|Monthly Tech-Tip |
Ceramic clays have a flat particle shape. Various factors determine the extent to which they can bind face-to-face in pugged clay in the presence of particles of other materials.
Key phrases linking here: particle orientation - Learn more
Clay particles are flat and prefer to orient or arrange in a lattice-like pattern during mixing and forming operations. In bodies where they mix with other non-clay ingredients, they continue to arrange to the extent to which they can tolerate the disruptions of the others. The smaller the clay particles are compared to the others, the better they can tolerate being mixed with other particles yet still impart good plasticity to the mass.
The pugging process, orients particles concentric to the center of the clay slug. Throwing a vessel on the potter's wheel lines up the particles in a similar manner. Rolling, casting, kneading operations affect particle orientation. Particle orientation imposes a big influence on a clay's drying shrinkage (a piece will shrink more along one dimension than another).
As particles draw closer and closer as clay dries, the mass becomes more and more dense. The clay particles fill in the space between the others (silica, feldspar, etc) in the matrix. When clay particles, and those of other minerals present, have a range of sizes (and shapes), the manner in which they can pack changes (and therefore drying shrinkage and dry strength). With sufficient testing on a specific body (having kaolin, ball clay, bentonite, silica, feldspar, etc), sweet spots of maximum density and minimum drying shrinkage be discovered and exploited. Without advanced testing equipment it is not possible to clearly describe why certain mixtures have so much better drying properties that others that are only slightly different, we simply speak of this as optimal particle packing.
When pugged clay freezes, then thaws, the manner in which it falls apart reveals particle orientation.
The way in which the walls of this bisque fired kaolin cup laminate reflect the plately and uniform nature of the kaolin particles. Because they are lining up during the wedging and throwing process, the strength to resist cracks is better along the circumference than perpendicular to it. The bonds are weak enough that it is very easy to break it apart by hand (even though it is bisque fired). The worst laminations were at the bottom where wall thickness was the most variable and therefore the most drying stresses occurred. However, if this kaolin were blended with feldspar and silica, this lamination tendency would completely disappear.
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.
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.
The surface area of a powder can be measured. It is the total surface area of all the particles in a gram of the material, and this number can be alot larger than you might think.
Laminations because of improper pugging of a clay body will cause separations and drying cracks in the ware.
|By Tony Hansen|
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