|Monthly Tech-Tip |
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.
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 bodies 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 of a clay is different. The differences in mineralogy, particle size, firing history, body preparation, and ware forming methods all influence the final fired product. Simple tests can indicate plasticity, 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.
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 test that we like most for clay bodies: The SHAB test, LDW test and DFAC test. While these are easy to make it does take a few weeks to bring them through all the stages of the process (of course we are running these tests on dozens of clays at any given time and so firing is done in batches).
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, how it dries, how it reacts visually with his glazes and fires in his kiln. He/she adjusts procedures, recipes and materials to compensate for perceived changes. On the other hand, a ceramic engineer may have never hand-made a piece of ceramic in his life or even kneaded a piece of clay. As a result, he/she may not fully appreciate what plasticity is, for example, viewing it merely in terms of how a clay reacts in machines. To him, dried and fired properties exist as numbers produced by test equipment.
Traditionally potters often had excellent all-around knowledge and intuitive abilities at evaluating clay bodies. Many potter's textbooks today are highly insightful and helpful. Yet there is no denying the value of good physical properties testing and the concrete it produces. 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 physical testing can explain strange behaviour in the process or kiln.
Many large manufacturers in the ceramic industry do not have a standard testing and quality control program in place. It is common to rely completely on suppliers and their tech support. Trouble-shooting 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, setting up a test program to accumulate some data is a good start.
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.
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.
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 documentation and paper, not understanding the physics of the materials.
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 make do with what is available, so standard methods are usually adjusted. This is not necessarily bad. Simple tests are sometimes most revealing and practical.
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.
We used to have a bunch of information on setting up tests here, but now it seems simpler to just recommend starting with one of the tests built in to Insight-Live. It predefines many tests 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. 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.
Data for hundreds fired clay test bars was logged into a portable Epson custom programmed HX-20 computer and uploaded to a Radio Shack TRS-80 Model III where it was stored first on cassette, then floppy disk, then a loop tape. That data was later migrated to the Digitalfire DOS 4Sight lab record keeping system (as SHAB specimens) where it lived for more than 27 years (expanding to more than 200,000 tests) until being imported to an insight-live.com account in 2014.
A batch of fired test bars that have just been boiled and weighed, from these we get dry shrinkage, fired shrinkage and porosity. Each pile is a different mix, fired to various temperatures. Test runs are on the left, production runs on the right. Each bar is stamped with a code number and specimen number (the different specimens are the different temperatures). The measurements have all been entered into our group account at insight-live.com. Now I have to lay out and photograph each pile and upload the picture into the code-numbered record. Upon doing so I compare color and tests results to make decisions on what to do next (documenting these in insight-live).
The ideal drying chamber is a tunnel. Starter tunnels pass wheeled-ware-carts single file. Hot dry air enters where the ware exits. The moving air touches all surfaces and picks up humidity as it moves toward the ware entrance. The tunnel must be calibrated so that air reaching the entrance, is still very warm, but of high humidity (laden with water it got from ware down the tunnel). When an equal volume of ware is passing constantly, manual calibration of cart movement, air volume and temperature is possible. But if flow is not constant then your "dynamic system" needs multi-location monitoring and intervention. Locating wireless thermometer/hygrometers and actuators is a good early-start to the project. ESP8266 controllers are revolutionizing industrial control. As cheap as $5, they are tiny but completely capable battery-powered WIFI servers. One of these little things can email you! Even display a web page. These communicate with a central dashboard online (in-plant control systems are now obsolete). There are many online dashboard services that talk to these devices and display results graphically. And it is easy to make your own. Hiring a technician on upwork.com to design a system for you is only a matter of a few thousand (even hundreds) of dollars. Shown here is an Amazon listing for a development kit of an 8266, sensor and cables. I included a listing for a ready-made one, but it is expensive, not well described. A similar product line sells under the name "SensorPush".
Material prices are sky rocketing. And, the more complex your supplier's supply chain the more likely they won't be able to deliver. How can you adapt to coming disruption, even turn it into a benefit? Learn to create base recipes for your glazes and even clay bodies. Learn now how to substitute frits and other materials in glazes (get the chemistry of frits you use now so you are ready). Even better: Learn to see your glaze as an oxide formula. Then calculate formula-to-batch to use whatever materials you can get. Learn how to adjust glazes for thermal expansion, temperature, surface, color, etc. And your clay bodies? Develop an organized physical testing regimen now to accumulate data on their properties, learn to understand how each material in the recipe contributes to those properties. Armed with that data you will be able to adjust recipes to adapt to changing supplies.
|Tests||Pyrometric Cone Equivalent|
|Tests||Sieve Analysis Wet|
|Tests||Dry Strenth (Round Bars)|
|Tests||Dry Strength (Square Bars)|
|Tests||Density (Specific Gravity)|
|Tests||Dry Strength (kgf/cm2)|
|Tests||Sieve Analysis Dry|
|Tests||Sieve Analysis 35-325 Wet|
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.
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.
Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
Stoneware Casting Body Recipes
Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them