All Glossary

Native Clay

A clay that a potter finds, tests and learns to process. To reduce the costs of importing materials, manufacturers, especially in Asia, also often develop processes to enable the use of clays mined in their locality.

Key phrases linking here: native clay, found clay, wild clays, wild clay - Learn more

Details

“Native clay” is a term commonly used by potters for clay collected directly from a natural deposit, also called “wild clay” or “found clay.” It may be used as dug or dried, crushed, slaked and screened before use. Unlike commercial pottery bodies, these materials usually come without published property data: Their working and firing qualities must be discovered through testing.

With access to a kiln and basic equipment, potters can learn a great deal about a native clay. The SHAB test measures firing shrinkage and water absorption across a range of temperatures (plus drying shrinkage). It reveals changes in color and appearance and helps establish a useful firing range. Preparing the bars provides an initial feel for plasticity and workability. The DFAC test compares resistance to cracking under uneven drying and can reveal soluble salts migrating to the surface.

Potters are especially well equipped to take the next step: Make something from it. Slake the clay, screen it if needed, dewater it to a workable consistency (e.g. by slurry up and plaster batt) and try it on the potter’s wheel or build a small piece by hand. Once test bars have established a promising firing temperature, glaze and fire trial pieces. Used together, measured tests and actual making reveal both what the clay can do and what might need adjusting.

Related Information

The oven bird knows how to find and use wild clay:

If she can do it, maybe you can

This picture has its own page with more detail, click here to see it.

She is likely getting it from nearby. She knows what works: A mix of sand, clay and organic matter. She isn't even the best mud-nester in the bird world. Some species can stick their nests on to walls and cliffs.

If a bird can find and use a native clay, should a potter or even hobbyist be intimidated by the prospect of doing the same? She doesn't overthink it; she likely just samples different spots and uses the one that works the best. The clay appears to be a typical terracotta; if her nest were fired, it could well come out like these overlaid SHAB test bars (the two types, a sandy and smooth, were fired at cone 4 and 04).

Finding a native clay for pottery can begin just as simply: collect small samples from different spots, see how each one works, and watch how it dries. Then take the step the bird has no reason to take—fire test bars. These show how I compare drying shrinkage, firing shrinkage and fired absorption. A nest proves that mud can be built with. Test bars help determine what that mud can become in a kiln.

Testing your own native clays is easier than you think

This picture has its own page with more detail, click here to see it.

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).

Step-by-step how to test and evaluate a new native clay

This picture has its own page with more detail, click here to see it.

"It Starts With a Lump of Clay", a step-by-step Insight-Live.com tutorial (from its help system and the link below) on how to document every step (in an account at insight-live.com) of testing a raw clay. You will learn about drying shrinkage, drying performance, particle size distribution, plasticity, firing shrinkage, fired porosity, fired color, soluble salt content, fired strength, etc. We will not just observe these properties, but measure them. In doing so we will characterize the material. We will answer simple questions about how the material forms, dries and fires across a range of temperatures. In doing the testing I will be generating a lot of data. No single factor is more intimidating to new technicians than what to do with this data, how and where to store it, how it can be searched, learned from, compared.

Niko Leung uses "wild clays" from construction sites in Hong Kong

This picture has its own page with more detail, click here to see it.

Niko is trained as a product designer so the science and physics of clays and firing require lots of self-learning. She is "salvaging" red burning stoneware materials and hand building these large brick forms for firing between cone 7-8. Drying and firing such shapes without cracking is very demanding. For drying the blocks evenly she made a drying cabinet and put a hot fan to blow in warm air, maintaining an interior temperature of 45C (with high humidity). She puts the blocks on a piece of flat tile and covers them with a cloth to further assist in bringing away the moisture. She is able to fire the pieces on the third day after they have been in the drying tent for 2 days. The drying tent is an idea from the EKWC (European Ceramic Work Centre), these are often used for drying sculptures. Her firing schedule is 80C/hr to 140 with a 1-hour hold, 85C/hr to 650 with a 5-minute hold, 150C/hr to 1130 with a 30-minute hold, 60C/hr to 1230 with a 15-minute hold, 500C/hr to 1130 with a 30-minute hold, 83C/hr to 760.

Giant pestle crusher tool: "The Shusher"

A handy DIY tool for potters

This picture has its own page with more detail, click here to see it.

This inch-thick iron plate welded to a five-foot-long heavy pipe produces an ideal “mortar and pestle” style tool to break down dry clay lumps on a cement floor. I sometimes add side rails to contain flying lumps, but when crushing softer materials, like the clay shown here, they are not needed. On a heavy iron plate (instead of the floor), I can crush rocks and bricks. By incorporating appropriate sieves, I can effectively create granular material down to 50 mesh or finer.

I discovered the utility of this during the time the movie “Home” was popular, so I dubbed it “The Shusher” after Captain Smec’s control staff (more precisely, the rock on the end of it).

A plaster table: Better than a pugmill, essential for testing

This picture has its own page with more detail, click here to see it.

This is an example of an angle iron utility table being made into a plaster table. The cardboard sides extend upward to make the slab thicker and create a buffer gap to prevent the expanding and setting plaster from pressing outward on the frame. 150 lbs plaster (92 lbs water) was poured into the plastic-lined space (the bottom cardboard sections were supported from below). In a dry enough climate, this table could make enough clay to support slurry-to-plastic production for a potter (a thicker slab and a fan would enable even more capacity). The slurry-up process is better than a pugmill for small operations. It's much cheaper and is an easier way to utilize scrap material and weigh out custom recipes. The clay quality and de-airing is better (without hard chunks and contamination common to pugmilling). The procedure generates much less dust and the tank is easily cleaned. Slurries are easily sieved, especially if you have a sieve shaker.

Inbound Photo Links

Max out a native clay in a new glaze recipe: Turning Plainsman 3B into a transparent base glaze

Links

PayPal	No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me.

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk