two are mixed thoroughly. This process of dividing and mixing should be repeated at least four times. This method is called quartering (fig. 147) and ensures that the final sample is representative of the bulk of the clay.
A sample of about 100 g is weighed on a scale. The weight Wm is recorded and the sample is heated to 110 - 200 °C for an hour so that all water evaporates. It is then put on the scale again immediately and the dry weight Wd recorded.
Moisture content in per cent = (Wm - Wd)/Wd x 100
When the clay is purchased by weight the moisture content shows how much water has been paid for. When weighing the clay according to recipes, excessive amounts of water in the clay should be compensated for.
The clay is mixed with water to normal plasticity and 5 - 10 test bars measuring 1x2x 12 cm are formed. A wooden mould makes this job more easy. Two parallel lines exactly 100 mm apart are marked across all the test bars. While drying, the test bars should be turned over now and then in order to avoid warping. When the test bars feel dry the distance between the two cross-lines are measured in mm on all bars and the amount of drying shrinkage is found:
Drying shrinkage in per cent
(100 - Dry length)/100 x 100
As the distance was 100 mm the shrinkage in mm is equal to shrinkage in per cent. After firing the test bar to the highest temperature possible additional shrinkage is measured in mm and recorded as:
Firing shrinkage in per cent = (Dry length - Fired length)/Dry length x 100
Total shrinkage in per cent = 100 - Fired length in mm.
The drying shrinkage indicates to some degree the plasticity of the clay. A large drying shrinkage means that the plastic clay could absorb much water, which in turn indicates fine clay particles. The figure for drying shrinkage should be compared with figures of former supplies to see if the present batch is of the same quality.
The firing shrinkage indicates how fusible the clay is. A high shrinkage normally means a lower melting point. The total shrinkage of refractory bodies tells us how much bigger we should make our moulds. In case we want our slab to measure 30x30 cm and the total shrinkage of the clay/grog mixture is 8% then our mould frame should measure:
30 + 30+8/100 cm = 32.4 cm on each side.
The test bars are placed in the kiln as shown in fig. 1-49. The test bars should be supported so that the free span equals the distance between the cross-lines of the test bar. If possible cones should be placed next to the test bars to show the temperature. After firing the amount of bending is compared with the cones and results from former tests. When testing a new clay the test bar should be placed so that it can be viewed through a spyhole and the approximate temperature at which bending starts is noted.
After measuring drying shrinkage some of the test bars can be used for measuring the amount of pore water. Pore water is the water that is left in the clay after the water of plasticity has evaporated. The pore water will only leave the clay above 100 °C during the smoking period of biscuit firing.
First the weight, Wd, of the dry test bar is found and recorded and the test bar is heated to 110-200 °C for one hour. Immediately after that the test bar is weighed again, weight Wp is recorded and the percentage of pore water can be computed:
Pore water in per cent = (Wd - Wp)/Wp x 100
The pore water percentage expresses the fineness of the clay particles or the plasticity of the clay. The test is simple and is good for ensuring that new supplies of clay do not contain too much sand.
The following pore water contents are typical: kaolin 1.5 %, fireclay 3.5 %, ball clay 6.1%, brick clay 2.2%, bentonite 14%
The results of drying shrinkage and pore water content tests discussed above are an accurate indicator of a clay's plasticity. However, the first and most simple test for any potter is to wet a small portion of the clay in the palm of his hand and get the "feel" of it. The clay is rolled into a pencil shape and the more this "pencil" can be bent into a ring without rupturing, the more plastic the clay is.
A quick test of new clay supplies can be done by making the clay into a thin slurry and screening it through one or more very fine sieves. A 200 mesh sieve holds back particles bigger than 0.0076 mm. The residue on the screen is dried and put on the scale. If the weight of this residue is called Wr and the dry weight of the total sample Wc,
Size less 200 mesh in per cent = (Wc - Wr)/Wc x 100
This figure can be used to check the amount of sand in the clay. Some fine sand will pass a 200 mesh sieve, but for comparing the quality of new batches of clay with former supplies it is accurate enough:
"Spalling count" test
Besides possessing refractoriness our refractory products such as firebricks and saggars should be able to withstand many cycles of heating and cooling without cracking or spalling. The ability to withstand thermal shocks is tested by heating a standard-size (appr. 23 x 11.5 x 6.5 cm) firebrick to around 900 °C. The hot firebrick is then picked out of the kiln and plunged into water of room temperature. This is repeated until half of the brick measured by weight has cracked away due to this shock treatment. If the brick can endure 10 cycles of such heating and cooling it is very satisfactory. The clay body for saggars or slabs is formed into bricks and tested in the same way.
If the clay body of saggars or bricks becomes too dense it will be more prone to cracking due to thermal shocks. The more dense a body is the less water it will absorb. So the density (or porosity) can be measured by soaking a piece of the fired clay body in water for at least 24 hours. It is then taken up and after its surface is wiped dry its weight, Ww, is found. The soaked test piece is then heated at 110-200 °C for one hour and its dry weight, Wd, is recorded. Porosity or more accurately the water absorption can be estimated.
Water absorption in per cent = (Ww -Wd)/Wd + 100
For saggars and slabs a figure of 18-25% is reasonable.