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The Physics of Clay Bodies
Section: Clay Bodies, Subsection: Testing
Learn to test your clay bodies and recording the results in an organized way and understanding the purpose of each test and how to relate its results to changes that need to be made in process and recipe.
If you have been working with glazes and glaze chemistry for some time, you may have developed a mindset that is too narrow when it comes to dealing with clay body formulation. Clay body analysis is much more of an adventure in mineralogy and physics than it is in oxide chemistry. Two clays of completely different physical properties can have very similar chemistry; two clays of radically different chemistry can have very similar physical and fired properties. Thus on the surface it would seem that chemistry is of little use in formulating and evaluating clay bodies. Actually, this is not quite the case, but it is not far from it.
When glazes melt everything usually goes into solution, but the vitrification process of a clay is quite different. The differences in mineralogy, particle size, firing history, body preparation, and ware forming methods all influence the final fired product. Thus the ability to measure physical clay properties is very important. In this section of the book, I am going to introduce you to some very simple clay tests that you can learn and do. They do not require advanced test equipment and they tell you an incredible amount about a material.
There is no time to waste in learning how to effectively test your clay bodies and materials. In this chapter I'll outline how you can go about getting a quality control program going on a low budget. Don't let anyone tell you that modern test equipment has supplanted this type of test. You can go into a lab full of million dollar test devices and ask the technicians to describe to you exactly what clay is and I'll bet few could do it in an understandable way that relates to the key reasons why we use clay in ceramics, namely plasticity and vitrification. They could likely show you thousands of numbers from DTA, CoE, XDF, etc. machines, but these are comparative measurements used for quality control, and technicians often lose sight of the reason some properties are even measured. While machines don't measure plasticity well and lab techs don't describe it well (the average potter could talk about the subject at length), it is directly related to shrinkage. The other reason for using clay is that it forms a rock when heated. Porosity and fired shrinkage measurements tell you how complete that process was.
Let us start quickly. Consider the three simple-looking specimens that make it all possible.
These three tests can completely change your view of clay bodies
As you will see, by making the above test specimens for a clay, you will be able to record its absorption and shrinkage over a range of temperatures, its water content, density, dry shrinkage, loss on ignition, soluble salts content, drying performance, glaze-over behavior, and dry strength. While these tests require very little investment in equipment (assuming you already have a gram scale and calipers), there is one testing device you really should buy: a good set of sieves. I will consider these in a separate section.
By doing these tests in a very standarized way, your data will be universal to all other tests that both you and others do (I'll explain what I mean by 'standardized' in a minute). It means you can compare clay properties using real numbers.
As most people have learned, glazes don't travel well. Still, we can compensate for this somewhat with calculations that attempt to preserve a glazes oxide formula into a new setting. But with clay bodies the added dimension of physical properties demands center stage. Unless you can test for them, you cannot even adjust a body let alone 'take it on tour'. For example, while you can usually exchange one kaolin for another in a glaze, such adjustments are likely to have considerable effects on a clay body's drying performance, green strength, fired color, and casting behavior, to mention only a few. Even changing the particle size of a constituent body material can have significant impact.
>This subject 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, or how successfully it dries with his ware and techniques. He evaluates it on how it reacts visually with his glazes and fires in his kiln; he dynamically adjusts procedures to compensate for changes he is able to perceive. On the other hand, a ceramic engineer may have never handmade a piece of pottery in his life. As a result, he may not fully appreciate what plasticity is, viewing it merely in terms of how the clay reacts in machines. To him, dried and fired properties exist as numbers produced by test equipment.
Potters often have excellent all-around knowledge; some have remarkable intuitive abilities at evaluating clay bodies; they like to look down at engineers whose cold numbers and charts keep them at a distance from the material. Some potter's textbooks are incredibly insightful and helpful. Yet there is no denying the value of good physical properties testing and hard test results. The ideal is probably a situation somewhere in between these two extremes. Many body properties are immediately evident in the hands of an experienced potter and not quickly shown by instruments. Likewise, differences shown by instruments can explain strange results in the potter's kiln.
