Simple Physical Testing of Clays

Description

Learn to test your clay bodies and clay materials and record the results in an organized way, understanding the purpose of each test and how to relate its results to changes that need to be made in process, recipe and materials.

Article

Many technicians have amindset that is too narrow when it comes to dealing with clay body formulation. Compared to glazes, clay bodies and engobes are much more of an adventure in the mineralogy and physical properties of the materials. When glazes melt everything usually goes into solution in the melt, but the vitrification process is different. The differences in mineralogy; particle size, shape, size distribution and surface characteristics; body preparation, and ware forming methods; and firing history are examples of things that influence the final fired product.

Lab testing a clay for its physical properties
It only takes a few minutes to make these. But you would be amazed at how much information they can give you about a clay! These are SHAB test bars, an LDW test for water content and a DFAC test disk about to be put into a drier. The SHAB bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The tin can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI (from the measured values).

Consider these three simple tests that we find most practical for clay bodies: The SHAB test, LDW test and DFAC test. While these are easy to do it is tricky to organize all the data they produce (e.g. we are running these on dozens of clays at any given time, all in various stages of completion). Feeding the data into our insight-live.com account is the key to progressively learning from it.

This subject of physics reveals an interesting comparison between potters and industrial technicians. On one hand, the potter judges a body by how it feels in his hands, how it bends, stretches, pulls, how it behaves on the wheel, how it trims, how it dries, how it reacts visually with his glazes and fires in his kiln. But that potter may not be able to provide any data about things like fired density, thermal expansion, drying shrinkage, % water content, etc. On the other hand, a factory technician may have never hand-formed a ceramic object or even kneaded a piece of clay but he/she might be able to quote dozens of statistics about (as generated by testing devices). We advocate a middle ground: Hands-on experience applying the knowledge accumulated from affordable practical testing methods. Many body properties are immediately evident in the hands of an experienced potter and not quickly shown by instruments. Likewise, data from a test can really provide direction to resolve a problem or adjust a property.

Strangely, many large manufacturers in the ceramic industry do not actually have a standard testing and quality control program in place. It is common to rely completely on suppliers and their tech support. Troubleshooting manuals they supply speak the language of production-line workers with simple "if this happens do that" style instructions. What about people and companies who want to understand the why questions, become more independent? As noted, while many potter's textbooks are highly insightful and helpful there is no substitute for setting up a test program to accumulate some data.

Testing Categories:

Universal Standards

An example is the 50-volume Annual Book of Worldwide ASTM Standards (American Society for Testing and Materials). One of the volumes deals with refractories, glaze, and ceramic materials. The books are well organized and describe all test procedures in great detail. Just reference a test by number and you convey all details about how you achieve your results. However, these are not for the faint-of-heart. And they are not for people without the lab equipment called for.

Industry Specific Standards

Individual industries like construction, ferrous metals and electrical porcelain have outlined standard testing guidelines more specific to their needs, for example, ANSI (American National Standards Institute). Companies publish data sheets and advertising material in a format that voluntarily recognizes these standards.

Customer Required

Customers sometimes require manufacturers to document product quality and compliance (e.g. ISO 9000 which requires documentation on how tests are done, tolerances, noncompliance procedures, procedure change mechanisms, test equipment calibration schedules, proof of certification, etc). Unfortunately, the emphasis of all of this is on the production of documentation, not understanding the physics of the materials.

Compiling test bar shrinkage and weights for Insight-live
A batch of fired test bars, organized by temperature, have already been weighed (the weight is written on the side of each bar). Now they will be measured and the SHAB test data (shrinkage/absorption) entered into each recipe record (in an account at insight-live.com). From this data Insight-live can calculate fired shrinkage and fired porosity, enabling you to compare the degree of vitrification of different materials and bodies. This is especially good for quality control purposes.

Internal

Many tests are internal to a company, intended to solve problems, maintain properties critical to production efficiency and cost, control reject rates, etc. In this situation, one is free to formulate any method that seems best for the circumstances. Technicians generally have to be flexible and make do with what is available, so standard methods are usually adjusted. These simple tests are sometimes the most revealing and practical.

Getting Started with Internal Testing

We recommend starting with one of the tests built into Insight-Live. It predefines many and the ones of interest to us here are the SHAB test (Shrinkage, Absorption), DFAC test (Drying Factor), SOLU test (Solubles) and LDW test (LOI, Density, Water Content). The procedures for these describe how to make and process the three simple specimens I showed you at the beginning of this chapter (shrinkage bars, H₂O bars, drying disk). These provide a framework within which to begin gathering data and relating that to production needs.

