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).

Related Pictures

Get a kitchen blender for mixing ceramic slurries

Blender mixing is invaluable in slurry preparation in small scale ceramics and testing. It is quick and so effective that not only are particle surfaces wetted much better but clay particles can actually be reduced in size (literally ground finer). Slurry rheology is also stabilized. For example, thixotropy can often be achieved naturally, without any additives. Materials that are otherwise impossible to mix into a slurry (e.g. bentonite, Veegum, CMC gum) or just difficult (e.g. titanium dixoide, tin oxide) are no problem. Even slurry-processed porcelains benefit, not only being more plastic, but firing to a more homogeneous surface and to greater density. Just visually, it is easy to see how much improved this MNP slurry is (a local clay with porcelaneous properties). It was mixed using our propeller mixer and seemed OK (on the left). But the improvement after only 20 seconds in the blender (right) is amazing.

These are behind the art center. A dirty secret. Or an opportunity!

Barrels of scrap clay behind the art center

When many clays are in use there is good reason to collect like this - you will see why in a moment. Each barrel is a unique product, a mix of random clays used by students. Each could be thus considered a wild clay, inconsistent throughout and with impurities. As such, each needs to be characterized, as a whole. For example, consider the front barrel: What does it look like when fired at various temperatures? At what temperature does it mature? What is the drying shrinkage and how plastic is it? Does it fit the glazes we use? This information not only describes the clay but also points to what needs to be added to make it useful (e.g. bentonite or feldspar). An account at Insight-Live.com is a way to organize the testing and post the results to a group (the SHAB test, for example, is perfect to describe the plastic, drying and firing properties). Once the clay in a barrel becomes predictable then it becomes useful.
To do the testing it is necessary to get a representative sample from each barrel. The method depends on how processing will be done. In a wet climate, the wet contents are likely to be thrown into a pugmill (with impurities), recycled until mixed and then bagged - a sample can be taken when complete. But the dry climate option is much better: Dewater each barrel, do quartering to get a representative sample and then characterize it. Finally, slake, slurry up batches using a propeller mixer, screen in a vibrating sieve and then dewater (e.g. on a plaster table).

Mother Nature's stoneware and glazes!

To make these cone 6 mugs I slake and slurry up raw lump Plainsman MNS clay and process it through a 325 mesh vibrating sieve. This natural porcelain vitrifies to zero porosity at cone 6, yielding incredibly strong ware. I also make these outside and inside glazes, GA6-C and GA6-B, from clays we mine in Montana and Alberta. Using glaze chemistry I can maximize the clay percentage to more than 70% of the recipe (for firing at 2200F). Of all the pieces I make, ones like these mean the most to me.

A must-have: Laboratory variable speed propeller mixer

If you are at all serious about testing glazes and clay bodies, you need one of these. There are other methods, but nothing else comes close to this. It is the most valuable and frequently used tool in any ceramic bodies and glazes testing lab or classroom. These are expensive new, this Lightnin 1/20 hp variable speed cost more than $1000 many years ago, now it could be $4000! But you can get them used on ebay.com, it uses a 7.9mm dia (5/16") shaft. I adapted a mount (to give it vertical adjustment) from a hardware store. Propellers are also expensive, but you can design and 3D print them yourself or have them printed at a place like shapeways.com.

Heavy duty mixer mounted on a steel pole

It is adjusted so the shaft is at an angle (rather than straight up and down) to pull less air bubbles into the slurry. It can mix up to 5 gallons of viscous glaze or body slurry. The motor is very powerful enabling the mixing of low water content slurries (this means that amounts of less than about 2 gallons of slurry can splatter quite a bit). The 1/2 inch shaft is 22 inches long and the propeller is mounted up from the end of the shaft.

Niko Leung finding and testing clays in Hong Kong

She is using the DFAC test and SHAB test to systematically characterize each clay. Based on the results she will be able to either blend them or incorporate other materials (e.g. bentonite to add plasticity, feldspar to aid vitrification, kaolin or ball clay to impede vitrification, barium carbonate to precipitate solubles salts, silica to help with glaze fit, etc.). She is using code numbering to organize testing and material location records.

Why you need to make your own glazes and have a test kiln

Oatmeal cones 6 glazes needs improvements

Are you a potter that depends on glazes made by others? Do you have your ware fired in someone else's kiln? Cannot mix clay body tests? Then the evolution of the quality and aesthetics of your work may be stunted. This mug is a good example of tests I need to do. This is G3933, made by adding iron, rutile and tin to a 75:25 blend of our base matte and glossy glazes. It is crawling at some of the sharp angles of the incised decoration, which means it needs a little CMC gum. And, I need to switch to an 80:20 blend for more matteness. Third, our red-burning body gives better color at cone 5, I want to test this glaze in the C5DHSC slow cool schedule. Finally, I want to test increases in the rutile and iron. All of these changes are on my radar because I have my own test kiln and an account at insight-live.com to document them.

Links

URLs	https://insight-live.com/insight/help/It+Starts+With+a+Lump+of+Clay-433.html Case Study: Testing a Native Clay Using Insight-Live.com
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.
Glossary	Characterization In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.
Glossary	Clay body A term used by potters and in the ceramic industry. It refers to the earthenware, stoneware or porcelain that forms the piece (as opposed to the engobe and covering glaze).
Glossary	Native Clay A clay that a potter finds, tests and learns to process and use himself. To reduce the costs of importing materials manufacturers, especially in Asia, often develop processes for clays mined in their locality.
Glossary	Slurry Up The process of slurrying a clay body powder and dewatering it on a plastic slab or table.
Glossary	Brick Making Brick-making is surprisingly demanding. Materials blending and processing, forming, drying and firing heavy and thick objects as fast as possible are like no other ceramic manufacturing challenge.
Glossary	Digitalfire Insight-Live A cloud-hosted ceramics-targetted LIMS (lab info management system) enabling collection, organization and learning from data to develop, adjust and study their recipes, materials and processes.
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.
Tests	Shrinkage/Absorption Test SHAB Shrinkage and absorption test procedure for plastic clay bodies and materials

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