There are still many companies in the ceramic industry that do not have a standard testing and quality control program in place. The question is, where does one start to test his clay bodies; how do you set up a simple but relevant program? One answer is FORESIGHT Ceramic Database software. It provides a way to define your own test procedures, variables, and equations in keeping with equipment you have. It acts as a platform from which to accumulate unlimited test results and allows you to search, query, and report these results as needed. Being a mature software solution it has a better chance of success than any effort to date.
In a few minutes, I will show you some reports generated by FORESIGHT for one of its predefined tests. But first, let us review the options you have with regard to setting up a test program.
An example is the 50-volume Annual Book of worldwide ASTM Standards (American Society for Testing and Materials, 1916 Race St, Philadephia, PA 19103). 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.
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, 1430 Broadway, New York, NY 10018). Companies publish data sheets and advertising material in a format that voluntarily recognizes these standards.
A customer will sometimes require that a manufacturer document quality and compliance of each product shipment. In this case, the client may reference a standard test or define his own test procedure for the manufacturer to carry out. With the advent of quality control standards like ISO 9000, customers are going to the next step and requiring documentation not only on how tests are done, but tolerances, noncompliance procedures, procedure change mechanisms, test equipment calibration schedules, and proof of certification.
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, the manufacturer is quite free to formulate any method that seems best for the circumstances.
Tests have typically required expensive equipment. In the real world, technicians generally have to make do with what is available, so standard methods are usually adjusted. This is not necessarily bad. Simple tests are sometimes most revealing (excellent examples are the FORESIGHT DFAC and SOLU tests). It is important to think a test through thoroughly, document it, and analyze the information it provides. If you can prove the value of the information, customers will respond positively and production yields and quality will improve.
Implementing a Test
Define the test
Document the test
Set up the software
Put the test into practice as documented
Accumulate tolerance samples
Analyse the results and take corrective action
FORESIGHT predefines many tests and the ones of interest to us here are the SHAB (Shrinkage, Absorption), DFAC (Drying Factor), SOLU (Solubles), and LDW (LOI, Density, Water Content). The procedures for these four 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). They also provide a framework within which to gather data.
This is the first part of the Test Procedure report for the SHAB test. It is formatted like a standard ISO 9000 style procedure.
PROCEDURE NO 02-012-002 REVISION NO: 2 DATE: 03/16/97 PAGE 1 OF 8
Title: SHAB - SHRINKAGE/ABSORPTION/H2O
1. Purpose of Test
1.1 This test is designed to measure dry shrinkage, absorption and fired shrinkage properties. Results from this test are repeatable if instructions are followed closely.
1.1.1 DRY SHRINKAGE As a clay dries the removal of interparticle water causes the mass to tighten up and pack together resulting in shrinkage. Clays of fine particle size and those of high plasticity have high shrinkage. Unfortunately the benefits of plasticity are offset by drying problems. Variation in drying shrinkage is an indicator of changes in a clays plasticity. However comparing the dry shrinkage of different types of clay is not necessarily in indicator of their comparative plasticity since some fine clays are not plastic. Note that higher water content also means greater dry shrinkage.
For typical modeling stiffnesses dry shrinkage for non-plastic clays is around while plastic clays which require care in drying are usually above 7.0%. High shrinkage can be reduced by the addition of an aggregate however this can produce a matrix where micro-cracks radiate outward from each of these larger particles creating a weaker dried and fired product. A low drying shrinkage is important to successfully dry larger items or ware of uneven cross section.
