|Monthly Tech-Tip |
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).
For potters and in industry, the term "clay" is not specific enough, thus the term "body" is used. Clays are ingredients that can be found in glazes, engobes and in the body (but obviously bodies have a 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 when referring to the body used as a commodity that needs to be purchased (e.g. "I am going to buy some clay"). Likewise, clays that are ingredients in bodies and glazes are referred to by their mineral names in commerce (e.g. kaolin, ball clay, bentonite).
A typical 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 vitrified bodies the larger grains of quartz and other refractory particles remain unmelted while the clay particles go into solution in the feldspar glass. In these, a transformation occurs in the matrix, a new amorphous solid is born having grains, crystals and a glassy glue binding the whole mass together. Sintered and earthenware bodies have strength by virtue of whatever particle bonding mechanism 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).
Some simple equipment is all you need. You can do practical tests to characterize a clay in your own studio or workshop (e.g. our SHAB test, DFAC test, SIEV test, LDW test). You need a gram scale (accurate to 0.01g) and set of calipers (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, and learn from, all the measurement data collected. An account at insight-live.com is perfect.
I am testing runs of clays we (Plainsman Clays) make for potters. We are doing too many small-run products, bodies that we want to discontinue because others we are really good at making are much better. I am using native bodies and porcelains but the native ones will turn out best. 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 (I am also using a black engobe here). I'll use a mix of base and cover glazes that I make myself (using recipes we publish) for food surfaces and decorate some using bottled commercial glazes.
"It Starts With a Lump of Clay" (link below) is a step-by-step Insight-Live.com tutorial (from its help system) on how to document every step (in an account at at insight-live.com) of analyzing a raw sample of clay. You will learn things 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 do to with this data, how to store it, where to store it, how it can be searched, learned from, compared. This tutorial will erase that question.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
In ceramics, glazes are made by weighing out dry ceramic powdered materials to fill a recipe. Batch recipes often are a combination of a base recipe and additions.
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.
In ceramics, glazes and bodies have a chemistry, a mineralogy and a physical presence. All of these need to be understood to adjust and fix issues.