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Simple Physical Testing to Compare Clay Materials

Section: Testing, Subsection: General


Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.

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Few things are as exciting as finding your own clay materials and testing and blending them into clay bodies. But how does one evaluate a clay material by simple means? is a good model. It has mined a wide range of its own clay materials since 1965 and has a very practical set of inexpensive lab tests to characterize them. My work there inspired the development and refinement of these tests and the creation of and the Digitalfire Reference Database.

The first thing to realize is that mixing clay bodies is not like mixing glazes. Glaze chemistry is a key predictor of fired results but with clay materials the physical working properties demand attention (e.g. plasticity, color, shrinkage, particle size). You can mentally model clay working properties by visualizing the particles, their mineralogy, sizes, shapes, surface area and interactivity with water and each other (clay particles differ from other mineral particles in their affinity for and resistance to settling out in water and drying bonding and water removal). Likewise, fired properties are much better explained by particle dynamics, their physical changes, decomposition and participation in the formation of new minerals in the matrix.

It is amazing how many questions you can answer about a clay material with simple observations. Often data sheets accompanying widely-used commercial materials fail to provide enough information to deduce answers to simple questions like: Will it survive fast firing or drying? Will common glazes shiver or craze on it? How does it perform and feel in plastic forming? Can it be slaked and used in the raw form? Will it effloresce? What are its drying properties? How does it compare to common similar materials?

Many potters have wanted to evaluate a clay that they have found in their area to find a way to use it in their process. They would generally want to take a practical hands-on approach to such a task in contrast to industrial technicians that generally evaluate materials based on their data sheets. While lab numbers are useful for ongoing quality control, in isolation they often do not paint a very practical picture of what a clay really is? However seeing the properties of one material relative to other known ones is a whole different matter. Imagine if you could describe the materials you know in concrete terms and use these as benchmarks for new ones. What materials do you know? The ones you have been using also along? But do you really know them? Likely they are just powders. Let's change that. The methods described here are a good starting point in learning to evaluate raw native materials but also common commercial clays of widely differing particle size, plasticity, flux content and homogeneity to create points of reference in your mind (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, a stoneware; learn more at the Digitalfire Reference Database). You will not believe the sense of control this kind of testing can give you!

Does it contain much soluble calcium?

This early field test on raw lump clays involves dripping some vinegar or 50% muriatic acid (HCl) on a specimen. Fizzing is a predictor of serious efflorescing and volatile firing properties. However if you are going to use the material a low temperatures, you might want to test it anyway.

Lump properties

If you have a raw representative lump specimen of the material some testing opportunities present themselves. If wet lumps smell bad it might contain decaying plant matter. If raw lumps are grey or black it might contain lignite (a classic sign of ball clay). Wash some of the slaked clay (see below) through a fine screen (e.g. 100 mesh) to see what it catches. During firing organic particles create gases that can bubble-damage glazes or bloat or black-core bodies. Dry about 1.5 lb of lumps in an oven around 100C and note if they crack into many smaller ones during drying (indicates bentonite). Using a hammer, break larger lumps into smaller ones and note how hard they are and whether they fracture or turn to powder (non-plastic clays can be very fragile, plastics can be amazingly hard). If there are stringers of higher iron material within the lumps the breaks should occur on these boundaries. If there are larger particles of iron and calcium impurities they should be evident on the broken surfaces (fire a small raw lump to cone 6 and 10 kiln to check for iron staining, speckles, melt-outs or even pop-outs). Gauge the fired maturity to the lump by breaking with a hammer, if the broken surfaces are bubbling it is over fired, if they absorb off your tongue vitrification has not been reached.

Prepare a plastic sample

For many of the tests below you need a plastic specimen and the act of preparing it will tell you a lot.

Slaking properties

‘Slaking’ refers to the breakdown that normally occurs when you immerse a dry clay lump in water. Typically the water attacks the clay lump leaving a pile of fine material (rather fascinating to watch). Clays that slake well will break down in minutes and stir or propeller mix to a smooth slurry. Only very plastic clays will not slake (surface-swell prevents water penetration). Damp lumps will likewise not generally slake because existing water creates a barrier to further penetration.