Dry shrinkage is simply the per cent change in length between wet and dry. The SHAB test provides the data for this property as follows:
Wet length - dry length / wet length * 100
or where a 10 cm marks are stamped on the bar it is simply:
100 - mm dry length
1.1.2 FIRED SHRINKAGE As a clay fires, it shrinks and particles continue to pack together. At some point, they begin to break down and react with each other, fluxes begin to melt and flow, and mineral grains seed the development of more stable forms. The amount of shrinkage during firing is thus an indication of the degree to which the complex "maturing" process has proceeded.
This report shows the variables defined for the SHAB test. Data is to be collected for each variable. It also displays calculated fields and the equations used to derive them from the variable data.
TEST DEFINITION REPORT ====================== Date: 03/16/97 Description: SHRINKAGE/ABSORPTION/H2O Four Letter ABBR: SHAB TESTID Code (TESTDATA LINK): ABPurpose: This test is designed to derive shrinkage and absorption data by drying and firing clay bars according to a detailed procedure. This test is recommended over the SAWL test since its fewer variables mean that you can route results reports to the screen and they will fit within 80 columns. This test does not account for LOI as does SAWL, however it is assumed that you will also use the LDW test.Much of the theory behind why this test is beneficial is dealt with in the Digitalfire book "The Magic of Fire II". Like other tests defined here, it is assumed that you have the FORESIGHT software to log results.This test is meshed with the SOLU, DFAC and LDW tests in that it is very convenient to perform all four at the same time.VARIABLES (each shown with VAR NAME & TESTDATA link) =====================================================1) Dry Length...................DRY-LEN " 1" The distance between the outer edges of two marks on the dried clay bar as measured with a set of calipers. These marks were pressed into the wet bar at exactly 10 cm apart on the outer edges.2) Fired Length.................FIR-LEN " 2" The length between two marks on the fired clay bar as measured with a set of calipers.3) Fired Weight.................FIRE-WT " 3" The weight in grams of the clay bar after firing.4) Boiled weight................BOIL-WT " 4" The weight in grams of the clay bar after boiling for 5 hours and soaking for 19 and being blotted on a towel.5) Fired cone...................CONE " 5" The Orton cone number to which the bar was fired. Take the highest cone to show deformation and interpret it as follows:Position Value ~~~~~~~~ ~~~~~ 1 oclock n-.4 2 oclock n-.3 For example, if cone 6 is at 3 oclock, then the 3 oclock n-.2 value is 6 minus .2 = 5.8. 4 oclock n-.1 5 oclock nIf the guard cones shows deformation you must increase the figure appropriately.6) Ring diameter................RING " 6" The diameter of a bullers ring which has been included in the firing. This ring must be on edge in a holder to allow heat access from all sides.7) Bar color....................COLOR " 7" The color of the bar (e.g. GOOD, DARK, LIGHT, SPECK, UNDERM, OVERM, etc.)CALCULATED FIELDS (each shown with ABBR, TESTDATA link, FIELDNAME) =================1) Fired Shrinkage..............F SHR " 1"EQUATION: IF(V>0 .AND. V>0, str((V-V)/V*100,6,2)+"%", " n/a")This is the fired component of total shrinkage. Note that dry+fired shrinkage does not equal total shrinkage because fired shrinkage is based on the dry length not the original 10 cm.2) Dry Shrinkage................D SHR " 2"EQUATION: IF(V>0, str(100-V,5,1)+"%"," n/a")This is the shrinkage due to drying only. Assuming 10 cm marks on the wet bar, drying shrinkage is simply 100 mm minus the dry length in mm.3) Aborption....................ABSORP " 3"EQUATION: IF(V>0 .AND. V>0, str((V-V)/V*100,5,1)+"%"," n/a")This is a measure of the clays fired maturity as interpreted from its pore space. The pore space is calculated from the increase in weight a bar experiences during boiling in water.