The end-product of all your clay body testing work is to generate 'real numbers' that mean something; that can be compared with others to reach conclusions. So my advice is simple. Set up a little lab for yourself and take control of the physical properties of your clay bodies and materials.

Related Information

Drying cracks in bricks - but no data to determine best response

These bricks were being extruded in India and the plant was suffering drying cracks. A consultant recommended a high percentage addition of lignosulphonate, at a cost of $800/ton, as a solution. But before adding such a large expense, some obvious changes seemed in order first. The technician knew the plasticity index of the clay (a measurement used for soils) but he did not have records of its drying shrinkage, water permeability, drying strength or drying performance - when problems like this arise such data provides direction and help answer questions. For example, is cracking happening because of lack of drying strength or plasticity or because drying shrinkage is too high. The splitting along the corner of the extrusion is a clue that plasticity could be lacking - that could be solved by a small bentonite addition or reduction in grog. If permeability is low an increase in grog might be needed (if the pugmill can still extrude slugs with a smooth edge and corner). Notice the cracks that start from those splits (lower left). But also notice how the top edge has shrunk while the center section has not. That indicates the drying process is not tuned to subject all surfaces to equal airflow (sure enough, these are being dried outside in the sun and wind). Another factor is cross-section: The round holes create variations in thickness that exceed 300%, square holes with rounded corners would be better. Given the location, economic realities and past success this one change might be enough to make a big difference.

Kaolin changes raw color in second shipment

These are SHAB test bars prepared from two different shipments of the same commercial kaolin (used in porcelain body production). The darker one is markedly more plastic also. This underscores the need to characterize the materials you use in production and maintain an ongoing testing program. This difference was actually easy to deal with: Reduction of the percentage of bentonite in the body.

Clay lab report. Is this really what you need?

If you are trying to use local clays for brick or tile or even pottery production, characterizing the available materials is the first step. But how? This is the kind of data a lab might return - perhaps you wonder about its value? We feel traditional ceramics technology is fundamentally relative. A history of many reports like these, in context with other data, might be good for mining companies to determine if new stockpiles have any shifts in certain specific properties. Or a tile company evaluating a new ball clay. But as a way to understand the utility of a clay for a specific ceramic purpose, this contextless report is of little use. For example, the physical properties, the whole reason for using a clay, are unrelated to the chemistry. This is also a tunnel vision view, looking at only one temperature. On the other hand, simple procedures, like the SHAB test, provide a hands-on way to understand what a clay actually is.

Links

Tests	Pyrometric Cone Equivalent Make a pyrometric cone out of a clay or material to see what temperature it bends at
Tests	Drying Shrinkage Measure the amount a clay test bar shrinks as it dries
Tests	Firing Shrinkage Measure the amount a clay test bar shrinks as it is fired in a kiln
Tests	Dry Strength (kgf/cm2) Measure the tensile strength of a dried clay test bar
Tests	LOI/Density/Water Content LDW LOI, density and water content test procedure for plastic clay bodies and porcelains
Tests	LOI (100-1000C) Measure how much weight a dried clay test bar loses as it is fired in a kiln
Tests	Sieve Analysis Dry A measure of particle size distribution by vibrating a powdered sample through a series of successively finer sieves
Tests	Shrinkage/Absorption Test SHAB Shrinkage and absorption test procedure for plastic clay bodies and materials
Tests	Sieve Analysis 35-325 Wet A measure of particle size distribution by washing a powdered or slaked sample through a series of successively finer sieves
Tests	Soluble Salts Evaluate and compare the solubles salts content in clay bodies and materials
Tests	Density (Specific Gravity) Measure the density of a dried clay test bar
Tests	Sieve Analysis Wet A measure of particle size distribution by washing a powdered or slaked sample through a series of successively finer sieves
Tests	Dry Strenth (Round Bars)
Tests	Dry Strength (Square Bars) Measure the tensile strength of a square cross section test bar
Projects	Tests
Articles	How to Find and Test Your Own Native Clays 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.
Glossary	Physical Testing In ceramics, glazes, engobes and bodies have chemistries and physics. To fix, formulate and adjust their relative importances in each situation need to be understood.

By Tony Hansen
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