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Few things are as exciting as finding your own clay materials, testing and blending them into clay bodies. But how can you easily evaluate a clay material? The first thing to realize is that mixing clay bodies is not like mixing glazes. With glazes, glaze chemistry is a key predictor of fired results. With clay materials, however, physical working properties like plasticity, color, shrinkage and particle size demand attention.
Lab results are useful for ongoing quality control purposes, but potters want to know what a new clay is actually useful for; seeing its properties relative to other known materials is the best way to accomplish this. Obviously, you need a body of experience of other known materials, so I recommend using the methods described here to evaluate some common commercial clays (e.g., a plastic and non-plastic kaolin, a low- and high-coal ball clay, a bentonite, a fireclay, a low-fire iron red clay, or a stoneware) of widely varying particle size, plasticity, flux content and homogeneity to create points of reference. Keep careful notes and records of your findings for later reference.
Sieve AnalysisKnowing the identity, amount, size, and proportions of sizes of apparent particles (clay and non-clay) in a sample is important, since the feel of a material can sometimes be misleading. Clays with a lot of +100-mesh impurity particles can be difficult to work with. For example, lime causes surface popping, while iron sulfate causes specking.
The apparent particle size of a material is determined by washing it through a series of progressively finer sieves. Wash 50 grams through a 100-mesh screen with a 200-mesh screen stacked below it. Dry the screens and weigh the material on each, then multiply by two to obtain a percentage. Compare the amounts with bodies of known performance. Examine the particles using a light scope or microscope. Are they quartz sand? Hard clay or shale? Lignite? Flakes of mica? Dark particles of iron or manganese material?
If the clay contains a lot of quartz sand, remember that these particles expand and contract suddenly as they pass upward and downward through the quartz inversion phase (about 1050°F) during firing, producing micro-cracks around each grain in the rigid fired matrix. Such materials must be fired more slowly, and the sand actually performs the valuable services of terminating cracks and helping to channel water and gases of decomposition during drying and firing.
What's in a Lump?Visually inspect a raw representative lump specimen of the material. If the lump smells bad, it might contain decaying plant matter. Lumps that are grey or black might contain lignite (a classic sign of ball clay). Wash some of the slaked clay through a fine screen (e.g., 100 mesh) to see what organics the screen catches. During firing, organic particles create gases that can bubble-damage glazes, or bloat or black-core bodies.
Dry about 1.5 lbs of lumps in an oven around 100°C and note if they crack into many smaller pieces during drying. If so, this indicates the presence of bentonite (a sign of potential drying problems). Using a hammer, break larger lumps into smaller ones. Note how hard they are, and whether they fracture or turn to powder. Non-plastic clays can be very fragile, while plastics can be amazingly hard. If the broken surfaces are bubbling, the material is over fired; if the pieces absorb on your tongue, the material has not vitrified.
If there are stringers of higher iron material within the lumps, the breaks should have occurred on these boundaries. If the material contains larger particles of iron and calcium impurities, they should be evident on the broken surfaces (e.g., iron staining, speckles, melt-outs or even pop-outs).
Drip some vinegar or 50% muriatic acid (HCl) on a lump. Fizzing is a predictor of serious efflorescing and volatile firing properties.
Prepare a Plastic SampleYou'll need a plastic specimen of the clay material for testing purposes. Slake the dried 1.5 lb of lumps. "Slaking" refers to the breakdown that normally occurs when you immerse a dry clay lump in water (lumps should not be bigger than about 1/2 in. on their largest dimension). Typically, the water attacks the clay lump, leaving a pile of fine material. Clays that slake can be stirred or mixed with a propeller to a smooth slurry. Very plastic clays will not slake because the surface swell prevents water penetration. Damp lumps will likewise not generally slake because existing water creates a barrier to further penetration.
Sprinkle the broken lumps into a 1-pint plastic container with enough water to immerse them completely. Leave them for an hour or so, and then propeller-mix the mixture to a smooth slurry with a blender and let it stand for a few minutes. If the slurry is lumpy, or if sand, shale and grit settle on the bottom, then normal screening or grinding will be needed during production processing. Does the slurry gel, indicating thixotropic behavior? This could preclude its use in casting bodies and signal problems with plasticity and stiffness maintenance during aging. Note any organic material coating the top at this stage as well.
Pour the slurry onto a big plaster batt and let it dewater to the right stiffness for plastic forming (not too soft), then wedge it thoroughly, flattening it back down on the batt if it's still too soft. Fine-particle clays have ultimate sizes much smaller than a micron, while large-particle clays, such as kaolin, might have 5-micron particles. Sandy and silty materials dewater in minutes or even seconds, even though they might appear to be composed of fine particles. Fine clays are easier to process and have fewer fired defects, but unfortunately they often experience drying problems.
Bentonitic, highly plastic clays can lie on a plaster slab for days during dewatering, and when you peel them up for wedging they are sticky and still too soft. Clay/non-clay hybrid materials often separate during dewatering; either the fines will layer against the plaster or the particulates will settle to the bottom quickly.
If a difficult-to-clean film forms on your plaster slab after dry-out, it likely means the clay contains soluble salts (e.g., calcium sulfate). Many native clays contain sulfates that come to the surface with the water and leave solute behind, which melts into a dirty, glassy surface layer. This universal problem in the ceramic industry is dealt with through the addition of barium carbonate (0.2-0.4%) during clay processing. The barium carbonate reacts with the sulfates to produce insoluble carbonates and barium sulfate.