This report is actually a form that can be given to your assistants to use to record data as it is collected. The data from these forms can then be keyed into
FILL IN FORM: SHRINKAGE/ABSORPTION/H2O ID: "AB" ABBR: "SHAB" ============================== Variables to measure are:
DRY-LEN - Dry length of unfired thoroughly dried bar (assuming that 10cm marks were pressed into bar when it was wet) FIR-LEN - Fired length of bar FIRE-WT - Weight of bar after firing BOIL-WT - Weight of bar after 5 hour boil and 19 hour soak CONE - Orton cone to which bar was fired RING - Bullers ring measure after firing COLOR - Color judgement on fired bar. DESCRIPTION ID NUMBER SPECIMEN DRY-LEN FIR-LEN FIRE-WT BOIL-WT CONE RING COLOR +-----------------+-----------+---------+--------+--------+--------+--------+--------+--------+--------+ | | | | | | | | | | | +-----------------+-----------+---------+--------+--------+--------+--------+--------+--------+--------+ | | | | | | | | | | | +-----------------+-----------+---------+--------+--------+--------+--------+--------+--------+--------+
This test results report was generated from a record for which test data has been accumulated.
TESTDATA REPORT FOR A RUN *TEST Definition reports are available for tests results shown ======================================== NUMBER: L2537E DESCRIP: JIM COOPER'S TRANSLUCENT C10R PORCELAIN CODENUM: DATE: 02/25/97 LOCATION: BD 749 ======================================== This is a series of bodies that were mixed after reading Dave Beumee's article in Ceramics Monthly January 1994. Nepheline syenite melts faster than the potash feldspars so this body may tend to warp more. Compared to the other porcelains tested, this has excellent workability, the best translucency, excellent whiteness, no absorption, slight slumping, 17.5% shrinkage at cone 10 and glaze fit of GTS-3-10 (see #L2538). I made one vase and transparent glazed it. The bare surface fired was probably smoothest and it was was the darkest fired color. It reacted visually with the glaze the same as the others. SHRINKAGE/ABSORPTION/H2O (ID-AB, ABBR-SHAB) DRY-LEN FIR-LEN FIRE-WT BOIL-WT CONE RING COLOR F SHR D SHR ABSORP +-------+-------+-------+-------+-----+-----+-----+ 6 |95.56 |86.65 |35.82 |35.83 |6.7 | | | 9.32 4.4% 0.0% 7 |95.65 |86.9 |35.42 |35.43 |6.9 | | | 9.15 4.3% 0.0% 8 |95.73 |87.16 |38.31 |38.31 |7.7 | | | 8.95 4.3% 0.0% 9 |95.51 |87.5 |41.27 |41.28 |9.2 | | | 8.39 4.5% 0.0% 10 |95.56 |88.55 |40.6 |40.6 |9.7 | | | 7.34 4.4% 0.0% 11 |95.75 |88.86 |41.86 |41.92 |10.9 | | | 7.20 4.3% 0.1% 12 |95.74 |86.11 |40.47 |40.47 |10R | | | 10.06 4.3% 0.0% +-------+-------+-------+-------+-----+-----+-----+ DRYING FACTOR (ID-DF, ABBR-DFAC) DRY_FAC - A000 LOI/Water Content (ID-LW, ABBR-LDW ) WET-WT DRY-WT FIRE-WT OIL-WT IMM-WT PERCENT LOI DENSITY +-------+-------+--------+-------+-------+ 1 |33.49 |26.34 |24.7 |26.6 7 |11.9 | 21.3% 6.2% 1.81 g/cc +-------+-------+--------+-------+-------+ SOLUBLES (ID-SL, ABBR-SOLU) FIRED GLAZED DRY +-----+------+-----+ 1 |NIL | | | +-----+------+-----+
The end-product of all your clay body testing work is to generate 'real numbers' that mean something;reliable numbers that can be compared with others to reach conclusions. While the above report may appear a little foreign, it all comes together when you see it in terms of the structured set of variables which are defined for each test. This is a basic report showing gathered data and the results of equations applied to that data. But it is a beginning of a flexible testing system on which much more graphical reports can be built.
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.
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