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Shrinkage and AbsorptionTesting prospective clay bodies for shrinkage and absorption can help eliminate problem batches before you form and fire your products. Bodies that have a high level of shrinkage might be subject to cracking problems, while extremely low shrinkage can cause the final product to be fragile. A stoneware body might exhibit satisfactory shrinkage between 9-14 percent, while an acceptable cone 6-10 body shrinkage might fall within in the 10-13 percent range. Porcelain and whiteware shrinkages are typically higher, but lower shrinkages can also be acceptable. You will need to go through some trial and error testing to find out what level of shrinkage works for you, your firing and your glazes.
The absorption of a clay body indicates its vitrification, or the amount of glass formed within the body. In general, the lower the absorption, the tighter the body and the more vitrified it is. Many potters consider an absorption of under 5 percent acceptable. At Ceramic Design Group's facilities, we use 3 percent or lower. However, an absorption that is too low--say, around 1 percent--might weaken a body rather than strengthen it.
1. Decide which temperature range you want to work in, and whether you want a casting body or a plastic body.
2. Choose a selection of materials for both the plastic and non-plastic parts of the body. Examples of body formulations can be obtained from books, magazine articles, suppliers and other potters.
3. Set up a chart similar to the one shown in Table 1. Note the arrangement of the materials, as well as the varying amounts of each material. Also note that any additions to the body, such as grog materials and refractory calcines, bentonites, coloring oxides, etc., are always added on top of the sample. This principle also applies when mixing larger batches.
4. Mix a 10-lb sample of each body to a slip-like consistency using an electric drill-powered mixer. Since a plastic body contains about 20 percent water, approximately a 1⁄2-gallon of water will yield about 1 gallon of slip for testing when using a 10-lb dry weight sample of a casting body. Screen each sample through a 60-mesh sieve. (Note that the screen residue is a graphic indication of what is referred to loss on ignition [LOI]--organic materials that are responsible for a number of glaze-related defects.) If you are testing bodies for slip casting, you will also need to check the mixture for proper viscosity and specific gravity at this point.
5. Dry the plastic body on a plaster slab to wedging consistency.
6. Prepare two small, flat bars or slabs approximately 2 in. wide by 5 in. long for each clay sample. Scribe a thin line on the surface using a needle tool, xacto knife or razor blade in the middle of the slab over its entire length. Using a metric ruler, place two thin vertical lines at either end of the main line, 10 cm apart (see Figure 1).
7. Identify these slabs with a reference indicating the test number, date and/or any other data that you might find useful. We use small metal stamps, as shown in Figure 2.
8. Dry these small slabs, turning them over periodically and keeping them as flat as possible.
9. Fire the slabs to maturity.
10. Measure the fired length of the 10 cm line and calculate the shrinkage of the clay from wet to fired using the formula shown in Figure 3. Calculate the average shrinkage by adding both shrinkage numbers and dividing by two. Record these numbers for each test.
11. Weigh each bar and record the dry weight. Then immerse the bars totally in a container of water for at least one hour or more. Some potters prefer to use boiling water for this part of the test. (It's possible that the boiling water enters the pores of the fired clay body better and provides more accurate test results.)
12. Dry the bars off, removing any water left on the surface, and weigh them on a small laboratory scale. Using the formula shown in Figure 3, calculate the absorption of the sample tiles and then calculate the average. Record these numbers for each test. Save the remaining clay bodies in plastic bags for further testing.
Many different approaches can be used to determine acceptable levels of shrinkage and absorption, and there are some exceptions to every rule. However, finding the right balance for your clay body can help you create a finished product that is strong and durable.
WorkabilityFor plastic bodies, the workability of the clay is another important factor. Workability can be tested by rolling a small amount of each clay body into a small coil and wrapping it around your finger. The clay should wrap easily without cracking. The samples used for this test do not have to be aged, but they should be wet-mixed as described under "Shrinkage and Absorption" so that all of the clay particles are thoroughly wetted. This relatively simple test is a good indication of how plastic the body might become with the proper aging. Throwing some small bowls or other small pieces is another way to get an immediate indication of the clay's workability.
Once you have developed a clay body with the perfect level of workability, measure its water content (also known as the "water of plasticity") using a penetrometer (see Figure 4). This plunger-type device provides a numerical reference of the hardness or softness of the clay. By keeping track of the amount of water used to prepare a given batch, you will easily be able to either repeat the same formulation or adjust the body for other forming processes. For example, adding less water will result in a stiffer clay body necessary for hydraulic pressing, while adding more water can generate a softer clay body for hand throwing or jiggering.