If you have a raw lump sample then you will be able to note slaking properties during preparation of a plastic sampe. Sprinkle 1.5 lb of that material into water (lumps should be less that about 1/2 inch on their smallest dimension). During the process take the opportunity to note if the slurry is lumpy or sandy or if shale and grit settle on the bottom (of course this will mean that screening or grinding will be needed during production processing (this is normal). Also not if the slurry gels, this indicates thixotropic behavior. Also, is there organic material floating on top?

However as a consistent test it is best to slake one of the dried test bars later in the process. This works well because you will have a specimen of consistent size and the slaking time can be compared. The best method is to set a bar upright inside a glass and then fill it with water. You can see how long it takes before the bar begins to slake and time the process effectively. Note that soluble salts on some surfaces of the bar may significantly slow the onset of slaking.

Dewater the slurry

Pour the slurry onto a big plaster bat and let it dewater to the right stiffness for plastic forming (not too soft) and wedge it thoroughly. This process may involve peeling it up, wedging it and flattening it back down on the batt several times.

Fine particle clays have ultimate sizes much smaller than a micron. Large particled clays, like kaolins, might have 5 micron particles. Sandy and silty materials dewater in minutes or even seconds even though they might be appear to be fine particled. Bentonitic highly plastic clays can lay on a plaster slab for days while 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 against the plaster 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) and will need some barium carbonate.

Plastic properties

Does it feel more or less plastic than a typical pottery clay body? Good plasticity is indicated if a rolled cigar shape can be bent without splitting. Tendency to allow water penetration and water splitting is indicated if you balance a cigar shaped piece on your finger, put a couple of drops on the top center and splits begin to form quickly (clays with coarser grains and lower plasticity have advantages but 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, start again and mix something with it to cut the plasticity. A standard in many labs is to mix it 50:50 with silica powder, this is commonly done with ball clays and bentonites. That means that every time you test a ball clay or a bentonite you will need to mix it with the silica to have a comparative test. However there is another option, but it takes more effort: Powderize some of the test material, calcine it and mix 50:50 calcine:raw.

Non-plastic clays present a similar challenge. It is common for labs to mix them with ball clay, perhaps 25% to make them workable. But again, there is another option that takes more effort but is potentially better: Even exceptionally non plastic clays can be formed into bars by adding a gum, by dust pressing or by a technique that involves preparing and handling the short clay and fragile bars with great care (see below).

Dried bars ready to measure and
fire at various temperatures

Make test bars

When the clay is suitable for forming, roll a 1cm thick slab (use 3/8” diameter metal bars as roller guides) and carefully lay the slab on a 12cm wide board. Cut along both sides to produce a 12 cm wide strip of clay. Use a wooden ruler and thin bladed knife to cut 2.5cm (1”) wide bars (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). If the clay is very non-plastic it will be difficult to form bars but it is usually possible by repeated cycles of rolling the non plastic clay between sheets of newspaper and peeling away the top layer and replacing it with a new piece, flipping the board, replacing that side and then carefully rolling it thinner. At the right thickness cut the bars using a compressing rather than cutting motion to prevent tearing the clay.

Make 5 or 6 bars. Stamp the identification and a sequential number on each. Carefully make 10cm-apart marks on each bar that can be accurately measured after drying and firing. Make a half length bar (for an LOI test).

Roll a 5 mm (3/16”) thick slab and cut a 12 cm diameter drying tester disk and put it 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.

Drying disk showing cracking
pattern and soluble salts

Soluble salts

Many native and commercial clays contain soluble materials (e.g. sulfates) that come to the surface with the water and leave solute behind that melts into a dirty glassy surface layer. This is a universal problem in the ceramic industry and is dealt with by the addition of barium carbonate (0.2-0.4%) during clay processing (it reacts with the sulfates to produce insoluble carbonates and barium sulphate).

On the circular drying disk (made above) solubles from the center migrate with the water to the outer exposed section and concentrate there during drying producing obvious differences in the surface coloration. Note the solubles deposits around the edges of the disk and fire it to see if they are harmful to the fired surface.

Note the nature and number of cracks on the circular drying disk and record (use method described for the SHAB test in the Digitalfire Reference Database.

