All 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.

Key phrases linking here: physical properties, physical testing, physics - Learn more

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In many circles today the chemistry of materials and compounds is over-emphasized - often to the exclusion of physics, which is typically more important. A long-term program of quality control measurements and documentation is an essential context in which to improve bodies, glazes and production processes and deal with anomalies.

While ceramic glazes fire the way they do mainly because of their chemistry, physical factors of the materials are also very important (e.g. particle size and shape and surface area, particle mineralogy and phase, solubles content, volatiles content, decomposition history). The chemistry of clay bodies is much less important - technicians in traditional ceramics want to know about the plasticity, casting properties, drying shrinkage and performance, fired shrinkage and warp and porosity across a range of temperatures, etc. These are physical factors that are very mostly unrelated to chemistry, they must be observed and measured methodically.

Supply and price issues are often a main focus. Recipes tend to be viewed as cast-in-stone and companies are afraid to use new materials where there is not a good context of testing and understanding of the variables that must be managed. Yet, while technicians study, compare, research and log a wide range of physical property data for their materials and recipes, a common oversight is an effective way to record, organize and use the data. Spreadsheets are the wrong tool, data belongs in a searchable database where many projects can be managed simultaneously. Consider an example: You are measuring and recording the weights of a few dozen SHAB test bars, would it make sense to hunt for and open all those spreadsheets to enter each? Or enter them into a dialog designed specifically for that in an account at Insight-live.com?

Ceramic materials for glazes source oxides to the chemistry of the fired product and physical properties to the slurry and laydown. Ceramic materials source physical and fired properties to bodies (e.g. plasticity, casting performance, drying characteristics) but also to the physical presence of the fired product (e.g. maturity, thermal expansion, durability, etc).

What if you do not have expensive testing equipment? This may not be the disadvantage that you think. Equipment that generates pages full of numbers that often have little to do with the practical use of the material in traditional ceramic production can be more a barrier to understanding that a help. Simple tests specific to your application, done diligently over time, and recorded in a database are better anyway.

Related Information

Specification for a Tapper Clay lacks physics

Often ceramic clays are described on data sheets primarily by their chemistry (and requested as such). This is typically done at the expense of physical properties data. For example, Tapper clay is employed to plug the drain hole of ladles used to melt metals in the smelting industry. The operators of that equipment confront, in the physical presence of the material, many properties that have no relation to the chemistry (e.g. plasticity, shrinkage, water content). Notice also that the chemistry is not correct anyway, it species calcined material yet does not total 100. It specifies no carbon, yet this chemistry is like a ball clay, all of which have some carbon.

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).

Why does Tony Hansen take months to unload his kilns?

I love making pottery, but I love the technical side more. I searched for all the test specimens in this 2020 load of cone 10 reduction ware first, then pushed it back in and forgot about it. For three months! I really anticipate the test results (I am developing and adjusting many of bodies and glazes at any given time). The data and pictures for them go into my account at insight-live.com, it enables me to compare the chemistry and physical properties of recipes and materials side-by-side. That teaches me which roads to abandon and which ones to pursue. My last kiln went back in for six weeks, so things are getting worse! If you are wondering, we use the C10RPL firing schedule.

We fight the dragon that others do not see

There are thousands of ceramic glaze recipes floating around the internet. People dream of finding that perfect one, but they often only think about the visual appearance, not of the usability, function, safety, cost or materials. That resistance to understanding your materials and glazes and learning to take control is what we personify as the dragon. Using the resources on this site you could be fixing, adjusting, testing, formulating your own glaze recipes. Start with your own account at insight-live.com.

Stamp used for stamping information onto clay test bars

This type of stamp is ideal for marking code number and ID information on SHAB test specimens (and many others) while in the plastic form. Set up the run or recipe number on the left and the specimen number on the right. You can find these stamps on Amazon by searching "12 digit rolling alphabet symbol number stamp".

Fired clay test bars ready for measuring

SHAB (Shrinkage, Absorption) test bars ready to unload. These are measured for length after drying and firing and for weight after firing and boiling. This data is plugged into my account at insight-live.com and it calculates shrinkage and porosity numbers. If you fire bars of a clay to a range of temperatures you can characterize key properties of a clay very effectively.

The kaolin arrives on a semi of 880 bags. First step: Record the date code.

A shipment EP Kaolin has arrived for use in some of our production porcelain and stoneware bodies. Of course, this needs to be tested before being put into product. But how? The first step is to create a new recipe record in my Insight-Live account, and find their production date code stamp on the bag. Hmmm. It does not have one! OK, then I need to record the date on which we received it. We need to save a bag on every pallet and sieve 50 grams through 100 mesh (to spot contamination). Then we'll make test bars (of all the samples mixed) to fire across a range of temperatures (to compare fired maturity with past shipments). We do a drying performance disk also to assess soluble salts.

