Clay body
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For potters and in industry, the term "clay" is not specific enough, thus the term "body" is used. Technically, "clays" are ingredients that can be found in glazes, engobes and in the body. Bodies have a much higher clay proportion than glazes. Thus there is danger of ambiguity when one refers to the body as "the clay". Notwithstanding this, the term "clay" is often used, e.g. "I am going to buy some clay". Clays that are ingredients in bodies and glazes are normally referred to by their mineral names in commerce (e.g. kaolin, ball clay, bentonite) and by their brand names.
A typical vitreous stoneware or porcelain body is mixture of raw and/or refined clays with likely additions of silica as a filler (and thermal expansion increaser) and feldspar as a flux. In bodies the larger grains of quartz and other refractory particles remain mostly unmelted while the clay particles go into solution in the feldspar glass or convert to mullite. Sintered and earthenware bodies have strength by virtue of whatever particle bonding mechanisms occur. When potters or technicians refer to the fired clay or porcelain matrix under the glaze they also call it "the body" (e.g. "the body is vitrified", "the glaze crazes on this body").
Plastic bodies require high percentages of ball clay (or ball-clay-containing materials) or additions of bentonite (or other plasticizer). Casting bodies can be made using only kaolin as the clay component of the recipe (since the body does not need high plasticity). That being said, casting bodies most often also contain ball clay to give them leather hard and green strength. Dust pressing bodies can be workable with even lower clay contents, relying on binders to impart dry strength. That being said, it is normal for such bodies to use high percentages of local clays with fillers to minimize firing shrinkage.
Testing your own native clays is easier than you think
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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).
Twenty six bodies. Porcelains and native. Which do I like best?
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I am testing runs of clays, at Plainsman Clays, both native bodies and porcelains. I got lots of s-cracks on the porcelains; the low humidity at this time of the year caught me by surprise. But I got zero cracks on native bodies. And I am using engobes on the natives, these can turn even a dark colored stoneware surface pure white (of a white body black). I use a mix of base and cover glazes that I make myself (using recipes I publish) for food surfaces and decorate some using bottled commercial glazes.
Step-by-step how to test and evaluate a new native clay
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"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.