Additives can also be used to enhance a clay's plasticity or workability. Adding vinegar, for instance, keeps the clay in a slightly acidic state that promotes the "water hull concept"
--i.e., the total surrounding of each clay particle in a packed condition with water. Other additives can also be used, depending on the desired effect.1
Balanced GlazesAssuming that the shrinkage, absorption and workability of the clay is acceptable, you will next want to test for glaze fit and durability. Make some test tiles from your remaining test bodies. Trim the pieces, fettle the seams, and bisque fire the pieces to your normal bisque firing temperature. Glaze the bisqued pieces and fire them to your working temperature. After the pieces have cooled, look at each one under a magnifying glass to see if it the glaze is crazed. Crazing will appear as a fine network of lines in the fired glaze. Shivering--in which the glaze buckles or flakes off the clay body in sheets--might also be apparent. Another method to determine whether the glaze is crazed is to rub the surface with a marker and quickly wipe it off. The black pigment will highlight any crazing.
The DilatometerUsing a dilatometer will provide you with a graphic footprint that will show linear change on heating and cooling, coefficient of expansion and, very importantly, any evidence of free silica in unstable forms upon heating and cooling (see Figures 5 and 6).
A dilatometer subjects a small bar or cylinder of a clay body to a specific heating cycle in either an oxidizing or reducing atmosphere (see Figure 7). It measures the small changes in dimension upon heating and cooling (also known as differential thermal analysis, or DTA), and plots a graph showing this information. If excess silica emerges at the top end of the cycle, the dilatometer will show this on the graph. It will also show the quartz inversions as the sample is heated and cooled since this relationship is reversible. If the graph plots a very large "blip" at quartz inversion temperatures, the clay body is not reliable.
If you make functional ware that is intended for daily use, your clay body must be able to withstand the DTA test. Anything less is not only irresponsible, but can also subject you to legal action if injury results. If you do not own a dilatometer, find a company that will perform this analysis for you. Be proactive and take responsibility for your ware. If your samples pass this crucial test, as well as the other tests described earlier in this article, you have successfully formulated a clay body that will meet your and your customers' specifications.
Material Variability and Pre-Mixed BodiesThe science behind clays, clay bodies, and ceramic chemical and physical reactions is quantifiable. Yet a number of challenges often stand in the way of obtaining a "perfect" clay body. For example, few potters have the necessary resources to wet-mix their clay bodies, screen them through a vibratory screen and filter press them before pugging. These processes are expensive and time-consuming, and they also require a large amount of space for equipment.
Additionally, since raw materials can exhibit variations from batch to batch, we have to operate with a certain degree of faith that what is in the bag as dry processed material (or in the box as a prepared body) will provide us with defect-free ware from the kiln. As small businesses, our ability to beneficiate a ceramic raw material might be limited to dry screening large-particle-sized fireclays or testing a few bags of the same material with an acidic solution to check for the presence of lime. Basically, we can only do so much.
Many clay body manufacturers and raw material suppliers work within similar constraints. No manufacturer can check each bag of raw materials, nor can they test every 50-lb pug of prepared clay that goes into each box. The materials and clays that potters use constitute a minute part of what is used by industry. However, potters can and should check with their material and clay suppliers to find out what types of testing are regularly done on their products.
Ideally, a testing program by a materials supplier or clay body manufacturer will include the following procedures:
• Spot-checking each shipment of raw materials with X-ray diffraction, a valuable tool for comparative analysis of raw materials from batch to batch. X-ray diffraction shows the angle of diffraction of the crystal shape of each material, and this information can be compared to other batches of the same material to provide quantifiable data.
• Testing the range of firing of a component clay or prepared clay body using a gradient furnace. This test also provides data on color, shrinkage and the overall material compatibility of the formulation.
• Testing the DTA of the clay body using a dilatometer. As discussed earlier in this article, a DTA test will show heating and cooling curves, quartz inversion and whether the necessary chemical reactions have yielded a correctly balanced clay body.
• Analyzing the green and fired strength of the clay body using a Dillon shear device or other comparable measurement technique.
• Testing the clay body for shrinkage, absorption and glaze fit.
Some suppliers undertake extensive testing programs, while others do a bare minimum of testing and adjusting from batch to batch. It's up to you as the end user to determine whether you can take on the additional testing or whether you need your supplier to verify their product quality.
Even if the above procedures have been performed, you might also want to ask how the clay bodies are physically mixed. Questions might include:
• Are the formulas computer-generated to minimize human error?
• How are the component batch weights checked and rechecked, and at what stages of the mixing process?
• Is the water of plasticity measured during blending, and is the pugged clay checked with a penetrometer?
• Are samples from each batch checked for absorption, shrinkage and color? Have these tests been fired according to a repeatable and specific profile?
• Are batch numbers generated so that if there are problems with the clay, the data can be rechecked to determine the cause?
• If the end user does encounter problems, how does the supplier handle these issues?