Evaluate PlasticityDoes the material feel more or less plastic than a typical pottery clay body? Good plasticity is apparent if a rolled cigar shape can be bent without splitting. Balance a cigar-shaped piece on your finger and put a couple of drops of water on the top center of the material. If splits quickly begin to form, the material has a tendency to allow water penetration. Clays with coarser grains and lower plasticity can have advantages; some potters, for example, like the tactile texture, better microcrack termination, and more earthy feel and look. However, they can be susceptible to water splitting.
If the clay is clearly so plastic that it is too sticky to roll or peel up after rolling, mix something with it to cut the plasticity. For low-fire clays, start by adding 25% talc. For high-fire, start at 25% silica. An even better option is to powderize some of the test material, calcine it, mix 50:50 calcined material with raw material, and start again.
Test BarsNon-plastic clays are difficult to wedge, and it may not be practical to make test bars from them. If this is the case, start by mixing in 25% ball clay. If the material is fragile but workable, roll it on top of some newspaper and place a 12-cm-wide board on top. Turn it over and peel off the newspaper (treat the dried bars with great care, as they will break easily).
When the clay is suitable for forming, roll a 1-cm-thick slab (use 3/8-in. diameter metal bars as roller guides) and carefully lay the slab on a 12-cm-wide board. Cut along both sides to produce a 12-cm-wide strip of clay. Use a wooden ruler and a thin-bladed knife to cut bars 2.5 cm (1 in.) wide. Be careful to cut vertically so the bars will stand on edge for firing. Note if the clay is sticky (e.g., the ruler sticks).
Make five or six bars. Stamp identification information and a sequential number on each. Carefully make marks 10 cm apart on each bar that can be accurately measured after drying and firing. Make a half-length bar for a loss on ignition (LOI) test. Roll a slab 5 mm thick (3/16 in.) and cut a 12-cm diameter drying tester disk. Put the disk upside down on the board with the bars and place a plaster-filled small soup can in the center, leaving only the outer part of the disk exposed. Dry the specimens completely.
At this point, any solubles from the center will migrate with the water to the outer exposed section of the circular drying disk and concentrate there during drying, producing obvious differences in the surface coloration. Note the soluble deposits around the edges of the disk and fire it to see if the deposits are harmful to the fired surface. Note the nature and number of cracks on the circular drying disk.
Clean the test bars using a knife to remove corner burs. Plastic clays are strong and hard to trim, and they chip at the corners. Highly plastic clays shrink a lot, which warps and cracks the bars during drying. Non-plastics cut to soft edges and are fragile. Kaolins feel smooth and soapy on cut edges. Break one of the bars and rate it based on your knowledge of the strength of kaolin, ball clay, fireclay, etc.
Record the weight of the half-width bar and calculate the material's water content as wet weight (dry weight divided by wet weight, times 100). Kaolin has water content of less than 20%; stoneware, 20-23%; and ball clay, 25-30%. Since bentonites and some ball clays need to be diluted to cut plasticity, water content numbers will be less meaningful for them. Record the average dry shrinkage of all the bars (shrinkage is simply 100 minus the dry length). High shrinkage (7% or more) can be tolerated if coarse particles are available to terminate micro-cracks and prevent warping. Low shrinkage (4% or less) is normally accompanied by poor dry strength and low plasticity.
Using your experience from firing the initial lumps, guess the maturing temperature and fire your test bars to one or two cones higher and lower. Place the bars on their edges on the shelf. Fire one of the bars at an accelerated rate (compared to your typical firing) up to 500°F to see if it can withstand a faster firing. Cracking or exploding indicates the presence of fine-particle bentonite or ball clay without interspaced water-channeling particles.
Fire the half-length bar to at least cone 02, record its weight and calculate loss on ignition (LOI) by subtracting the fired weight from the dry weight, dividing by the dry weight and then multiplying by 100. Clays that contain organic matter lose weight (often 7% or more) on firing as the organics and water burn out. Fire all of the bars to your intended temperatures, being sure to include guide, guard and firing cones in each firing near the bars. Record the temperature reached according to the degree of bend of the three cones.
Shrinkage and PorosityMeasure the distance between your marks on each fired bar and record the fired shrinkage (subtract the fired length from the dry length, divide by the dry length, and then multiply by 100). Record the weight of each bar, boil all of the fired bars for five hours, and then soak them for 19 hours. Blot them dry and record the weight again. Porosity is determined by subtracting the dry weight from the wet weight, dividing by the dry weight, then multiplying by 100.
Plot a chart showing percentage porosity and shrinkage against temperature. Compare this chart with one for a standard commercial body you routinely use with success. These two-line charts are a visible manifestation of fired maturity; over time you will learn to use them to spot inconsistencies in fired behavior and evaluate firing performance.
Vitreous stoneware bodies are best tuned to fire at the point where porosity and shrinkage curves are leveling out before reversing direction. Red earthenware gives best results at least 2-3 cones before it suddenly changes color to dark brown. Sculpture bodies need a compromise between fired strength and resistance to fired warping.
Further EvaluationNow that you know how the material behaves in relation to kaolin, ball clay, red clay, stoneware and bentonite, put it into a recipe accordingly. Don't be tempted to use it alone as a body; it should at least be diluted to dampen changes in properties over time. Feel free to use as much of the material as possible in a body, however, along with other materials according to their complementing strengths and weaknesses.
For more information regarding clay evaluation, contact Plainsman Clays Ltd. at Box 1266, Medicine Hat, AB T1A 7M9 Canada; (403) 527-8535; fax (403) 527-7508; e-mail firstname.lastname@example.org ; or visit http://www.plainsmanclays.com .