|Monthly Tech-Tip |
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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 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.
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.
Let's suppose you need strength and density for utilitarian ware. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, water tight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet-spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low fire transparent base that works fine on this up to cone 2: G1916Q.
|Tests||Sieve Analysis 35-325 Wet|
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.
A database website where potters and ceramic technician account holders enter their recipes, materials, pictures, test procedures, firing schedules, etc.
Particle Size Distribution
Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.
Clays used in ceramics shrink when they dry because of particle packing that occurs as inter-particle water evaporates. Excessive or uneven shrinkage causes cracks.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
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.