Do not rely on material data sheets, do the testing

The cone 6 porcelain on the left uses Grolleg kaolin, the right uses Tile #6 kaolin. The Grolleg body needs 5-10% less feldspar to vitrify it to zero porosity. It thus contains more kaolin, yet it fires significantly whiter. Theoretically this seems simple. Tile #6 contains alot more iron than Grolleg. Wrong! According to the data sheets, Grolleg has the more iron of the two. Why does it always fire whiter? I actually do not know. But the point is, do not rely totally on numbers on data sheets, do the testing yourself.

How many simultaneous testing projects can you manage at once?

This two-inch pile of lab test body and glaze mix tickets is about half of what I have tested in the past year. I have added thousands of pictures too (using my smartphone). I just realized why I am doing a lot more testing. It is so much easier to organize the record keeping in my Insight-live.com account. I can manage so many separate testing projects. I have so much more of a sense that I am progressing. And it feels great to build such an organized set of records that I can refer back to.

Can you afford to completely trust supplier body quality control reports?

This body is used in the sanitary ware industry in China, the supplier sends this report with each shipment. The chemistry and assorted values for porosity, shrinkage, particle size are provided. The factory receiving this report accepts it as gospel and goes into production. However engineers at the plant need to think twice about such reports. These tests are being done at one temperature, they say nothing about what that body is doing above and below that temperature. Is it being employed in a volatile range of the porosity or firing shrinkage curves? Zero porosity bodies of this type are best when fired to a point near where the porosity curve descends to reach the x-axis. However that curve remains at zero while the shrinkage one tops out and reverses direction. At some point the porosity curve sharply rises. Only by firing and testing at a range of temperatures in your own lab can you where your body is on the curve.

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.

Identifying throwing tests of clay bodies

If you are doing testing, and everyone should be testing body and glaze variations, then your ware needs to be identified. Do that with a code number that cross references into your documentation in your account at Insight-live.com.

Testing your own native clays is easier than you think

Some simple equipment is all you need. You can do practical tests to characterize a local clay in your own studio or workshop (e.g. our SHAB test, DFAC test, SIEV test, LDW test). You need a gram scale (preferably accurate to 0.01g) and a set of callipers (check Amazon.com). Some metal sieves (search "Tyler Sieves" on Ebay). A stamp to mark samples with code and specimen numbers. A plaster table or slab. A propeller mixer. And, of course, a test kiln. And you need a place to put all the measurement data collected and learn from it (e.g. an account at insight-live.com).

A kiln load going in with no tests. What a shame.

Next kiln: Had tests. But no documentation system to learn from those tests. What a shame! Even if you are potter or hobbyist, it is always good to be testing something new. An oxide or stain added to a base (or a variegator like rutile or titanium). A new base clear or matte that fits your clay bodies better. A new stain or commercial glaze. A found-clay. A new body/glaze combination. But every time you test something new, be sure to keep a record (with notes, recipes, pictures) in your account at insight-live.com.

Is industry misdirected by chemical analyses?

In response to a query about the suitability of a clay-feldspar material for use in our porcelain bodies this company asked for our "index requirements". They want to know what chemistry I want (crediting all pertinent properties to the chemistry). But physical properties like plasticity, drying performance, green strength, soluble salts are unrelated to chemistry; and fired properties like shrinkage, deformation and density are imprecisely related. Yes, Fe₂O₃ and TiO2 percentages do relate to whiteness and translucency for different shipments of the same material but much more poorly when comparing products from different manufacturers. The same is true of feldspars, because one has a higher flux content on a data sheet by no means indicates it will vitrify a porcelain better (it depends what minerals are delivering the fluxes and what other minerals are present).

Ball clay data sheet demonstrates the merit of physical testing

Purchasing agents do not know about ceramics so they like to compare numbers on data sheets when second sourcing materials. But this data sheet demonstrates an issue with that. Although they have dramatically different plasticities (as suggested by drying strength and drying shrinkage) the chemistry of these two ball clays gives no indication of this (nor can it). The No. 1 has a finer size, which means it should be more plastic, right? But it isn't. Ball clays are used in ceramics principally for their plasticity and only actual physical handling of the plastic material enables comparing this important physical property.

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.

#6 Tile Kaolin 2021 vs EP Kaolin 2020: Fired properties tell an unexpected story

These test bars are fired at cone 10 reduction (top) and 10, 9 and 8 oxidation (downward) - they demonstrate the importance of measuring physical properties. The charts for each show data for drying shrinkage, firing shrinkage and porosity (water absorption). The left bars are #6 Tile kaolin and the right are EPK. These two kaolins had almost the same drying shrinkages in this SHAB test, that suggests the same plasticity. And the EPK fires whiter. So it should make a better plastic porcelain, right? Not so. In reality, #6 Tile kaolin is far more plastic - EPK requires the addition of significant bentonite to equal it (bentonite is dirty and that compromises whiteness). And EPK fires less vitreous - when feldspar is added color darkens. And, although both have extremely high firing shrinkages, the EPK is much higher than the #6 Tile (even though it is not as vitreous). Bottom line: #6 Tile is a better kaolin for clay bodies.

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

Historical data on drying and firing casting slip performance

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