Evaluate dry bars

Clean the bars using a knife to remove corner burs. Plastic clays are strong and hard to trim and they chip at corners. Highly plastic ones shrink a lot and warp and crack 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 water content as wet weight – dry weight / wet weight * 100. Kaolins have water contents of less than 20%, stonewares 20-23% and ball clays 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). 7% shrinkage is high but can be tolerated if coarse particles are available to terminate micro-cracks and prevent warping. 4% shrinkage is very low and normally accompanied by poor dry strength and low plasticity.

Seven particle size ranges showing
relative amounts of each

Seive analysis

Knowing the identity (non-clay particles are important too), amount, sizes, and proportions of sizes of apparent particles in a sample is important (the feel of a material can actually be misleading). Apparent particles and ultimate particles are vastly different (apparent particles are agglomerations of ultimate particles). Clays with a lot of harmful +100 mesh impurity particles (e.g. lime causes surface popping, iron sulfate causes specking) may be usable if you have a way of fine grinding or screening. If the clay contains a lot of quartz sand remember that these particles expand and contract suddenly as they pass upwards and downwards through quartz-inversion (about 1050F) producing micro-cracks around each grain in the rigid fired matrix. Such materials obviously must be fired slower (the sand performs the valuable services of terminating cracks and helping to channel water and gases of decomposition during drying and firing).

The apparent particle size is determined by washing material through a series of progressively finer sieves. Wash 50 grams through a 100 and 200 mesh screen (piled on top of each other). Dry the screens and weigh the material on each (and multiply by 2 for percentage). Examine the particles on the 100 under and 30X lightscope or microscope (available at Radio Shack). Are they quartz sand? Hard clay or shale? Lignite? Flakes of mica? Dark particles of iron or manganese material?

Fire the test bars

Using your experience of firing the initial lumps, guess the maturing temperature and fire bars to one or two cones higher and lower. Place bars on edge on the shelf.

If needed in your application fire one of the bars at an accelerated rate (compared to your typical firing) up to 500F to see if it can withstand faster firing (cracking or explosion indicates the presence of fine-particled bentonite or ball clay without interspaced water-channeling particles).

Fire the half-length bar to at least cone 02, record its weight and calculate LOI (Loss On Ignition) as dry weight – fired weight / dry weight * 100. Clays that contain organic matter lose weight on firing for obvious reasons (often 7% or more). Much of this amount is crystal bound water, but it is surprising how much smoke can be present in a rising 500F kiln.

Fire all the bars to intended temperatures being sure to include guide, guard and firing cones in each firing near the bars. Record the temperature reached according to degree of bend of the three cones (see the article in the Digitalfire Reference Database about interpreting the bend of cones) .

Evaluate porosity and shrinkage

Measure the distance between marks on each fired bar and record fired shrinkage as dry length – fired length / dry length * 100.

Record the weight of each bar, then boil all fired bars for 5 hours and soak for 19, blot dry and record the weight again. Record porosity is wet weight – dry weight / dry weight * 100. Plot a chart showing percentage porosity and shrinkage against temperature. Compare this chart with one for a standard commercial body you use with success. Vitreous stoneware bodies are best tuned to fire at the point where porosity and shrinkage curves are leveling out before reversing direction. Red earthenwares give best results at least 2-3 cones before they suddenly change color to dark brown. Sculpture bodies need a compromise of fired strength with resistance to fired warping.

Thermal expansion

The above testing should indicate the best firing temperature for the material. Apply a typical transparent glaze to a dry and bisque fired bar of the clay, fire and then stress its fit with two alternate two minute immersions in ice and boiling water. If it crazes the clay has a low thermal expansion, and is likely low in quartz. If it shivers the clay is likely high in quartz and would be a good body ingredient to prevent crazing.

Further evaluation

Now that you know what the material is in relation to kaolin, ball clay, fireclay, 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. To use as much as possible in a body, find other materials with complementing strengths and weaknesses.

Fired bars and glaze fired
drying/solubles tester

Making your own report

Do you want to go to the next step and really learn how to test materials and compile the test results in an organized way? There is a lot more information in the Digitalfire Reference Database (including a glossary). Our 4Sight Server software can complete change your quality control and lab testing program.

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By Tony Hansen

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